Review Article Drought Tolerance in Modern and Wild...

Hindawi Publishing CorporationThe Scientific World JournalVolume 2013 Article ID 548246 16 pageshttpdxdoiorg1011552013548246

Review ArticleDrought Tolerance in Modern and Wild Wheat

Hikmet Budak Melda Kantar and Kuaybe Yucebilgili Kurtoglu

Biological Sciences and Bioengineering Program Faculty of Engineering and Natural Sciences Sabanci University 34956 TuzlaIstanbul Turkey

Correspondence should be addressed to Hikmet Budak budaksabanciunivedu

Received 5 March 2013 Accepted 3 April 2013

Academic Editors J Huang A Levine and Z Wang

Copyright copy 2013 Hikmet Budak et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The genus Triticum includes bread (Triticum aestivum) and durum wheat (Triticum durum) and constitutes a major source forhuman food consumption Drought is currently the leading threat on worldrsquos food supply limiting crop yield and is complicatedsince drought tolerance is a quantitative trait with a complex phenotype affected by the plantrsquos developmental stage Droughttolerance is crucial to stabilize and increase food production since domestication has limited the genetic diversity of crops includingwild wheat leading to cultivated species adapted to artificial environments and lost tolerance to drought stress Improvement fordrought tolerance can be achieved by the introduction of drought-grelated genes and QTLs to modern wheat cultivars Thereforeidentification of candidate molecules or loci involved in drought tolerance is necessary which is undertaken by ldquoomicsrdquo studies andQTLmapping In this sense wild counterparts of modern varieties specifically wild emmer wheat (T dicoccoides) which are highlytolerant to drought hold a great potential Prior to their introgression to modern wheat cultivars drought related candidate genesare first characterized at themolecular level and their function is confirmed via transgenic studies After integration of the toleranceloci specific environment targeted field trials are performed coupled with extensive analysis of morphological and physiologicalcharacteristics of developed cultivars to assess their performance under drought conditions and their possible contributions toyield in certain regions This paper focuses on recent advances on drought related geneQTL identification studies on droughtrelated molecular pathways and current efforts on improvement of wheat cultivars for drought tolerance

1 Introduction

Current climate change is projected to have a significantimpact on temperature and precipitation profiles increasingthe incidence and severity of drought Drought is the singlelargest abiotic stress factor leading to reduced crop yieldsso high-yielding crops even in environmentally stressfulconditions are essential [1 2] This is not the first time weface this situation in which increasing demands on existingresources are not feasible and higher-yielding crops arerequired to balance crop production with increasing humanfood consumption A similar scenario occurred 50 years agodue to the high rate of population growth and it was over-come by selective breeding of high grain yielding semidwarfmutants of wheat a process coined Green Revolution [3]In relation to current development of cultivars which arehigher yielding even in water-limited environments one ofthe major targets is Triticum species being one of the leading

human food source accounting for more than half of totalhuman consumption [2 4]

The increasing incidence and importance of droughtin relation to crop production has rendered it as a majorfocus of research for several decades However studyingdrought response is challenged by the complex and quanti-tative nature of the trait Drought tolerance is complicatedwith environmental interactions In the analysis of a plantrsquosdrought response the mode timing and severity of thedehydration stress and its occurrence with other abiotic andbiotic stress factors are significant [5] Furthermore differentspecies subspecies and cultivars of crops show variation intheir drought tolerance under same conditions emphasizingthe importance of genetic diversity as an underlying factorof drought and its significance in drought-related researchPlants exhibiting high drought tolerance are the most suit-able targets of drought-related research and are the mostpromising sources of drought-related gene and gene regions

2 The Scientific World Journal

to be used in the improvement of modern crop varietiesThese include the natural progenitors of cultivated crops andfor wheat improvement Ae tauschii which is more droughttolerant thanTriticum andwild emmerwheat (T dicoccoides)which harbors drought tolerance characteristics lost duringcultivation of modern lines is of great importance [6]

Although development of higher-yielding crops underwater-limited environments is the most viable solution tostabilizing and increasing wheat production under currentclimatic conditions it is challenged by the nature of droughtresponse as a trait and the complex genomic constitutionof wheat [16] However recently the utilization of droughttolerant wild species and the rapid advances in molecularbiological functional genomics and transgenics technolo-gies have facilitated drought-related studies resulting insignificant progress in the identification of related genesand gene regions and dissection of some of its molecularaspects This paper summarizes the current state of drought-related research in Triticum species focusing on the iden-tification and functional characterization of drought-relatedmolecules analysis of their interactions in the complexnetwork of drought response and applications of thesedata to improve wheat cultivars utilizing molecular based-technologies

2 T dicoccoides and Drought Tolerance

Wild emmer wheat (T turgidum ssp dicoccoides (korn)Thell) is the tetraploid (2119899 = 4119909 = 28 genome BBAA)progenitor of both domesticated tetraploid durum wheat (Tturgidum ssp durum (Desf) MacKey) and hexaploid (2119899 =6119909 = 42 BBAADD) bread wheat (T aestivum L) It isthought to have originated and diversified in the Near EastFertile Crescent region through adaptation to a spectrum ofecological conditions It is genetically compatible with durumwheat (T turgidum ssp durum) and can be crossed withbread wheat (T aestivum L) [17] Wild emmer germplasmharbors a rich allelic pool exhibiting a high level of geneticdiversity showing correlation with environmental factorsreported by population-wide analysis of allozyme and DNAmarker variations [18ndash24]

Wild emmer wheat is important for its high drought tol-erance and some of T dicoccoides genotypes are fully fertilein arid desert environments Wild emmer wheat accessionswere shown to thrive better under water-limited conditionsin terms of their productivity and stability compared todurum wheat The wild emmer gene pool was shown tooffer a rich allelic repertoire of agronomically important traitsincluding drought tolerance [23 25ndash28]HenceT dicoccoidesis an important source of drought-related genes and highlysuitable as a donor for improving drought tolerance incultivated wheat species

Wild emmer wheat being a potential reservoir ofdrought-related research has been the source of severalidentified candidate drought-related genes with the devel-opment of ldquoomicsrdquo approaches in the recent decades Inrecent years transcript profiling of leaf and root tissuesfrom two T dicoccoides genotypes originating from Turkey

TR39477 (tolerant variety) TTD-22 (sensitive variety) wasperformed by our group in two separate studies utilizingdifferent methodologies In one report subtractive cDNAlibraries were constructed from slow dehydration stressedplants and over 13000 ESTs were sequenced In anotherstudy Affymetrix GeneChip Wheat Genome Array was usedto profile expression in response to shock drought stress[1 29] Wild emmer wheat was shown to be capable ofengaging in known drought responsive mechanisms harbor-ing elements present in modern wheat varieties and also inother crop species Additionally several genes or expressionpatterns unique to tolerant wild emmer wheat indicative ofits distinctive ability to tolerate water deficiency were alsorevealed Transcript and metabolite profiling studies werealso undertaken for two T dicoccoides genotypes originatingfrom Israel Y12-3 (tolerant variety) and A24-39 (sensitivevariety) under drought stress and nonstress conditions Leaftranscript profiling indicated differential multilevel regula-tion among cultivars and conditions [30] Integration of roottranscript and metabolite profiling data emphasized droughtadaptation through regulation of energy related processesinvolving carbon metabolism and cell homeostasis (Table 1)[14] Recently in wild emmer wheat our group also profileddrought induced expression of microRNA (miRNAs) smallregulatory molecules known to be involved in several cellularprocesses including stress responses In this study leaf androot tissues of resistant wild emmer wheat varieties TR39477and TR38828 were screened via a microarray platformand 13 differentially expressed miRNAs were found to bedifferentially expressed in response to drought (Table 1)[15]

Following the identification of T dicoccoides drought-related gene candidates as discussed previously a numberof these potential drought resistant genes were clonedand further characterized In one of the recent reportsTdicTMPIT1 (integral transmembrane protein inducible byTumor Necrosis Factor-120572 TNF-120572) was cloned from wildemmer root tissue and shown to be a membrane pro-tein associated with the drought stress response exhibitingincreased levels of expression specifically in wild emmerwheat upon osmotic stress [31] In a different study TdicDRF1(DRE binding factor 1) conserved between crop species wascloned for the first time from wild emmer wheat Its DNAbinding domain AP2ERF (APETALA2ethylene-responsiveelement binding factor) was shown to bind to droughtresponsive element (DRE) using an electrophoretic mobilityshift assay (EMSA) It was revealed to exhibit cultivar and tis-sue specific regulation of its expression throughmechanismsinvolving alternative splicing [32] Moreover the relationsbetween autophagy and drought response were analyzed inanother line of research by the cloning of TdATG8 (autophagyrelated protein 8) and its further functional investigationwith yeast complementation assay and virus induced genesilencing (VIGS) of plants In this study autophagy wasshown to be induced in drought-stressed plants in an organ-specific mode and silencing of ATG8 was shown to decreasedrought tolerance of plants revealing it as a positive regulatorof drought stress [33] (Tables 2 and 3)

The Scientific World Journal 3

Table 1 Transcript protein metabolite profiling studies conducted in the last three years

Species Cultivars Tissue Drought stress application Method Reference

T aestivum Drought tolerance Plainsman Vtolerant Kobomugi sensitive Root Moderate drought stress applied on

tillering stage cDNA microarray [7]

T aestivum Drought tolerance information cannot be accessed Grain Short water shortage in early grain

development cDNA microarray [8]

T aestivumEfficiency of stem reserve

mobilization in peduncles N49tolerant N14 sensitive

Stem Progressive drought stress afteranthesis 2D gel and MS [9]

T aestivum Cultivar Vinjett Grain Drought applied at terminal spiklet orat anthesis 2D gel and MS [10]

T aestivumYield under drought Excaliburtolerant RAC875 tolerant

Kukri sensitiveLeaf

Cyclic drought applied after first flagleaf formation mimicking field

conditions

SCX columnHPLC and MS [11]

T durum Able to acquire drought toleranceOfanto tolerant Leaf

Drought applied at booting stage(controls SWC irrigated when itdecreases 50 of field capacitydrought SWC irrigated when itdecreases 125 of field capacity)

cDNA-AFLP [12]

T durum Drought tolerance Om Rabia3tolerant Mahmoudi sensitive Embryo Drought applied at final development

stage of seed maturity2D gel and HPRPcolumn and MS [13]

T dicoccoides Yield under drought conditionsY12-3 tolerant A24-39 sensitive Leaf Terminal drought applied at

inflorescence emergence stage Transcript profiling [14]

T dicoccoides Yield under drought conditionsY12-3 tolerant A24-39 sensitive Leaf Drought applied after germination at

fivesix leaf stage

Transcript andmetaboliteprofiling

[14]

T dicoccoides Drought tolerance TR39477tolerant TR38828 tolerant Leafroot Shock drought stress miRNA profiling [15]

T Triticum SWC soil water content 2D 2-dimensional SCX strong cation exchange HPLC high performance liquid chromatography MS massspectrometry cDNA complementary DNA AFLP amplified fragment length polymorphisms HPRP human prion protein

