CHapter 1 Biotechnological applications to improve ...
Transcript of CHapter 1 Biotechnological applications to improve ...
Plant-Environment Interaction: Responses and Approaches to Mitigate Stress, First Edition. Edited by Mohamed Mahgoub Azooz and Parvaiz Ahmad.
© 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd.
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1.1 Introduction
For food, humans rely on approximately 275 crops
(Tilman et al., 2011). Out of these, three crops, wheat,
maize and rice, are significant cereal crops that con
tribute to major dietary requirements as staple foods for
humans – a reason why they are collectively termed the
‘big three cereal crops’ (Shewry, 2009). Comparatively,
wheat is the most important cereal crop that contributes
a major portion of the daily diet for humans (Slade
et al., 2012). It is estimated that wheat is a source for
one‐fifth of total calories utilized by humans globally
(Waines & Ehdaie, 2007). Wheat grains contain vital
constituents such as carbohydrates, including 60–70%
starch (Slade et al., 2012) and 8–15% protein such as
glutenin (Shewry et al., 1995) and gliadin (D’Ovidio &
Masci, 2004). From the total wheat grain produced
globally, 65% is utilized as food by humans while the
remaining 35% is distributed among livestock feed
(21%), seed material (8%) and raw material (6%) in
industries such as the production of vitamins and anti
biotics, manufacturing of paper; it is also used as a
fermentation substrate or as adhesives in various prod
ucts (Shewry & Jones, 2005).
1.1.1 History of wheat: from domestication to revolutionsIn ancient times, wheat was a product of the activities
of hunter‐gatherers but about 10,000 years ago, the
Neolithic Revolution laid the basis for domestication of
various crops (Waines & Ehdaie, 2007). This domestica
tion process focused mainly upon cereal crops, and
wheat is considered the originator of domesticated crops
(Peleg et al., 2011). With the passing of time, problems
arising in the domestication process compelled scientists
to analyse and study various concerns such as local con
ditions, yield maximization, development of improved
cultivars and storage techniques (Cavanagh et al., 2013).
Eventually, these findings resulted in major events such
as the Agricultural Revolution in the 19th century
(Godfray et al., 2010) and the Green Revolution in the
20th century (Waines & Ehdaie, 2007).
Wheat domestication followed by major revolutions
and scientific achievements contributed to speciation
and initiation of new varieties (Shewry, 2009). The
factors involved in such speciation primarily include
adaptations to the ecology of an area as soon as wild‐
type wheat cultivars were moved for domestication
purposes (Chaudhary, 2013). These adaptations under
the influence of epigenetics offered the opportunity to
select the desired traits in wheat such as yield, grain
quality, grain size and many other phenotypic attributes
(Burger et al., 2008). Thus, wheat evolved into many
varieties in response to human cultivation practices,
selection procedures and the phenomena of epigenetics
(Fuller, 2007).
Since the Green Revolution, technologies have
been incorporated into crop improvement practices,
specifically wheat, in various ways (Schmidhuber &
Tubiello, 2007). These include successful development
of hybrids with enhanced desired traits, development of
pathogen‐resistant plants, enhanced yield, improved
nutrient contents, affordable fertilizer requirements
and improved irrigation systems (Godfray et al., 2010).
Biotechnological applications to improve salinity stress in wheatSami ullah Jan1, Ghulam Kubra1, Mehreen Naz2, Ifrah Shafqat2, Muhammad Asif Shahzad1, Fakiha Afzal1 and Alvina Gul Kazi1
1 Atta‐ur‐Rahman School of Applied Biosciences (ASAB), National University of Sciences and Technology (NUST), Islamabad, Pakistan2 Department of Bioinformatics and Biotechnology, International Islamic University, Islamabad, Pakistan
CHapter 1
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COPYRIG
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ATERIAL
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The consequences of all aspects of the Green Revolution
increased yield to fulfil the world’s food requirements
(Tilman et al., 2011).
1.1.2 Wheat genomeModern wheat includes six sets of genomes, called hexa
ploidy, and is a result of domestication and scientific
processes practised by man. Polyploid genomes of wheat
cultivars evolved after crossing or hybridization, selec
tion procedures and cultivation practices in domestication.
The wild wheat ancestor Triticum turgidum sp. dicoccoides is
considered as the first domesticated wheat species in
the Near East region (Maier, 1996). This wheat species
was spread across Europe and gave rise to new varieties
like Triticum turgidum sp. dicoccum and Triticum turgidum
sp. durum (Buckler et al., 2001). Durum wheat is still
widely grown in the Near East crescent around the
Mediterranean Sea (Thuillet et al., 2005).