3 Phenotyping for Drought Tolerance inWheat with Physiological Traits

For screening out transgenic wheat lines with desirabledrought tolerance the physiological traits and processeswhich can be genetically manipulated to improve wheatadaptation to drought have to be taken into account Thegenetic basis of drought tolerance in wheat is still elusive Atpresent the physiological traits (PTs) linked to heat toleranceappear to be a superlative accessible tool since they exhibitthe favorable allele combination for drought tolerance Suchalleles interact with the environment and genetic backgroundwhich includes variation in gene expression and hence arestill poorly understood through the QTL approach [50]Hybridization of heat tolerance PTs may not always havea predictable outcome related to net crop yield particularlyin varying environmental conditions but breeding suchvarieties with complementary PTs could augment the cumu-lative gene effect [51] Thus the physiological phenotypingalong with gene discovery can be valuable to pin downdesired alleles and understand their genetic mechanism [50]Cossani and Reynolds have proposed a model based on thisconcept of genetically characterized PT for improved droughttolerance of wheat [52]Themodel focuses on 3major genetic

parameters of yield when water and nutrients are not limitingfactorsThe genetic parameters are discussed in the following

31 Light Interception (LI) Traits

311 Canopy Architecture Since increase in temperature islinked with a decrease in green area duration and leaf areaindex light interception or LI traits can be manipulated bystudying the variation in the rapid ground cover (RGC) andleaf senescence of wheat RGC shows genotypic variabilityin relatively heritable and simple breeding targets such asembryo and grain size specific leaf area or seedling emer-gence rate [53] Optimized distribution of light may improveradiation use efficiency (RUE) and LI traits since wheatdisplays a vast diversity in canopy structure Furthermoreleaves are more erect and smaller in size in many moderncultivars thereby facilitating RUE and allowing more lightpenetration to lower leaves

312 Hindrance of Leaf Senescence Leaf senescence duringdrought can be hindered by delayed expression of senescencerelated green thereby giving stay-green (SG) genotypes withnormal photosynthesis [54] Stay green is thus identifiedas an important adaptive PT for drought stress conditions


Table 2 List of identified and characterized drought related genes in the last three years

Gene Function Related mechanismstress ReferenceTaPIMP1 Transcription factor R2R3 type MYB TF Drought [34]

TaSRG Transcription factor Triticum aestivum saltresponse gene Drought [35]

TaMYB3R1 Transcription factor MYB3R type MYB TF drought [36]TaNAC(NAMATAFCUC)

Transcription factor plant specific NAC(NAMATAFCUC) TF Drought [37]

TaMYB33 Transcription factor R2R3 type MYB TF Drought [38]TaWRKY2TaWRKY19 Transcription factor WRKY type TF Drought [39]

TdicDRF1 Transcription factor DRE binding protein Drought [32]

TaABC1 Kinase protein kinase ABC1 (activity ofbc(1) complex) Drought [40]

TaSnRK24 Kinase SNF1 type serinethreonine proteinkinase Drought [41]

TaSnRK27 Kinase SNF1 type serinethreonine proteinkinase drought [42]

TdTMKP1 Phosphatase MAP kinase phosphatase Drought [43]

TaCHP CHP rich zinc finger protein with unknownfunction

ABA-dependent and-independent pathways [44]

TaCP Protein degradation cysteine protease Drought [45]

TaEXPR23 Cell wall expansion expansin Water retention ability andosmotic potential [46]

TaL5 Nucleocytoplasmic transport of 5Sribosomal RNA ribosomal L5 gene Drought [47]

TdPIP11 TdPIP12 Protective protein aquaporin Drought [48]TdicATG8 Autophagy autophagy related gene 8 Drought [33]

TdicTMPIT1 Autophagy integral transmembrane proteininducible by TNF-120572 Drought [31]

Era1 Sal1 Enhanced response to ABAinositol polyphosphate 1-phosphatase Drought [49]

Ta Triticum aestivum Td Triticum durum Tdic Triticum dicoccoides DRE drought related element SNF Sucrose nonfermenting MAP mitogen activatedproteinABA abscisic acid CHP cysteine histidine proline TNF-120572 tumornecrosis factor120572 PIMP pathogen inducedmembrane protein CP cysteine proteaseEXPR expansin PIP plasma membrane intrinsic proteins

but its role in improving grain yield in drought is still amatter of extensive research However some correlationswere shown between SG and yield and identified QTLs inmapping populations [55] Since chlorosis in plants is notexpressed homogenously in plant organs aboveground manyapproaches have been developed to estimate SG includingspectral reflectance but these also need to be more specificto functional SG

32 Radiation Use Efficiency Traits

321 Photosynthesis and Photorespiration According to Cos-sani and Reynolds once the LI traits are optimized the focuson increased crop biomass will depend on RUE traits whichinclude dark respiration photorespiration and other photo-synthetic strategies A central player of the photosyntheticpathway Rubiscowas observed to show lower affinity forCO

2

over O2in higher temperatures [52] Thus increasing the

affinity of Rubisco is especially significant for adaptation towarm conditionsThe importance of CO

2fixation by Rubisco

for high temperature adaptation is also emphasised by theobservation that C4 plants adapt to warm conditions byconcentrating CO

2 Present transgenic attempts to convert

C3 plants into C4 plants are still in progress and requiremore knowledge of the maintenance of the C4 pathwayStudies of the Rubisco kinetic properties of Limonium gibertiimay be used in transgenics in wheat even though wheatRubisco has an excellent CO

2affinity One model shows

12 increase in net assimilation when substrate specificityfactor of wheat Rubisco was replaced from L gibertii [56]Rubisco activase active sites become inactive progressivelyunder drought thus associating the activase with heat shockchaperone cpn60120573 could provide Rubisco protection [57]This has great potential since thermotolerant types of Rubiscoin tropical species and diverse optimum temperature ofRubisco have been found in nature [58] By exploitingthis fact a chimeric enzyme was created thus increasingthe heat resistance in Arabidopsis by combining the Ara-bidopsis Rubisco recognition domain and tobacco activase[55]


4 Identification of Drought-RelatedGenes and QTLs

Prior to focusing on individual drought-related componentsdrought response due to its complex nature must be viewedas a whole system for which large scale identification ofprobable dehydration stress-related genes or QTLs is neces-sary Potential markers for stress tolerance can be identifiedeither through ldquoomicsrdquo studies or QTL mapping of yieldrelated traits under drought prone environments In thelong run these markers can aid in screening cultivars fordrought tolerancesensitivity andor improvement of droughttolerance in wheat

41 Drought-RelatedGene Identification by ldquoOmicsrdquo ldquoOmicsrdquotechniques examine all or a representative subset of anorganismrsquos genes transcripts proteins or metabolites Aswell as accumulating genomic sequence knowledge datafrom profiling studies is also crucial in understanding thedrought response which is largely mediated by differentialaccumulations of drought-related components

In the recent decades high-throughput profiling tech-niques have been utilized for the identification of potentialdrought tolerance markers from different wheat species(Table 1) Some of the large scale profiling studies undertakenin wild emmer wheat were mentioned in Section 2 [1415 59] ldquoOmicsrdquo studies were also performed to monitordehydration induced transcripts and proteins of bread anddurum wheat cultivars with differing sensitivities to droughtboth in stress and nonstress conditions Methodologies usedin transcript profiling studies range from cDNA microar-rays to cDNA-AFLP (amplified fragment length polymor-phism) For differential protein identification the commonprocedures used include 2D (2-Dimensional) gels vari-ous chromatography techniques and mass spectrometryIn these recent high-throughput studies molecular mech-anisms behind various drought induced physiological ormorphological events were targeted using related tissues andappropriate modetimingseverity of stress treatments foreach profiling experiment In two of these studies underlyingmolecular mechanisms of early grain development uponshock dehydration response and root functional responsesuponmoderate drought at tilleringwere investigated in breadwheat by transcript profiling [8 60] Proteome profiling wasestablished in several breadwheat tissues grain upondroughtat terminal spikelet or at anthesis leaf under field like cyclicdrought conditions after first flag leaf formation stem uponprogressive drought stress after anthesis were establishedThelatter research was conducted to understand the underlyingmolecular mechanisms of mobilization of stem carbohydratereserves to grains a process that contributes to yield underterminal drought conditions and its findings pointed outto the involvement of senescence and protection againstoxidative stress in effectiveness of the mobilization process[9] In recent years transcript profiling in durum wheatflag leaf upon field like drought at booting was performed[12] In a different line of research proteomic profiles of Tdurummature embryos were established which is especially

important since embryos are goodmodel systems for droughtstudies sustaining germination in extreme conditions ofdesiccation [13] An overview of recently established profilingstudies is provided in Table 1

42 QTL Mapping Dissection of drought tolerance a com-plex quantitative phenotype affected bymultiple loci requiresthe identification of related quantitative trait loci (QTLs)QTL cloning is a large effort in terms of the technologyresources and time required but determination of QTLs isproceeded by great advantages in applications of marker-assisted selection (MAS) and better yielding cultivar devel-opment Identification of QTLs takes advantage of molecularmaps developed by the use of DNA markers The estab-lishment of these molecular maps has been enabled by therecent advances in functional genomics which have sup-plied bacterial artificial chromosomes (BACs) gene sequencedata molecular marker technology and bioinformatic toolsfor comparative genomics Mapping and fine mapping forthe identification of candidate regions for a trait prior topositional cloning requires suitable mapping populationsrecombinant inbred lines (RILs) and near isogenic lines(NILs) several of which have been established for wheatvarieties However up to now only a limited number ofstudies has succeeded in the positional cloning ofwheatQTLsand none in the context of drought [2 4]

In recent years several yield QTLs were identified inwheat through linkage analysis and association mappingSince yield is the most crucial trait to breeders most QTLsfor drought tolerance inwheat have been determined throughyield and yield related measurements under water-limitedconditions [64ndash68] However these studies are challengedby the factors that yield and drought are both complextraits involving multiple loci and showing genotype andenvironment interactions Yield is difficult to be describedaccurately with respect to water use and its accurate phe-notyping is a challenge since QTLs established in one envi-ronment may not be confirmed in other For this reasonlarge scale phenotyping trials carried out in multiple fieldstaking into consideration the environmental varieties arecrucial Until now a number of studies have identified QTLsassociated with specific components of drought responseusing T durum T aestivum and Tdurum X T dicoc-coides mapping populations however the genomic regionsassociated with individual QTLs are still very large andunsuitable for screening in breeding programmes Howeverin recent years several yield related QTLs were mappedusing (T aestivum L) RAC875Kukri doubled haploid pop-ulations grown under a variety of environmental conditionsincluding nonirrigated environments In one study inbredpopulation was assessed under heat drought and highyield potential conditions to identify genetic loci for grainyield yield components and key morphophysiological traits[69] In another study regions associated with QTLs forgrain yield and physical grain quality were assessed under16 field locations and year combinations in three distinctseasonal conditions [70] In a third study QTLs were iden-tified for days to ear emergence and flag leaf glaucousness


Table 3 List of genes confirmed to function in drought by transgenic studies in last three years

Type of transgenicstudy Source of the gene Gene Function Related mechanism

stressReference

Overexpression inA thaliana From T Aestivum WRKY2

WRKY19Transcription factor WRKY type

TF Drought [39]

Overexpression inA thaliana From T Aestivum MYB33 transcription factor R2R3 type

MYB TF Drought [38]

Overexpression inN tabacum From T Aestivum PIMP1 Transcription factor R2R3 type

MYB TF Drought [34]

Overexpression inN tabacum From T Aestivum

NAC(NAMATAF

CUC)

Transcription factorplant-specific NAC

(NAMATAFCUC) TFDrought [37]

Overexpression inA thaliana From T Aestivum ABC1 Kinase protein kinase ABC1

(activity of bc(1) complex) Drought [40]

Overexpression inA thaliana From T Aestivum SnRK24 Kinase SNF1-type

serinethreonine protein kinase Drought [41]



Overexpression inA thaliana From T Aestivum CP Protein degradation cysteine

protease Drought [45]

Overexpression inA thaliana From T Aestivum CHP CHP rich zinc finger protein

with unknown functionABA-dependent and

-independent pathways[44]

Overexpression inN tabacum From T Aestivum EXPR23 Cell wall expansion expansin Water retention ability

and osmotic potential[46]

Overexpression inA thaliana From T Aestivum TaSIP Salt induced protein with

unknown function Drought and salinity [61]

Overexpression inN tabacum From T Durum PIP11 PIP12 Protective protein aquaporin Drought [48]

Overexpression inT aestivum From H Vulgare HVAI Protective protein LEA Drought [62]

Transgenicubiquitin TaCHP mdash CHP CHP rich zinc finger protein



TaABA081015840OF1deletion line mdash ABA08 ABA catabolism

ABA 81015840-hydroxylase Drought [63]

VIGS silencing inT dicoccoides mdash ATG8 Autophagy autophagy related

gene 8 Drought [33]

VIGS silencing inT aestivum mdash Era1 Sal1

Enhanced response to ABAinositol polyphosphate

1-phosphataseDrought [49]

ABA abscisic acid CHP cysteine histidine proline SNF sucrose nonfermenting PIMP pathogen induced membrane protein CP cysteine protease EXPRexpansin PIP plasmamembrane intrinsic proteins LEA late embryogenesis abundant HVAHordeum vulgare aleurone TaSIPTriticum aestivum salt inducedprotein VIGS virus induced gene silencing

under southern Australian conditions [71] Another multi-environmental analysis provided a basis for fine mappingand cloning the genes linked to a yield related QTL [72]These recent studies are promising and along with therecent advances in DNA sequencing technology and newapproaches of coupling linkage analysis with ldquoomicsrdquo studiesthese data will find their way into practical wheat breedingprogrammes in relation to drought [2 4] Drought-relatedQTLs identified in these studies are listed in SupplementaryTable 1 (see Supplementary Materials available online athttpdxdoiorg1011552013548246)

5 Identification of Molecular MechanismsRelated to Drought

Probable drought-related genes and QTLs identified inldquoomicsrdquo and ldquoQTL mappingrdquo studies should be furthercharacterized prior to their use in the development of betteryielding cultivars Elucidation of these components includesanalyzing their gene and protein structure and determiningtheir roles and interactions in the complex network of stressresponse signaling Their functional relevance to droughtshould be shown and eventually confirmed with transgenic


studies This section summarizes the recent research regard-ing the characterization of drought-related genes in detaildissection of drought-related molecular pathways and func-tional genomics studies

51 Characterization of Drought-Related Genes Prior to itsutilization for drought tolerance improvement for eachputative drought-related gene region or molecule identifiedin ldquoQTL mappingrdquo or ldquoomicsrdquo study the immediate step isthe cloning and in detail characterization of the gene andits protein This process exploits a variety of in silico andbasicmolecular biologymethods and involves several aspectsthat differ on the nature of the research including analysisof gene and protein structure phylogeny-based studies anddetermination of gene chromosomal localization protein-protein and protein-DNA interactions and transcript andprotein subcellular localizations Further characterizationinvolves transcript and protein monitoring in response tostress conditions and functional analysis of the proteinUtilizing these strategies in recent years several drought-related proteins were elucidated the majority being stress-related transcription factors (TFs) and signal transducers

Drought is known to be regulated at the transcriptionallevel and TFs have been the focus of attention for theimprovement of better yielding cultivars since targeting asingle TF can affect several downstream-regulatory aspectsof drought tolerance Classically two transcriptional reg-ulatory circuits induced by drought have been studiedABA-dependent and DREB-(dehydration-responsive ele-ment binding protein-) mediated (ABA-independent) path-ways These pathways are schematically depicted in Figure 1One of the major classes of TFs involved in ABA-dependentstress responses is MYB TFs and in the recent years therehas been a focus on the elucidation of bread wheat R2R3and MYB3R type MYB TFs known to be involved in ABAsignaling of drought In three different lines of researchdrought responsive MYBs TaPIMP1 (pathogen inducedmembrane protein) TaMYB33 and TaMYB3R1 were clonedand studied via the analysis of their domains determinationof their nuclear subcellular localizations and assessment oftranscriptional activation function to proteins [34 36 38]Phylogenetic analysis of their protein sequences classifiedTaMYB3R1 asMYB3R type and the others as R2R3 typeMYBTFs The R2R3 type MYB TaPIMP1 was originally describedas the first defense related MYB in wheat however detailedanalyses indicated that TaPIMP1 is also induced by abioticstresses particularly drought In addition the induction ofits expression by ABA and its inability to bind to the DRE-box element as indicated by EMSA suggest that TaPIMP1 actsin the ABA-dependent pathways of drought response [73]Similarly TaMYB33 another drought responsive R2R3 typeMYB was shown to be induced by ABA treatment and theoverexpression in Arabidopsis plants could not detect a sig-nificant increase in DREB2 suggesting that TaMYB33 is alsoinvolved inABA-dependentmechanisms [38] BothTaPIMP1and TaMYB33 appear to enhance drought tolerance throughROSdetoxification and reinforcement of osmotic balance Anelevated level of proline or proline synthesis common to both

ABA-dependent ABA-independent

TaPIMP1

TaMYB3R1

GSTSOD

Proline

TaWRKY2TaWRKY19

TaNAC4a

TaNAC6TaNAC2a

AntioxidantsOsmoprotectants

Drought stress response

Dehydrins

LEA proteins

Others

DRECRT box

TdicDRF1

Protein foldingOsmotic adjustmentROS scavenging

MYS motif

TaMYB33

Membrane stability

Figure 1 ABA-dependent andABA-independent pathways of stressresponse MYB and DREB TFs are given as examples to ABA-dependent and-independent routes While ABA-dependent path-ways appear to recruit antioxidant and osmoprotectantmechanismsABA-independent pathways generally involve protective proteinsNAC and WRKY TFs provide crosstalk between these pathwayswhere some members such as TaNAC4 and TaNAC6 may pre-dominantly act in an ABA-dependent fashion some members maybe closer to ABA-independent pathways In several cases such asTaWRKY19 both pathways are employed It should be noted thatboth pathways are highly intermingled and functions of severalregulators such as TaNAC2a as well as entire pathways are yet tobe elucidated

TaPIMP1 and TaMYB33 mediated response is noteworthy[34 38] MYB3R type MYB TFs are less pronounced classof MYB proteins in stress response TaMYB3R1 one of thefew examples of MYB3R type MYBs in wheat has beenimplicated in drought stress response and is also responsiveto ABA similar to TaPIMP1 and TaMYB33 However thedownstream events of TaMYB3R1-mediated drought stressresponse remains to be elucidated [36]

ABA-independent also called DREB-mediated pathwaysare largely governed by dehydration-responsive element-binding (DREB)C-repeat-binding (CBF) proteins whichrecognize dehydration-responsive element (DRE)C-repeat(CRT) motifs through a conserved AP2 domain WhileDREB1 TFs are mainly responsive against cold stress DREB2TFs are more pronounced in drought stress responsealthough functional overlaps are possible where a certainDREB responds to multiple stresses [74] It should be notedthat DREB-mediated stress responses may also cooperate oroverlap with ABA-dependent stress responses (Figure 1) Anumber of DREB homologs have deen identified in wheatand althoughDREB2-mediated drought response is not fullyelucidated yet enhanced drought tolerance through DREB-mediated pathways is considered to involve LEA proteins[75] As noted in Section 2 recently a DREB2 homologTdicDRF1 was identified cloned and characterized for the


first time in wild emmer wheat Comparison of drought-stressed resistant and sensitive genotypes revealing differen-tial expressions of TdicDRF1 suggested not only a conservedrole on drought stress response but also a promising mecha-nism that can be utilized in improvement of wheat cultivarsfor drought tolerance [32]

In recent years evidence has accumulated that thereis crosstalk between classical ABA-dependent and ABA-independent pathways (Figure 1) The best known exampleof such occurrence is NAC TFs which regulate droughtstress response through both ABA-dependent and ABA-independent pathways In a recent study T aestivum NAC(NAMATAFCUC) TFs were identified in silico phylogenet-ically classified and characterized and their expression pro-files were monitored in response to ABA and drought stressIn response to these treatments TaNAC4a and TaNAC6exhibited similar expression trends suggesting an ABA-dependent regulation of drought while in the case of TaNTL5and TaNAC2a the changes in the expression were notparallel [37] In another study WRKY type transcriptionfactors (TaWRKY2 and TaWRKY19) which are known to beinvolved in plant abiotic stress response and ABA signalingwere identified computationally localized to the nucleus andshown to bind specifically to cis-element W box This reportrevealed that WRKY19 as a component of both ABA andDREB pathways showing WRKY19 expression level wasresponsive to ABA application and in transgenic WRKY19deficient plants the expression levels of DREB pathwaycomponents were altered [39]

In addition to these known players of ABA-dependentand DREB-mediated pathways other novel TFs are alsodiscovered and one such TF is the recently discovered Taestivum salt response gene TF (TaSRG) which was shown tobe induced in response to drought and ABA [35]

Othermajor targets of recent research have been enzymesthat aid in reversible phosphorylation of signaling moleculesin drought-related network of protein interactions Althoughthe major classes of stress-related kinases and phosphatasestaking part in these cascades are known namely mito-gen activated protein kinases (MAPKs) SNF-1-like kinases(SnRKs) calcium-dependent protein kinases (CDPKs) andMAP kinase phosphatases (MKPs) information regarding tothese components is far from complete For this purposeknown components should be further investigated since theirexact positions and interactions in the complex signalingnetwork are currently unknown This has been appliedrecently in two separate studies in which SNF-1-like kinasesnamely TaSnRK24 TaSnRK27 from bread wheat werecharacterized further in detail [41 42] In a different researcha MAP kinase phosphatase TdTMKP1 was cloned fromdurum wheat and its specific interaction with two MAPKsTdTMPK3 and TdTMPK6 was verified in accordance of itsrole as a negative regulator ofMAPKs [43] In an independentline of research a priorly poorly characterized kinase TaABC1(T aestivum L protein kinase) was investigated and shown tobe involved in drought [40]

Although transcription factors and signal transducershave been the major focus of research in terms of char-acterization in recent years other putative drought-related

molecules were also isolated from wheat varieties investi-gated and supporting evidence for their roles in droughtstress was obtained [31ndash33 44ndash48] Information regardingthese studies is listed in Table 2 Most of the drought-related genes identified were confirmed either by usingoverexpressor plants or wheat deletion lines or silencing thegene of interest via virus induced gene silencing revealing itsfunction [33 39 63] (Table 3)

52 Studies of Drought-Related Pathways Plants in environ-ments prone to drought stress have developed several toler-ance strategies resistance and avoidance mechanisms whichenable them to survive and reproduce under conditions ofwater scarcity