In reference to common bread wheat, this is an
allopolyploid consisting of three genomes designated
as A, B and D originating from wild wheat grasses of
the genera Triticum and Aegilops (Zohary et al., 1969).
Modern wheat is hexaploid, existing in three sets,
A‐Genome, B‐Genome and D‐Genome. The ancestor
of A‐genome wheat Triticum urartu contained 14
chromosomes in two sets, and was crossed with Aegilops
speltoides (B‐genome) that resulted in a hybrid which
contained both genomes (AB) which after doubling
yielded a viable tetraploid containing 28 chromosomes
(AABB). This hybrid, known as wild emmer (Chen
et al., 2013), upon further crossing with Aegilops squar-
rosa (a diploid grass), produced a new hybrid with 21
chromosomes (42 chromosomes in diploid form). The
later hybrid produced is the hexaploid wheat utilized
today and contains genomes from three ancestors
(AABBDD) (Levy & Feldman, 2002).
1.1.3 Wheat production and concernsDuring the past 50 years, research and technological
applications in the cultivation of wheat have increased
its yield to a rate of 41 kg per hectare (Ewert et al.,
2005). But the world’s population is increasing all the
time (Godfray et al., 2010). If this continues, by the mid‐
century, the world’s population is estimated to be 9–10
billion (DeLong et al., 2010). Simultaneously, demands
for more food and energy resources will also be raised
such that, by the middle of the century, necessary food
production will be double that of the present (Ray et al.,
2013). Numerically, the required rate of increase in food
production by the year 2050 is 100–110% compared to
the current rate of production (Tilman et al., 2001).
About 600 million metric tons of wheat is produced per
year worldwide but with the increment in population,
by 2020 we would require an estimated yield of 1 billion
metric tons (Shewry, 2009). In 2005, calculated yield
per hectare of wheat was 2.5 tons which was forecasted
to reach a figure of 4.0 t/ha by 2020 (Rajaram, 2005).
Despite these important facts, only 3 billion hectares
of land out of 13.4 billion hectares is available for crop
cultivation (Smith et al., 2010). One solution to over
coming the world’s food requirements is to turn more
land over to arable in order to increase wheat global pro
duction (Gregory et al., 2002). It has been estimated that
by utilizing only 20% of untilled land, we could increase
crop yields up to 67% (Bruinsma, 2003). In 2007, total
yield of cereal crop was 3.23 tons per hectare which
could be increased to 4.34 tons per hectare by increasing
land under cultivation to 25% (Bruinsma, 2009). The
actual figure for per capita arable land is continuously
decreasing due to industrialization, housing and defor
estation as well as some environmental concerns
(Gregory & George, 2011). However, environmental
concerns are among the major problems that cause the
loss of yield such that only 50–80% yields are achieved
(Lobell et al., 2009). Various scientific communities con
tribute to minimize the gap between actual and potential
yields (Jaggard et al., 2010) but the problems remain the
same and the environmental concerns are important,
such as abiotic (salinity, drought, temperature) and
biotic stresses (Atkinson & Urwin, 2012).
1.2 Salinity stress is a striking environmental threat to plants
Agricultural production all over the world is constrained
by salinity stress and it is becoming a growing universal
issue that affects almost 20% of cultivated land globally
(Flowers & Yeo, 1995). From the agricultural point of
view, salinity is the aggregation of dissolved salts within
soil or agricultural water to an extent which adversely
affects plant growth (Gorham, 1992). High salinity
influences the physiological mechanism that adversely
affects plant growth and development which necessi
tates detailed investigation of tolerance mechanisms in
salinity (Abogadallah, 2010).
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Biotechnological applications to improve salinity stress in wheat 3
Salinity‐induced stress increases the accumulation of
salts in plant roots (Zhang et al., 2014). Such hyperac
cumulation of salts in roots restricts water absorption
from the soil surface and thus also causes water stress, in
spite of available water at the root zone. Water absorption
from saline soils requires extra energy expenditure.
Thus, higher salinity will always lead to decreased levels
of water as well as inducing analogous stresses like
water and osmotic stress (Bauder & Brock, 1992).
1.2.1 Statistics of salinity stress‐affected landSaline soils are widespread in arid and semiarid regions,
especially in areas where heavy irrigation or overfertili
zation is common (Reynolds et al., 2005). It is estimated
that 800–930 million hectares (7%) of the world’s total
arable land is influenced by salt stress (Shannon, 1997;
Szabolcs, 1994) while 230 million hectares of irrigated
land are affected by salts (Oldeman et al., 1991).