The fundamental plant drought responses include growthlimitation changes in gene expression altered hormonallevels induced and suppressed signaling pathways accumu-lation of compatible solutes and osmoprotectant proteinssuppression of metabolism increased lipid peroxidation withhigher levels of ROS and counter-acting increased levelsof antioxidant activity Drought is regulated both at thetranscriptional level and posttranscriptional level the latterincluding the action of miRNAs and posttranslational mod-ifications for proteosomal degradation [76] (SupplementaryTable 2)

521 Compatible Solutes Compatible solutes are nontoxicmolecules that accumulate in the cytoplasm upon droughtstress Common compatible solutes are sugars sugar alco-hols glycine betaine amino acids and proline They areknown to be involved in osmotic adjustment function asROS scavengers protect proteins and cell structures andexhibit adaptive value in metabolic pathways In a recentstudy compatible solutes in T aestivum leaves were screenedin response to water deficit at the reproduction stage Majorcontributors to osmotic adjustment were revealed to beK+ in the early stages of stress and molecules includingglycinebetaine proline and glucose in the late stress [77]Recently compatible solutes were also assessed in T aestivumcultivars under different irrigation regimes Drought applica-tion decreased the levels of inorganic solutes but increasedthe levels of organic solutes [78]

522 Protective Proteins Protective proteins known to beinvolved in drought stress response include LEA proteinsaquaporins heat shock proteins and ion channels Recentlyin T aestivum cultivars changes in the transcript and pro-tein levels of dehydrins and LEA proteins in response toprogressive drought applied at early vegetation and duringits recovery were monitored [79] In a different study twoaquaporins were overexpressed providing direct evidence oftheir drought-related functions [48] Additionally in anotherresearch SNP and InDEL (insertiondeletion) repertoire of aNa+K+ transporter HKT-1 was assessed and observed to becompromised of mostly missense mutations predominantlypresent in the tolerant wheat variety [80]


523 Signaling Drought signaling includes signal percep-tion and transduction Known drought-related signal trans-ducers and some recent reports on their characterization andfunctional assessment are summarized in Sections 51ndash53[40ndash42] Some of these studies were performed on calciumdependent protein kinases (CDPK) which sense and respondto Ca2+ an important secondary messenger of signal trans-duction cascades In a recent study two bread wheat CDPKsCPK7 and CPK12 were revealed to be molecularly evolvedthrough gene duplication followed by functional diversifi-cation In this study they were shown to contain differentputative cis-element combinations in their promoters andtwo CDPKs were shown to respond differently to droughtPEG salt (NaCl) cold hydrogen peroxide (H

2O2) and ABA

applications [81] Other suggested signaling molecules ofdrought stress network are SA (salicylic acid) and NO (nitricoxide) In recent studies NO was shown to be present inhigher levels in drought tolerant wheat variety and may havea possible role in the drought induced limitation in rootgrowth [82] Recently SAwas shown to aid drought tolerancethrough increasing accumulation of solutes [78]

524 Photosynthesis and Respiration Upon dehydrationconditions basal metabolic activities including photosyn-thesis and respiration are known to be altered in plantsRecently T aestivum and T durum genotypes with differingsensitivities to osmotic stress were evaluated in terms of theirgas exchange in response dehydration Photosynthesis wasanalyzed in depth in relation to ROS levels and physiologicalparameters for elucidation of the mechanisms by which thetolerant cultivar sustains a high performance of water-useefficiency maintaining respiration rate and photosynthesiseven under stress conditions [83] PS II (Photosystem II) isa key protein pigment complex of photosynthesis which aidsin light harvesting Upon dehydration PS II repair cycle isimpaired affecting oxygen evolving process of PS II reactioncenter leading to photooxidative damage PS II has a biphasicprimary phytochemistry kinetics which is referred to as PSII heterogeneity In recent studies photosynthetic efficiencyof PSII complex was measured in different wheat varietiesunder different environmental conditions and the extent andnature of this heterogeneity was assessed in detail in relationto osmotic stress [84]

525 Growth Stress response of plants in relation to growthdifferentiates based both on the tissue and the severitytiming and mode of stress applied The degree of osmoticstress induced limitation of plant organ growth also differsfrom cultivar to cultivar and does not have a correlationwith drought tolerance [82 85 86] Recently dehydrationinduced retardation of root and leaf growth was studiedin detail in relation to several components of the droughtresponse pathway In some of these studies cell wall-boundperoxidases ROS and NO were shown to be unfavorablefor root expansion and suggested to have possible rolesin retarding root cell wall extension [85] Another reportanalyzed the underlying mechanisms of T aestivum rootelongation in detail showing that even during plasmolysis

upon stress although root elongation is retarded new roothair cell formation is sustained [87] In a different studymonitoring of T durum leaves for ABA water status and leafelongation rates upon drought stress and recovery revealeda retardation in leaf elongation even after stress recoverysuggesting that a rapid accumulation of ABA during stressmay have caused the loss of cell wall extensibility [86]

Additionally recently expansin known to be involved incell wall loosening was overexpressed in tobacco confirmingits role in plant water retention ability and osmotic potential[83]

526 Transcriptional Regulation and PosttranscriptionalTranslational Modifications The expression of severaldrought-related gene products is regulated at the trans-criptional level Known drought-related TFs and somerecent reports on their characterization and functionalassessment are summarized in Sections 51ndash53 [32 34ndash39]Additional studies undertaken in the recent years includepreliminary research drought induced expression profilingof MYB transcripts revealing TaMYBsdu1 as a potentialdrought-related TF and characterization of SNP and INDELrepertoire of T durum DREB1 and WRKY1 [80 88]

As well as on the genomic level drought stress is alsoregulated at the posttranscriptional and posttranslationallevelsThemajor players of posttranscriptional regulation aremiRNAs which have been identified in a variety of cropsincluding Triticeae species using both computational andexperimental approaches and their expression profiling wasperformed in wild emmer wheat and crop model speciesincluding Brachypodium [15 76 89ndash92] Posttranslationalmodifications include protein degradation mostly via ubiq-uitination In a recent study the leaves of resistantT aestivumcultivar were shown to exhibit a relatively small increasein cysteine protease function upon dehydration limitingprotein loss of this cultivar In this study a cysteine proteasepresent only under drought conditions was detected [93] Ina different report the functional role of a cysteine proteaseprotein was confirmed by its overexpression in Arabidopsis[45]

527 ROS and Antioxidants Upon drought stress ROS gen-eration occurs mainly in the chloroplast and mitochondriawhich results in oxidative damage and lipid peroxidationNonenzymatic and enzymatic antioxidants are produced bythe plant to detoxify ROS In a recent report drought inducedROS generation and antioxidant activity was screened com-paratively in the root whole cells and mitochondria ofdrought acclimated and nonacclimatedT aestivum seedlingsA special role of mitochondrial scavenging mechanisms washighlighted by the finding that in specifically nonacclimatedseedlings mitochondrial antioxidants were found to be pre-dominantly active In the same study a quick increase inantioxidant mechanisms was observed with stress recoverythus it was proposed that accumulation of high levels ofH2O2upon stress can inhibit antioxidant mechanisms [94]

Another study was undertaken in two T aestivum cultivars


differing in their drought tolerance focusing especially onthe level and activity of ascorbate glutathione cycle relatedenzymes in stem and leaf tissues It was shown that ascorbateprocessing and oxidation were differentially changed in thetwo cultivars [7] Recently several other studies of ROS wereperformed to determine its role in the drought responsenetwork in relation to photosynthesis NO root growth ABAand BABA (120573-aminobutyric acid) [82 95 96]

528 Abscisic Acid ABA is a plant hormone that is knownto be involved in plant developmental processes and was alsoshown to be an inducer of stress-related pathways Recentlythe role of ABA catabolism in drought was supported bydirect evidence from a wheat deletion line [63] ABA is amajor hot topic of drought research and recently severalstudies are performed to determine its role in droughtin relation to several drought-related molecules protectiveproteins NO ROS leaf and root growth osmotic adjustmentBABA ROS and antioxidants [79 82 86 95] Additionallyin a recent study alginate oligosaccharides (AOS) preparedfrom degradation of alginate were shown to play a role inenhancing drought resistance in T aestivum growth periodby upregulating the drought tolerance related genes involvedin ABA signal pathway such as LEA1 SnRK2 and pyrroline-5-carboxylate synthetase gene (P5CS) [97]

53 Transgenic Studies for Identification of Gene FunctionTransgenic plants provide the most straightforward wayto demonstrate the functional relevance of the potentialdrought-related gene Using functional genomics methodsmodification of a single gene can be achieved in an identicalgenetic background Analyzing the functional role of aprotein of interest is achieved by the creation of overexpressorplants or loss-of-function mutants These studies are mostoften carried out in model species Arabidopsis thalianaor Nicotiana benthamiana The advantages of these speciesare basically their rapid reproduction time and the ease ofgenetic transformation However other transgenic modelsystems which hold the advantage of being phylogeneti-cally more similar to monocot crop species are also beingdeveloped most importantly Brachypodium distachyon Theuse of even a phylogenetically very close model plant infunction verification of a gene of interest does not excludethe possibility that its role can differ in the crop of interestTherefore transgenic studies applied directly on wheat arebeing developed but currently still more time and laborconsuming Other systems which hold the advantage ofstraightforward analysis of gene function in the target cropsare deletion lines and virus induced gene silencing (VIGS)which aids in functional characterization through silencingof targeted transcripts

531 Overexpression Studies Direct evidence for the func-tional role of several drought response candidates was estab-lished via their overexpression in A thaliana or tobaccoby Agrobacterium mediated transformation To gain insightinto the mechanisms the molecule of concern exerts a rolein drought and these studies are often coupled by the

profiling of stress-related genes and characterization of theoverexpressor plants in terms of their altered morphologicaland physiological properties In recent years drought-relatedmolecular function of several transcription factors signaltransducers and some other proteins were confirmed viatheir overexpression in Arabidopsis or N tabacum Informa-tion regarding these studies is summarized in Table 3

Recently in four independent studies overexpressionof transcription factors (TaWRKY2 TaWRKY19 TaMYB33TaPIMP1 and TaNAC) was shown to confer elevated droughttolerance in targetmodel organisms [34 37ndash39]With the useof these overexpressor plants WRKY proteins were shown tobe involved in the DREB pathway [39] and MYB proteinswere revealed to function in ROS detoxification [34 38]MYB TaPIMP1 overexpressors were also shown to exhibitincreased levels of ABA synthesis and its restricted signaling[38] Additionally recently introgression of three kinases(TaABC1 TaSnRK27 and TaSnRK24) into Arabidopsis inseparate studies was shown to improve drought toleranceevident by the water content and related measurements ofoverexpressor plants All three kinases were observed toimprove photosynthetic efficiency TaABC1 was shown tobe involved in DREB and ABA pathways [40] TaSnRK27was revealed to be involved in carbohydrate metabolism andmechanisms involving root growth [42] TaSnRK24 overex-pressors displayed differences in development and showedstrengthened cell membrane stability [41] In addition totranscription factors and kinases overexpression studies ofother proteins were also performed in model organisms inrelation to drought and drought-related capabilities Cysteineprotease TaCP transgenics were observed to have higher sur-vival rates under drought [45] CHP rich zinc finger proteinTaCHP was revealed to be involved in ABA-dependent and-independent signaling pathways [44] Expansin TaEXPR23was shown to increase water retention ability and decreaseosmotic potential [46] Durum wheat aquaporins (TdPIP1 1and TdPIP2 1) were revealed to regulate root and leaf growth[48] Besides in a recent study T aestivum salt inducedprotein (TaSIP) was shown to have a role in drought andsalt tolerance via overexpression of the gene in A thalianaresulted superior physiological properties [61] (Table 3)

532 Wheat Deletion Lines and Virus Induced Gene SilencingIn recent years a number of other transgenic studies werealso performed for function determination In such a studyT aestivum deletion lines which lack ABA 81015840-hydroxylasegene involved in ABA catabolism were used to study therole of ABA metabolism in the reproductive stage droughttolerance of cultivars This study revealed a parallel betweensensitivity to osmotic stress and higher spike ABA levels [63]In a different study ubiquitinTaCHP transgenic wheat lineswere used in studies in relation to the role of CHP richzinc finger protein in drought stress [44] Additionally VIGSvia barley stripe mosaic virus (BSMV) derived vectors wasundertaken to silence ATG8 in wild emmer wheat revealingit as a positive regulator of drought stress [33] (Table 3)VIGSwas also used in another research in order to investigatethe roles of Era1 (enhanced response to abscisic acid) Sal1


(inositol polyphosphate 1-phosphatase) and Cyp707a (ABA81015840-hydroxylase) in response to limiting water conditions inwheat When subjected to limiting water conditions VIGS-treated plants for Era1 and Sal1 resulted in increased relativewater content (RWC) improved water use efficiency (WUE)Compared to other tested genesEra1was found to be themostpromising as a potential target for drought tolerance breedingin wheat [49]

6 Improvement of Modern Wheat

Recent advances in molecular biological functional andcomparative tools open up new opportunities for the molec-ular improvement of modern wheat Recently developedtechniques enable faster identification and characterization ofdrought-related gene(s) and gene region(s) Natural variantsof modern species harbor a large repertoire of potentialdrought-related genes and hold a tremendous potential forwheat improvement Introduction of drought-related com-ponents of wheat can be performed either with breedingthrough marker-assisted selection or transgenic methodsRecent increase in sequence availability due to recently devel-oped high-throughput sequencing strategies has providedseveral high quality genetic markers for breeding Transgenicstrategies with enhanced transformation and selection meth-ods are currently being developed

61Marker-Assisted Selection Molecular breeding approach-es based on specific traits utilize molecular markers for thescreening of drought tolerance in cultivars Loci that aretargeted in marker-assisted selection (MAS) are most oftenderived fromQTLmapping studies of quantitative traits [98]MAS is most often performed based on physiomorphologicalcharacteristics related to yield under drought conditionsMarkers that are utilized in such a context include SSR(simple sequence repeat) markers Xgwm136 and NW3106which are linked to genes that effect tillering capacity andcoleoptile length respectively [99] Other selection markersare linked to Rht (reduced height) genes which are known tobe associated with harvest index Additionally transcriptionfactor-derivedmarkers especiallyDREBproteins hold a greatpotential as PCR-based selection markers that can be usefulin MAS [100] However the isolation of transcription factorsis a challenge since they belong to large gene families contain-ing members with high sequence similarities Identificationand successful isolation of a single drought-related loci iscompelling also in general due to the complex genomicstructure of wheat However in the near future completion ofwheat genome sequencing will pace identification of specificloci and the development of markers to be used in selectionduring breeding processes [98 101]

62 Use of Transgenics An alternative to ongoing breedingprogrammes is transgenicmethods which enable the transferof only the desired loci from a source organism to elitewheat cultivars avoiding possible decrease in yield due tothe cotransfer of unwanted adjacent gene segments Untilnow transcription factors have been the most appealing

targets for transgenic wheat improvement due to their rolein multiple stress-related pathways In two different lines ofresearch overexpression of cotton and A thaliana DREBwas performed in wheat resulting in transgenic lines withimproved drought tolerance [102ndash104] In another study abarley LEA protein HVA1 was also overexpressed in wheatand overexpressors were observed to have better droughttolerance [105] Transgenic wheat obtained with ArabidopsisDREB and HVA1 protein overexpression was also shown toproduce higher yield in the field under drought conditionsbut further studies are required to confirm their performanceunder different environments [105]

It is not unreasonable to predict in the following decadesGM (genetically modified) wheat will be transferred to thefields as a common commercial crop However to pace thisprocess new transgenicsmethodologies should be developedsince the currentmethods are laborious and time consumingIn a recent study drought enhancement of bread wheat wasestablished with the overexpression of barley HVA1 using anovel technique which combines doubled haploid technol-ogy andAgrobacteriummediated genetic transformation [62](Table 3)

63 Use of Proteomics Despite the impressive technologicalbreakthroughs in the genomics of drought resistant cultivarsthe overall scenario is not so promising and new dimensionshave to be explored for the exact elucidation of the wheatdrought response process Hence new studies are focusing tostudywheat tolerance at the proteomic level to target differentproteins and understand their role in stress One particularstudy during grain development used comparative proteomicanalysis and used 2 varieties of wheat resistant (CIMMYTwheat variety Kauz) and sensitive (Janz to drought) Theyapplied linear and nonlinear 2-DE and MALDI-TOF massspectrometry and elucidated that non-linear 2-DE showeda high resolution and identifies 153 spots of proteins thatwere differentially expressed 122 of which were detectedby MALDI-TOF The characterized proteins were primarilymetabolism proteins (26 carbohydrate metabolism) pro-teins involved in defense and detoxification (23) and therest of 17 were storage proteins The study successfullyshowed the differential expression of various proteins indrought resistant and tolerant varieties Kauz wheat varietyshowed high expression of LEA and alpha-amylase inhibitorsand catalase isozyme 1 WD40 repeat protein whereas theseproteins were either unchanged or downregulated in Janzvariety Vice versa ascorbate peroxidase G beta-like proteinand ADP glucose pyrophosphorylase remained unchangedin Kauz but were all downregulated in Janz Proteins such astriticin precursor and sucrose synthase showed a consider-ably higher expression in Kauzwater deficit variety comparedto Janz water deficit plants Thus the differential expressionshows that biochemical and protein level expression couldbe a simpler approach to understanding and manipulatingdrought stress in plants [106]

A parallel approach to understanding the protein expres-sion and posttranslational modification in wheat was carriedout by Budak et al in which 2 wild varieties of emmer wheat


Triticum turgidum ssp dicoccoides TR39477 and TTD22were used along with one modern wheat cultivar Triticumturgidum ssp durum cv KızıltanThe complete leaf proteomeprofiles of all three genotypes were compared by 2-DEgel electrophoresis and nanoscale liquid chromatographicelectrospray ionization tandem mass spectrometry Insteadof using only drought tolerant and drought resistant varietiesanother third intermediate variety (modern) was also usedAlthough many proteins were common in all 3 cultivars bothmodern and durum but 75 differentially expressed proteinswere detected [107] Consequently comparative proteomicsmay provide a clearer picture and alternate way to evaluateand characterize drought resistant genes and proteins inwheat varieties

7 Conclusion and Future Perspectives

Drought stress is one of the major limitations to cropproduction To develop improved cultivars with enhancedtolerance to drought stress identification of osmotic stress-related molecules and determination of their roles andlocations in several physiological biochemical and generegulatory networks is necessary Several QTLs for keymorphopysiological characteristics and yield were identifiedunder water-limited conditions through creation of linkagemaps using parentals with different drought coping abilitiesIn recent decades application of high-throughput screen-ing ldquoomicsrdquo strategies on Triticum species with differentialdrought tolerance coping abilities has revealed several stress-related candidate gene(s) or gene block(s) Furthermoreusing a variety of bioinformatics molecular biology andfunctional genomics tools drought-related candidates werecharacterized and their roles in drought tolerance werestudiedMajor drought-relatedmolecules were revealed to besignal transduction pathway components and transcriptionfactors Several osmoprotectants compatible solutes ROSand antioxidants were shown to accumulate in responseto dehydration Drought stress was found to alter variousongoingmetabolic processes such as growth photosynthesisand respiration

Analysis of drought response has been complicated inthe absence of wheat genomic sequence data However withthe recent advances in sequencing technologies genomesequence of bread wheat is almost complete by the effortsof ITMI (The International TriticeaeMapping Initiative) andIWGSC (International Wheat Genome Sequencing Consor-tium) Availability of whole wheat genome sequence willcontribute to the ongoing studies of exploring the extensivereservoir of alleles in drought tolerant wild germplasm andthis also enables bettermarker development genome analysisand large scale profiling experiments ldquoOmicsrdquo strategies haveespecially contributed to drought research since osmoticstress response is not only genomic based but also regulated atthe posttranscriptional and posttranslational levels Advancesin transformationselection strategies have paced moleculartransformation of wheat which has an advantage to conven-tional andmarker-assisted breeding for targeted introductionof only the desired loci

It is reasonable to predict that in the following yearshigher yieldingwheat under drought conditionswill be devel-oped through breeding or molecular transformation of novelgenes obtained from screening of wheat germplasms and willbe commercially grown to balance the production with theconsumption of the increasing human population Researchexploiting recent advances in genomics technologies hasmade it possible to dissect and resynthesize molecular reg-ulation of drought andmanipulate crop genomes for droughttolerance The future efforts will be to integrate and translatethese resources into practical higher yielding field products

References

[1] N Z Ergen and H Budak ldquoSequencing over 13 000 expressedsequence tags from six subtractive cDNA libraries of wild andmodern wheats following slow drought stressrdquo Plant Cell ampEnvironment vol 32 no 3 pp 220ndash236 2009

[2] D Fleury S Jefferies H Kuchel and P Langridge ldquoGenetic andgenomic tools to improve drought tolerance in wheatrdquo Journalof Experimental Botany vol 61 no 12 pp 3211ndash3222 2010

[3] M Tester and P Langridge ldquoBreeding technologies to increasecrop production in a changing worldrdquo Science vol 327 no 5967pp 818ndash822 2010

[4] D Z Habash Z Kehel and M Nachit ldquoGenomic approachesfor designing durum wheat ready for climate change with afocus on droughtrdquo Journal of Experimental Botany vol 60 no10 pp 2805ndash2815 2009

[5] M P Reynolds ldquoDrought adaptation in wheatrdquo in DroughtTolerance in Cereals J M Ribaut Ed chapter 11 pp 402ndash436Haworthrsquos Food Products Press New York NY USA 2006

[6] MAshrafMOzturk andHRAthar Salinity andWater StressImproving Crop Efficiency Berlin Germany Springer 2009

[7] M Secenji E Hideg A Bebes and J Gyorgyey ldquoTranscrip-tional differences in gene families of the ascorbate-glutathionecycle in wheat duringmild water deficitrdquo Plant Cell Reports vol29 no 1 pp 37ndash50 2010

[8] A Szucs K Jager M E Jurca et al ldquoHistological and microar-ray analysis of the direct effect of water shortage alone orcombined with heat on early grain development in wheat(Triticum aestivum)rdquo Physiologia Plantarum vol 140 no 2 pp174ndash188 2010

[9] MM Bazargani E Sarhadi A A Bushehri et al ldquoA proteomicsview on the role of drought-induced senescence and oxidativestress defense in enhanced stem reserves remobilization inwheatrdquo Journal of Proteomics vol 74 no 10 pp 1959ndash1973 2011