Extensive salts in soil arise due to natural processes such
as rainfall containing salts as well as irrigation practices
such as the use of fertilizers, resulting in poor water
quality (Reynolds et al., 2005).
1.2.2 Causes of salinity stressSalinity is a primary abiotic stress that hinders plant
growth. Numerous causes are responsible for salinity.
Some prominent causes include extensively irrigated
lands, use of improper waters, inefficient drainage,
practising inappropriate irrigation, standing water for
prolonged time and water seepage from reservoirs.
Underground leakage or seepage from water reservoirs
tends to raise the water table which mobilizes salt and
thus causes salinity stress (Awad, 1984). Further,
increment of the saline water table to 2 meters speeds
up the evaporation process, leaving excessive salt in the
soil, resulting in waterlogged soil. Due to the limited
oxygen in water‐logged soils, the survival of vegetation
is endangered. Another important reason causing
salinity is heavy rainfall which drains salts away with it
and may saturate cultivated land.
Despite such diverse causes, the main reasons for
salinity are (1) natural (primary) salinity, and (2) human‐
induced (secondary) salinity.
Primary salinity or natural salinity is a result of
a prolonged natural build‐up of salt in water or soil
which occurs by the natural process of breakdown of
rocks containing SO4
2–, Ca2+, CO3
2–, Cl–, Na+ and Mg2+.
Furthermore, evidence of ocean salt conveyed by wind
and downpour is likewise a reason, which changes with
the types of soil.
More specifically, the main reason for natural salinity
is the excess accumulation of salts at the soil surface.
Among all these causes, studies have revealed that those
salts which accumulate and cause salinity stress are com
posed of a particular set of ions, including Na+, K+, Ca2+,
Mg2+ and Cl–. However, among these, Na+ is more domi
nant and its excess makes soil sodic. Sodic soil is more
challenging because it has a poor architecture, thus
limiting or preventing infiltration of water as well as
drainage. Some soils harbour large amounts of salt which
are flushed away along with rain or irrigation water
drainage. Salt accumulation in the form of precipitation
or resulting from weather changes as well as mineral
degradations also leads to salinity. Salt accumulation in
dry lands is very common and rapidly causes salinity
conditions (Brinkman, 1980).
Secondary salinity or human‐induced salinity is
induced because of anthropogenic activities that disturb
the hydrological stability of the soil between water
connected (watering system or precipitation) and water
utilized by plants (transpiration) (Garg & Manchanda,
2008). The water table is increased because of the large
quantity of water supplied to the irrigated fields with
poor drainage systems. Globally all these watering sys
tems may result in secondary salt stress and saturation
of soil with water (Garg & Manchanda, 2008).
1.2.3 types of salinity stressSalinity stress can be classified on the basis of its initia
tion process, mobilization means and mode as well as
their impacts. Regardless of the sources or types of
salinity, the effects of salinity on plant development
remain constant but these effects are variable in terms
of salt concentration encountered by plants. A compre
hensive classification of salinity is presented as follows.
1.2.3.1 Dry land salinity stressDry land salinity refers to a type of salinity that occurs in
unirrigated land. Such land is usually colonized by
shallow‐rooted plants instead of deep‐rooted crops
because the shallow‐rooted plants can withstand less
water and their growth cycle is short. Dry land, as com
pared to normal or moist land, increases seepage rates to
ground water and induces mobilization of salts pre‐
stored in soil. In low areas or slopes, ground water along
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with its salts is exposed to evaporation, leaving salts
at the soil surface and thus increasing salinity–plant
interactions. Likewise, ground water may also finally
be added to streams and rivers, in which case, salt
concentration in water resources is increased and when
these resources are utilized for irrigation purposes, they
will cause salinity stress to plants (Dias & Thomas, 1997).
Dry land salinity is dependent upon type and compo
sition of soil when subjected to irrigation. Predominantly
sodic or naturally saline clays fall under the category of
dry land salinity which may also spread if such soil is
transported by any means and/or mixed with other
soils. However, when sodic soil becomes wet, it dis
perses and causes blockage of pores within the soil
which are common routes of water seepage towards
ground water. In other cases, when sodic soil is dry, it
becomes dense and hard and forms a crust on the soil
surface. In the latter case, soil structure loses its capacity
for infiltration of water, leaving no or little salt at the
root. In some conditions, subsodic soils can produce a
perched water table which creates water logging at the
root zone. In view of such conditions, if large amounts
of water are applied in irrigation, it will increase the
ground water level which eventually causes irrigation
salinity. This problem will tend to increase if such soils
are accompanied with crops that utilize lower amounts
of water. The worst condition of irrigated salinity can
occur if water for irrigation purposes is obtained from
salty ground water or salty rivers (Dumsday et al., 1983).