[10] F Yang A D Joslashrgensen H Li et al ldquoImplications of high-temperature events and water deficits on protein profiles inwheat (Triticum aestivum L cv Vinjett) grainrdquo Proteomics vol11 no 9 pp 1684ndash1695 2011

[11] K L Ford A Cassin and A Bacic ldquoQuantitative proteomicanalysis of wheat cultivars with differing drought stress toler-ancerdquo Frontiers in Plant Science vol 2 article 44 2011

[12] P Rampino GMita P Fasano et al ldquoNovel durumwheat genesup-regulated in response to a combination of heat and droughtstressrdquo Plant Physiology and Biochemistry vol 56 pp 72ndash782012

[13] S Irar F Brini A Goday K Masmoudi and M Pages ldquoPro-teomic analysis of wheat embryos with 2-DE and liquid-phasechromatography (ProteomeLab PF-2D)mdasha wider perspective


of the proteomerdquo Journal of Proteomics vol 73 no 9 pp 1707ndash1721 2010

[14] T Krugman Z Peleg L Quansah et al ldquoAlteration in expres-sion of hormone-related genes in wild emmer wheat rootsassociated with drought adaptation mechanismsrdquo Functional ampIntegrative Genomics vol 11 no 4 pp 565ndash583 2011

[15] M Kantar S J Lucas and H Budak ldquomiRNA expressionpatterns of Triticum dicoccoides in response to shock droughtstressrdquo Planta vol 233 no 3 pp 471ndash484 2011

[16] S Farooq ldquoTriticeae the ultimate source of abiotic stresstolerance improvement in wheatrdquo in SalInity and Water StressM Ashraf Ed chapter 7 pp 65ndash71 Springer Berlin Germany2009

[17] M Feldman and E R Sears ldquoThe wild genetic resources ofwheatrdquo Scientific American vol 244 pp 102ndash112 1981

[18] P Dong Y M Wei G Y Chen et al ldquoEST-SSR diversitycorrelated with ecological and genetic factors of wild emmerwheat in Israelrdquo Hereditas vol 146 no 1 pp 1ndash10 2009

[19] T Fahima M S Roder K Wendehake V M Kirzhner and ENevo ldquoMicrosatellite polymorphism in natural populations ofwild emmer wheat Triticum dicoccoides in Israelrdquo Theoreticaland Applied Genetics vol 104 no 1 pp 17ndash29 2002

[20] T Fahima G L Sun A Beharav T Krugman A Beilesand E Nevo ldquoRAPD polymorphism of wild emmer wheatpopulations Triticum dicoccoides in Israelrdquo Theoretical andApplied Genetics vol 98 no 3-4 pp 434ndash447 1999

[21] E Nevo and A Beiles ldquoGenetic diversity of wild emmer wheatin Israel and Turkeymdashstructure evolution and application inbreedingrdquo Theoretical and Applied Genetics vol 77 no 3 pp421ndash455 1989

[22] E Nevo E Golenberg A Beiles A H D Brown and DZohary ldquoGenetic diversity and environmental associations ofwild wheat Triticum dicoccoides in Israelrdquo Theoretical andApplied Genetics vol 62 no 3 pp 241ndash254 1982

[23] Z Peleg Y Saranga T Krugman S Abbo E Nevo and TFahima ldquoAllelic diversity associated with aridity gradient inwild emmer wheat populationsrdquo Plant Cell amp Environment vol31 no 1 pp 39ndash49 2008

[24] J R Wang Y M Wei X Y Long et al ldquoMolecular evolution ofdimeric 120572-amylase inhibitor genes in wild emmer wheat and itsecological associationrdquo BMC Evolutionary Biology vol 8 no 1article 91 2008

[25] Z Peleg T Fahima S Abbo et al ldquoGenetic diversity for droughtresistance in wild emmer wheat and its ecogeographical associ-ationsrdquo Plant Cell amp Environment vol 28 no 2 pp 176ndash1912005

[26] J PengD Sun and ENevo ldquoDomestication evolution geneticsand genomics in wheatrdquo Molecular Breeding vol 28 no 3 pp281ndash301 2011

[27] J Peng D Sun and E Nevo ldquoWild emmer wheat Triticumdicoccoides occupies a pivotal position in wheat domesticationprocessrdquo Australian Journal of Crop Science vol 5 no 9 pp1127ndash1143 2011

[28] J H Peng D F Sun Y L Peng and E Nevo ldquoGene discovery inTriticum dicoccoides the direct progenitor of cultivated wheatsrdquoCereal Research Communications vol 41 no 1 pp 1ndash22 2013

[29] N Z Ergen J Thimmapuram H J Bohnert and H BudakldquoTranscriptome pathways unique to dehydration tolerant rel-atives of modern wheatrdquo Functional and Integrative Genomicsvol 9 no 3 pp 377ndash396 2009

[30] T Krugman V Chague Z Peleg et al ldquoMultilevel regulationand signalling processes associated with adaptation to terminaldrought in wild emmer wheatrdquo Functional and IntegrativeGenomics vol 10 no 2 pp 167ndash186 2010

[31] S Lucas E Dogan and H Budak ldquoTMPIT1 from wild emmerwheat first characterisation of a stress-inducible integral mem-brane proteinrdquo Gene vol 483 no 1-2 pp 22ndash28 2011

[32] S Lucas E Durmaz B A Akpnar and H Budak ldquoThe droughtresponse displayed by a DRE-binding protein from Triticumdicoccoidesrdquo Plant Physiology and Biochemistry vol 49 no 3pp 346ndash351 2011

[33] D Kuzuoglu-Ozturk O Cebeci Yalcinkaya B A Akpinar et alldquoAutophagy-related gene TdAtg8 in wild emmer wheat plays arole in drought and osmotic stress responserdquo Planta vol 236no 4 pp 1081ndash1092 2012

[34] H Liu X Zhou N Dong X Liu H Zhang and Z ZhangldquoExpression of a wheat MYB gene in transgenic tobaccoenhances resistance to Ralstonia solanacearum and to droughtand salt stressesrdquo Functional amp Integrative Genomics vol 11 no3 pp 431ndash443 2011

[35] X He X Hou Y Shen and Z Huang ldquoTaSRG a wheat tran-scription factor significantly affects salt tolerance in transgenicrice andArabidopsisrdquo FEBS Letters vol 585 no 8 pp 1231ndash12372011

[36] H Cai S Tian C Liu andHDong ldquoIdentification of aMYB3Rgene involved in drought salt and cold stress in wheat (Triticumaestivum L)rdquo Gene vol 485 no 2 pp 146ndash152 2011

[37] Y Tang M Liu S Gao et al ldquoMolecular characterization ofnovel TaNAC genes in wheat and overexpression of TaNAC2aconfers drought tolerance in tobaccordquo Plant Physiology vol 144no 3 pp 210ndash224 2012

[38] Y Qin M Wang Y Tian W He L Han and G XialdquoOver-expression of TaMYB33 encoding a novel wheat MYBtranscription factor increases salt and drought tolerance inArabidopsisrdquoMolecular Biology Reports vol 39 no 6 pp 7183ndash7192 2012

[39] C F Niu W Wei Q Y Zhou et al ldquoWheat WRKY genesTaWRKY2 and TaWRKY19 regulate abiotic stress tolerance intransgenic Arabidopsis plantsrdquo Plant Cell amp Environment vol35 no 6 pp 1156ndash1170 2012

[40] C Wang R Jing X Mao X Chang and A Li ldquoTaABC1 amember of the activity of bc1 complex protein kinase familyfrom common wheat confers enhanced tolerance to abioticstresses in Arabidopsisrdquo Journal of Experimental Botany vol 62no 3 pp 1299ndash1311 2011

[41] X Mao H Zhang S Tian X Chang and R Jing ldquoTaS-nRK24 an SNF1-type serinethreonine protein kinase of wheat(Triticum aestivum L) confers enhanced multistress tolerancein Arabidopsisrdquo Journal of Experimental Botany vol 61 no 3pp 683ndash696 2010

[42] H Zhang XMao R Jing X Chang andH Xie ldquoCharacteriza-tion of a commonwheat (Triticum aestivum L) TaSnRK27 geneinvolved in abiotic stress responsesrdquo Journal of ExperimentalBotany vol 62 no 3 pp 975ndash988 2011

[43] I Zaıdi C Ebel M Touzri et al ldquoTMKP1 is a novel wheat stressresponsive MAP kinase phosphatase localized in the nucleusrdquoPlant Molecular Biology vol 73 no 3 pp 325ndash338 2010

[44] C Li J Lv X Zhao et al ldquoTaCHP a wheat zinc finger proteingene down-regulated by abscisic acid and salinity stress plays apositive role in stress tolerancerdquo Plant Physiology vol 154 no 1pp 211ndash221 2010


[45] Q W Zang C X Wang X Y Li et al ldquoIsolation and char-acterization of a gene encoding a polyethylene glycol-inducedcysteine protease in common wheatrdquo Journal of Biosciences vol35 no 3 pp 379ndash388 2010

[46] Y Y Han A X Li F Li M R Zhao and W WangldquoCharacterization of a wheat (Triticum aestivum L) expansingene TaEXPB23 involved in the abiotic stress response andphytohormone regulationrdquo Plant Physiology and Biochemistryvol 54 pp 49ndash58 2012

[47] G Z Kang H F Peng Q X Han Y H Wang and T C GuoldquoIdentification and expression pattern of ribosomal L5 gene incommonwheat (Triticum aestivum L)rdquoGene vol 493 no 1 pp62ndash68 2012

[48] M Ayadi D Cavez N Miled F Chaumont and K MasmoudildquoIdentification and characterization of two plasma membraneaquaporins in durum wheat (Triticum turgidum L subspdurum) and their role in abiotic stress tolerancerdquo Plant Physi-ology and Biochemistry vol 49 no 9 pp 1029ndash1039 2011

[49] H Manmathan D Shaner J Snelling N Tisserat and NLapitan ldquoVirus-induced gene silencing of Arabidopsis thalianagene homologues inwheat identifies genes conferring improveddrought tolerancerdquo Journal of Experimental Botany vol 64 no5 pp 1381ndash1392 2013

[50] M Reynolds and R Tuberosa ldquoTranslational research impact-ing on crop productivity in drought-prone environmentsrdquoCurrent Opinion in Plant Biology vol 11 no 2 pp 171ndash179 2008

[51] M Reynolds and G Rebetzke ldquoApplication of plant physiologyin wheat breedingrdquo in The World Wheat Book A History ofWheat Breeding A P Bonjean W J Angus and M van GinkelEds vol 2 Paris France 2011

[52] C M Cossani and M P Reynolds ldquoPhysiological traits forimproving heat tolerance in wheatrdquo Plant Physiology vol 160no 4 pp 1710ndash1718 2012

[53] R A Richards and Z Lukacs ldquoSeedling vigour in wheatmdashsources of variation for genetic and agronomic improvementrdquoAustralian Journal of Agricultural Research vol 53 no 1 pp 41ndash50 2002

[54] P O Lim H J Kim and H G Nam ldquoLeaf senescencerdquo AnnualReview of Plant Biology vol 58 pp 115ndash136 2007

[55] U Kumar A K Joshi M Kumari R Paliwal S Kumar andM S Roder ldquoIdentification of QTLs for stay green trait inwheat (Triticum aestivum L) in the ldquoChirya 3rdquo times ldquoSonalikardquopopulationrdquo Euphytica vol 174 no 3 pp 437ndash445 2010

[56] M A J Parry M Reynolds M E Salvucci et al ldquoRaising yieldpotential of wheat II Increasing photosynthetic capacity andefficiencyrdquo Journal of Experimental Botany vol 62 no 2 pp453ndash467 2011

[57] M E Salvucci and S J Crafts-Brandner ldquoInhibition of pho-tosynthesis by heat stress the activation state of Rubisco as alimiting factor in photosynthesisrdquo Physiologia Plantarum vol120 no 2 pp 179ndash186 2004

[58] I Kurek K CThom SM Bertain et al ldquoEnhanced thermosta-bility of Arabidopsis rubisco activase improves photosynthesisand growth rates under moderate heat stressrdquo The Plant Cellvol 19 no 10 pp 3230ndash3241 2007


[60] M Secenji A Lendvai P Miskolczi et al ldquoDifferences in rootfunctions during long-term drought adaptation comparison of

active gene sets of two wheat genotypesrdquo Plant Biology vol 12no 6 pp 871ndash882 2010

[61] H-Y Du Y-Z Shen and Z-J Huang ldquoFunction of thewheat TaSIP gene in enhancing drought and salt tolerance intransgenic Arabidopsis and ricerdquo Plant Molecular Biology vol81 no 4-5 pp 417ndash429 2013

[62] H Chauhan and P Khurana ldquoUse of doubled haploid tech-nology for development of stable drought tolerant bread wheat(Triticumaestivum L) transgenicsrdquoPlant Biotechnology Journalvol 9 no 3 pp 408ndash417 2011

[63] X Ji B Dong B Shiran et al ldquoControl of abscisic acidcatabolism and abscisic acid homeostasis is important forreproductive stage stress tolerance in cerealsrdquo Plant Physiologyvol 156 no 2 pp 647ndash662 2011

[64] A A Diab R V Kantety N Z Ozturk D Benscher M MNachit and M E Sorrells ldquoDroughtmdashinducible genes and dif-ferentially expressed sequence tags associated with componentsof drought tolerance in durum wheatrdquo Scientific Research andEssays vol 3 no 1 pp 9ndash27 2008

[65] M Maccaferri M C Sanguineti S Corneti et al ldquoQuantitativetrait loci for grain yield and adaptation of durum wheat(TriticumdurumDesf) across awide range ofwater availabilityrdquoGenetics vol 178 no 1 pp 489ndash511 2008

[66] K L Mathews M Malosetti S Chapman et al ldquoMulti-environment QTL mixed models for drought stress adaptationin wheatrdquo Theoretical and Applied Genetics vol 117 no 7 pp1077ndash1091 2008

[67] C L McIntyre K L Mathews A Rattey et al ldquoMoleculardetection of genomic regions associated with grain yield andyield-related components in an elite bread wheat cross evalu-ated under irrigated and rainfed conditionsrdquo Theoretical andApplied Genetics vol 120 no 3 pp 527ndash541 2010

[68] Z Peleg T Fahima T Krugman et al ldquoGenomic dissection ofdrought resistance in durumwheattimeswild emmerwheat recom-binant inbreed line populationrdquo Plant Cell amp Environment vol32 no 7 pp 758ndash779 2009

[69] D Bennett M Reynolds D Mullan et al ldquoDetection of twomajor grain yield QTL in bread wheat (Triticum aestivum L)under heat drought and high yield potential environmentsrdquoTheoretical and Applied Genetics vol 125 no 7 pp 1473ndash14852012

[70] D Bennett A Izanloo M Reynolds H Kuchel P Langridgeand T Schnurbusch ldquoGenetic dissection of grain yield andphysical grain quality in bread wheat (Triticum aestivum L)under water-limited environmentsrdquo Theoretical and AppliedGenetics vol 125 no 2 pp 255ndash271 2012

[71] D Bennett A Izanloo J Edwards et al ldquoIdentification ofnovel quantitative trait loci for days to ear emergence andflag leaf glaucousness in a bread wheat (Triticum aestivumL) population adapted to southern Australian conditionsrdquoTheoretical and Applied Genetics vol 124 no 4 pp 697ndash7112012

[72] J Bonneau J Taylor B Parent et al ldquoMulti-environmentanalysis and improved mapping of a yield-related QTL onchromosome 3B ofwheatrdquoTheoretical andAppliedGenetics vol126 no 3 pp 747ndash761 2013

[73] Z Zhang X Liu X Wang et al ldquoAn R2R3 MYB transcriptionfactor in wheat TaPIMP1 mediates host resistance to Bipolarissorokiniana anddrought stresses through regulation of defense-and stress-related genesrdquo New Phytologist vol 196 no 4 pp1155ndash1170 2012


[74] P K Agarwal P Agarwal M K Reddy and S K SoporyldquoRole of DREB transcription factors in abiotic and biotic stresstolerance in plantsrdquo Plant Cell Reports vol 25 no 12 pp 1263ndash1274 2006

[75] C Egawa F Kobayashi M Ishibashi T Nakamura C Naka-mura and S Takumi ldquoDifferential regulation of transcriptaccumulation and alternative splicing of a DREB2 homologunder abiotic stress conditions in common wheatrdquo Genes ampGenetic Systems vol 81 no 2 pp 77ndash91 2006

[76] M Kantar S J Lucas andH Budak ldquoDrought stress moleculargenetics and genomics approachesrdquo Advances in BotanicalResearch vol 57 pp 445ndash493 2011

[77] S A Nio G R Cawthray L JWade and T D Colmer ldquoPatternof solutes accumulated during leaf osmotic adjustment asrelated to duration of water deficit for wheat at the reproductivestagerdquo Plant Physiology and Biochemistry vol 49 no 10 pp1126ndash1137 2011

[78] N Loutfy M A El-Tayeb A M Hassanen M F MoustafaY Sakuma and M Inouhe ldquoChanges in the water status andosmotic solute contents in response to drought and salicylicacid treatments in four different cultivars of wheat (Triticumaestivum)rdquo Journal of Plant Research vol 125 no 1 pp 173ndash1842012

[79] I I Vaseva B S Grigorova L P Simova-Stoilova K NDemirevska and U Feller ldquoAbscisic acid and late embryoge-nesis abundant protein profile changes in winter wheat underprogressive drought stressrdquo Plant Biology vol 12 no 5 pp 698ndash707 2010

[80] L Mondini M Nachit E Porceddu and M A PagnottaldquoIdentification of SNPmutations inDREB1 HKT1 andWRKY1genes involved in drought and salt stress tolerance in durumwheat (Triticum turgidum L var durum)rdquo OMICS vol 16 no4 pp 178ndash187 2012

[81] S Geng Y Zhao L Tang et al ldquoMolecular evolution of twoduplicated CDPK genes CPK7 and CPK12 in grass species acase study in wheat (Triticum aestivum L)rdquo Gene vol 475 no2 pp 94ndash103 2011

[82] I Tari A Guoth D Benyo J Kovacs P Poor and B WodalaldquoThe roles of ABA reactive oxygen species and Nitric Oxide inroot growth during osmotic stress in wheat comparison of atolerant and a sensitive varietyrdquo Acta Biologica Hungarica vol61 no 1 pp 189ndash196 2010

[83] VVassileva C Signarbieux I Anders andU Feller ldquoGenotypicvariation in drought stress response and subsequent recovery ofwheat (TriticumaestivumL)rdquo Journal of Plant Research vol 124no 1 pp 147ndash154 2011

[84] R Singh-Tomar S Mathur S I Allakhverdiev and A JajooldquoChanges in PS II heterogeneity in response to osmotic andionic stress in wheat leaves (Triticum aestivum)rdquo Journal ofBioenergetics andBiomembranes vol 44 no 4 pp 411ndash419 2012

[85] J Csiszar A Galle E Horvath et al ldquoDifferent peroxidaseactivities and expression of abiotic stress-related peroxidases inapical root segments of wheat genotypes with different droughtstress tolerance under osmotic stressrdquo Plant Physiology andBiochemistry vol 52 pp 119ndash129 2012

[86] M Mahdid A Kameli C Ehlert and T Simonneau ldquoRapidchanges in leaf elongation ABA and water status during therecovery phase following application of water stress in twodurum wheat varieties differing in drought tolerancerdquo PlantPhysiology and Biochemistry vol 49 no 10 pp 1077ndash1083 2011

[87] M Volgger I Lang M Ovecka and I Lichtscheidl ldquoPlasmol-ysis and cell wall deposition in wheat root hairs under osmoticstressrdquo Protoplasma vol 243 no 1ndash4 pp 51ndash62 2010

[88] M Rahaie G P Xue M R Naghavi H Alizadeh and P MSchenk ldquoA MYB gene from wheat (Triticum aestivum L) isup-regulated during salt and drought stresses and differentiallyregulated between salt-tolerant and sensitive genotypesrdquo PlantCell Reports vol 29 no 8 pp 835ndash844 2010

[89] T Unver andH Budak ldquoConservedmicrornas and their targetsin model grass species brachypodium distachyonrdquo Planta vol230 no 4 pp 659ndash669 2009

[90] H Budak and A Akpinar ldquoDehydration stress-responsivemiRNA in Brachypodium distachyon evident by genome-widescreening of microRNAs expressionrdquoOMICS vol 15 no 11 pp791ndash799 2011

[91] S J Lucas and H Budak ldquoSorting the wheat from the chaffidentifying miRNAs in genomic survey sequences of Triticumaestivum chromosome 1ALrdquo PLoS One vol 7 no 7 Article IDe40859 2012

[92] M Kantar B A Akpınar M Valarik et al ldquoSubgenomicanalysis of microRNAs in polyploid wheatrdquo Functional ampIntegrative Genomics vol 12 no 3 pp 465ndash479 2012

[93] L Simova-Stoilova I Vaseva B Grigorova K Demirevska andU Feller ldquoProteolytic activity and cysteine protease expressionin wheat leaves under severe soil drought and recoveryrdquo PlantPhysiology and Biochemistry vol 48 no 2-3 pp 200ndash206 2010

[94] D S Selote and R Khanna-Chopra ldquoAntioxidant response ofwheat roots to drought acclimationrdquo Protoplasma vol 245 no1ndash4 pp 153ndash163 2010

[95] Y-L Du Z-Y Wang J-W Fan N C Turner T Wangand F-M Li ldquoBeta-Aminobutyric acid increases abscisic acidaccumulation anddesiccation tolerance anddecreaseswater usebut fails to improve grain yield in two spring wheat cultivarsunder soil dryingrdquo Journal of Experimental Botany vol 63 no13 pp 4849ndash4860 2012

[96] IMHuseynova ldquoPhotosynthetic characteristics and enzymaticantioxidant capacity of leaves from wheat cultivars exposed todroughtrdquoBiochimica et BiophysicaActa vol 1817 no 8 pp 1516ndash1523 2012

[97] H Liu Y H Zhang H Yin W X Wang X M Zhao and Y GDu ldquoAlginate oligosaccharides enhanced Triticum aestivum Ltolerance to drought stressrdquo Plant Physiology and Biochemistryvol 62 pp 33ndash40 2013

[98] J R Witcombe P A Hollington C J Howarth S Readerand K A Steele ldquoBreeding for abiotic stresses for sustainableagriculturerdquo Philosophical Transactions of the Royal Society Bvol 363 no 1492 pp 703ndash716 2008

[99] S Gulnaz M Sajjad I Khaliq A S Khan and S HKhan ldquoRelationship among coleoptile length plant height andtillering capacity for developing improved wheat varietiesrdquoInternational Journal of Agriculture and Biology vol 13 no 1pp 130ndash133 2011

[100] B Wei R Jing C Wang et al ldquoDreb1 genes in wheat (Triticumaestivum L) development of functional markers and genemapping based on SNPsrdquoMolecular Breeding vol 23 no 1 pp13ndash22 2009

[101] E Nevo and G Chen ldquoDrought and salt tolerances in wildrelatives for wheat and barley improvementrdquo Plant Cell ampEnvironment vol 33 no 4 pp 670ndash685 2010

[102] P Guo M Baum S Grando et al ldquoDifferentially expressedgenes between drought-tolerant and drought-sensitive barley


genotypes in response to drought stress during the reproductivestagerdquo Journal of Experimental Botany vol 60 no 12 pp 3531ndash3544 2009

[103] A Pellegrineschi M Reynolds M Pacheco et al ldquoStress-induced expression in wheat of the Arabidopsis thalianaDREB1A gene delays water stress symptoms under greenhouseconditionsrdquo Genome vol 47 no 3 pp 493ndash500 2004

[104] D Hoisington and R Ortiz ldquoResearch and field monitoring ontransgenic crops by the Centro Internacional de MejoramientodeMaız y Trigo (CIMMYT)rdquo Euphytica vol 164 no 3 pp 893ndash902 2008

[105] A Bahieldin H T Mahfouz H F Eissa et al ldquoField evaluationof transgenic wheat plants stably expressing the HVA1 gene fordrought tolerancerdquo Physiologia Plantarum vol 123 no 4 pp421ndash427 2005

[106] S-S Jiang X N Liang X Li et al ldquoWheat drought-responsivegrain proteome analysis by linear and nonlinear 2-DE andMALDI-TOF mass spectrometryrdquo International Journal ofMolecular Sciences vol 13 no 12 pp 16065ndash16083 2012

[107] H Budak B A Akpinar T Unver and M Turktas ldquoProteomechanges in wild and modern wheat leaves upon drought stressby two-dimensional electrophoresis and nanoLC-ESI-MSMSrdquoPlant Molecular Biology 2013

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Anatomy Research International

PeptidesInternational Journal of


Hindawi Publishing Corporation httpwwwhindawicom

International Journal of

Volume 2014

Zoology


Molecular Biology International

GenomicsInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


BioinformaticsAdvances in

Marine BiologyJournal of



Signal TransductionJournal of


BioMed Research International

Evolutionary BiologyInternational Journal of



Biochemistry Research International

ArchaeaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Genetics Research International


Advances in

Virolog y

Hindawi Publishing Corporationhttpwwwhindawicom

Nucleic AcidsJournal of

Volume 2014

Stem CellsInternational



Enzyme Research



Microbiology


to be used in the improvement of modern crop varietiesThese include the natural progenitors of cultivated crops andfor wheat improvement Ae tauschii which is more droughttolerant thanTriticum andwild emmerwheat (T dicoccoides)which harbors drought tolerance characteristics lost duringcultivation of modern lines is of great importance [6]

Although development of higher-yielding crops underwater-limited environments is the most viable solution tostabilizing and increasing wheat production under currentclimatic conditions it is challenged by the nature of droughtresponse as a trait and the complex genomic constitutionof wheat [16] However recently the utilization of droughttolerant wild species and the rapid advances in molecularbiological functional genomics and transgenics technolo-gies have facilitated drought-related studies resulting insignificant progress in the identification of related genesand gene regions and dissection of some of its molecularaspects This paper summarizes the current state of drought-related research in Triticum species focusing on the iden-tification and functional characterization of drought-relatedmolecules analysis of their interactions in the complexnetwork of drought response and applications of thesedata to improve wheat cultivars utilizing molecular based-technologies

2 T dicoccoides and Drought Tolerance

Wild emmer wheat (T turgidum ssp dicoccoides (korn)Thell) is the tetraploid (2119899 = 4119909 = 28 genome BBAA)progenitor of both domesticated tetraploid durum wheat (Tturgidum ssp durum (Desf) MacKey) and hexaploid (2119899 =6119909 = 42 BBAADD) bread wheat (T aestivum L) It isthought to have originated and diversified in the Near EastFertile Crescent region through adaptation to a spectrum ofecological conditions It is genetically compatible with durumwheat (T turgidum ssp durum) and can be crossed withbread wheat (T aestivum L) [17] Wild emmer germplasmharbors a rich allelic pool exhibiting a high level of geneticdiversity showing correlation with environmental factorsreported by population-wide analysis of allozyme and DNAmarker variations [18ndash24]

Wild emmer wheat is important for its high drought tol-erance and some of T dicoccoides genotypes are fully fertilein arid desert environments Wild emmer wheat accessionswere shown to thrive better under water-limited conditionsin terms of their productivity and stability compared todurum wheat The wild emmer gene pool was shown tooffer a rich allelic repertoire of agronomically important traitsincluding drought tolerance [23 25ndash28]HenceT dicoccoidesis an important source of drought-related genes and highlysuitable as a donor for improving drought tolerance incultivated wheat species

Wild emmer wheat being a potential reservoir ofdrought-related research has been the source of severalidentified candidate drought-related genes with the devel-opment of ldquoomicsrdquo approaches in the recent decades Inrecent years transcript profiling of leaf and root tissuesfrom two T dicoccoides genotypes originating from Turkey

TR39477 (tolerant variety) TTD-22 (sensitive variety) wasperformed by our group in two separate studies utilizingdifferent methodologies In one report subtractive cDNAlibraries were constructed from slow dehydration stressedplants and over 13000 ESTs were sequenced In anotherstudy Affymetrix GeneChip Wheat Genome Array was usedto profile expression in response to shock drought stress[1 29] Wild emmer wheat was shown to be capable ofengaging in known drought responsive mechanisms harbor-ing elements present in modern wheat varieties and also inother crop species Additionally several genes or expressionpatterns unique to tolerant wild emmer wheat indicative ofits distinctive ability to tolerate water deficiency were alsorevealed Transcript and metabolite profiling studies werealso undertaken for two T dicoccoides genotypes originatingfrom Israel Y12-3 (tolerant variety) and A24-39 (sensitivevariety) under drought stress and nonstress conditions Leaftranscript profiling indicated differential multilevel regula-tion among cultivars and conditions [30] Integration of roottranscript and metabolite profiling data emphasized droughtadaptation through regulation of energy related processesinvolving carbon metabolism and cell homeostasis (Table 1)[14] Recently in wild emmer wheat our group also profileddrought induced expression of microRNA (miRNAs) smallregulatory molecules known to be involved in several cellularprocesses including stress responses In this study leaf androot tissues of resistant wild emmer wheat varieties TR39477and TR38828 were screened via a microarray platformand 13 differentially expressed miRNAs were found to bedifferentially expressed in response to drought (Table 1)[15]

Following the identification of T dicoccoides drought-related gene candidates as discussed previously a numberof these potential drought resistant genes were clonedand further characterized In one of the recent reportsTdicTMPIT1 (integral transmembrane protein inducible byTumor Necrosis Factor-120572 TNF-120572) was cloned from wildemmer root tissue and shown to be a membrane pro-tein associated with the drought stress response exhibitingincreased levels of expression specifically in wild emmerwheat upon osmotic stress [31] In a different study TdicDRF1(DRE binding factor 1) conserved between crop species wascloned for the first time from wild emmer wheat Its DNAbinding domain AP2ERF (APETALA2ethylene-responsiveelement binding factor) was shown to bind to droughtresponsive element (DRE) using an electrophoretic mobilityshift assay (EMSA) It was revealed to exhibit cultivar and tis-sue specific regulation of its expression throughmechanismsinvolving alternative splicing [32] Moreover the relationsbetween autophagy and drought response were analyzed inanother line of research by the cloning of TdATG8 (autophagyrelated protein 8) and its further functional investigationwith yeast complementation assay and virus induced genesilencing (VIGS) of plants In this study autophagy wasshown to be induced in drought-stressed plants in an organ-specific mode and silencing of ATG8 was shown to decreasedrought tolerance of plants revealing it as a positive regulatorof drought stress [33] (Tables 2 and 3)













Kukri sensitiveLeaf


conditions




cDNA-AFLP [12]






fivesix leaf stage


[14]

































2




















stressReference



TF Drought [39]


MYB TF Drought [38]


MYB TF Drought [34]


NAC(NAMATAF

CUC)


























gene 8 Drought [33]













TaPIMP1

TaMYB3R1

GSTSOD

Proline

TaWRKY2TaWRKY19

TaNAC4a

TaNAC6TaNAC2a



Dehydrins

LEA proteins

Others

DRECRT box

TdicDRF1


MYS motif

TaMYB33

Membrane stability

















2O2) and ABA


































References


























































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology













Kukri sensitiveLeaf


conditions




cDNA-AFLP [12]






fivesix leaf stage


[14]

































2




















stressReference



TF Drought [39]


MYB TF Drought [38]


MYB TF Drought [34]


NAC(NAMATAF

CUC)


























gene 8 Drought [33]













TaPIMP1

TaMYB3R1

GSTSOD

Proline

TaWRKY2TaWRKY19

TaNAC4a

TaNAC6TaNAC2a



Dehydrins

LEA proteins

Others

DRECRT box

TdicDRF1


MYS motif

TaMYB33

Membrane stability

















2O2) and ABA


































References


























































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology

























2




















stressReference



TF Drought [39]


MYB TF Drought [38]


MYB TF Drought [34]


NAC(NAMATAF

CUC)


























gene 8 Drought [33]













TaPIMP1

TaMYB3R1

GSTSOD

Proline

TaWRKY2TaWRKY19

TaNAC4a

TaNAC6TaNAC2a



Dehydrins

LEA proteins

Others

DRECRT box

TdicDRF1


MYS motif

TaMYB33

Membrane stability

















2O2) and ABA


































References


























































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology












stressReference



TF Drought [39]


MYB TF Drought [38]


MYB TF Drought [34]


NAC(NAMATAF

CUC)


























gene 8 Drought [33]













TaPIMP1

TaMYB3R1

GSTSOD

Proline

TaWRKY2TaWRKY19

TaNAC4a

TaNAC6TaNAC2a



Dehydrins

LEA proteins

Others

DRECRT box

TdicDRF1


MYS motif

TaMYB33

Membrane stability

















2O2) and ABA


































References


























































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology






TaPIMP1

TaMYB3R1

GSTSOD

Proline

TaWRKY2TaWRKY19

TaNAC4a

TaNAC6TaNAC2a



Dehydrins

LEA proteins

Others

DRECRT box

TdicDRF1


MYS motif

TaMYB33

Membrane stability

















2O2) and ABA


































References


























































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology














2O2) and ABA


































References


























































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology

























References


























































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology













































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology

Review Article Drought Tolerance in Modern and Wild...

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Transcript of Review Article Drought Tolerance in Modern and Wild...