1.2.3.2 Urban salinity stressUrban salinity includes the activities involved in urban
ization and its development as well as excessive use of
water for gardens and other low‐level cultivation prac
tices. Other phenomena include leakage from pipelines,
tanks and drainage systems or changing water flow
direction from its normal routes. All these factors
support the rise of the water column in the ground
water table. Sources of salt arising from these processes
include material used in buildings, waste from indus
tries, use of chemicals and fertilizers, salt effluents as
well as naturally occurring salt.
Industrial waste and effluents are excreted from
industrial and some domestic areas; those which have
high salt concentrations have proved more dangerous
compared to other categories in urban salinity. Industries
using coal for fire also use huge amounts of water for
cooling purposes which results in evaporation of water
while salts are deposited and finally added to soil.
Similarly, mining activities also play a role in causing
urban salinity (Murray, 1999).
1.2.3.3 River salinity stressRivers are the major source of irrigation but this source
is very much contaminated with salts. All the drainage
from domestic and industrial systems, as well as affected
dry lands, finally end in a river. The same water is recy
cled for irrigation purpose which creates a stress
environment for plants. With the passage of time, the
quality of river water becomes more saturated as water
level decreases in saline rivers which in turn irrigates
plants, resulting in impaired plant survival (Awang
et al., 1993).
1.2.3.4 Irrigation salinity stressWhen the source of water used for plants is a saline
water reservoir, a condition arises known as irrigation
salinity. It is different from river salinity because river
salinity refers to salinity caused by river water used for
irrigation purposes while irrigation salinity encompasses
the salinity caused by any water source for plants (Ayres &
Westcot, 1985). Sources of salts in such conditions are
mostly similar to those mentioned in rivers and dry land
salinity. However, in addition, climate and routes of irri
gation system also determine the irrigation salinity
levels (Bauder & Brock, 2001).
1.3 effects of salinity stress on wheat
Wheat can tolerate salt to some extent but as the salinity
concentration increases, more serious risks of damage
are likely to occur. The vast dimensions of threats to
wheat and other plants by salinity stress range from its
physiological characteristics to vital biochemical path
ways and its genetic make‐up. These effects include
hindrance of seed germination and seedling growth and
dry mass accumulation in wheat (Shirazi et al., 2001;
Sourour et al., 2014), disturbed biochemical pathways
(Radi et al., 2013) and disorganized patterns of gene
expression. Many investigators have screened numerous
wheat cultivars, including salt‐tolerant varieties, to
check the effects and variations due to salinity on var
ious growth levels (Munns & Termaat, 1986). Some of
the patterns of salt effects on numerous attributes of
wheat are discussed below.
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Biotechnological applications to improve salinity stress in wheat 5
1.3.1 effects of salinity stress on phenological attributes of wheatTo assess the impact of salinity on some physiological
(Yang et al., 2014) and phenological qualities in durum
wheat, a factorial investigation was directed at time of
stem lengthening, time to heading and development,
chlorophyll content and chlorophyll fluorescence of
leaf. Results demonstrated that the relation between
salinity and cultivars (61130 and PGS) causes expansion
in time to heading (Kahirizi et al., 2014). Distinctive
attributes like grain number obtained on a meter square
area, grain yield, 1000 grains weight, leaf’s Na+ and
K+ ion contents and plant’s height had been measured
in field analyses for bread wheat. Numerous quantitative
trait loci (QTL) were determined by ecological contacts
which demonstrate several phenological characteristics
for salt resistance. Numerous QTL for biomass of seed
lings and end of Na+ particles were seen in hydroponic
and field tests (Genc et al., 2013).
Excessive salinity has always diminished development,
yield and related yield characteristics, photosynthetic
traits (Mathur et al., 2013), ionic substances and
biochemical activities in all wheat cultivars. Nonetheless,
application of potash composts in soil and also in foliar
application counteracts the unfavourable impacts of salt
stress on all wheat cultivars and the application of sul
phate of potash (SOP) in soil and as a foliar splash was
more compelling in triggering salt stress tolerance. No
unfavourable impact of chloride on plant development
was noticed. Among the cultivars, ‘S‐24’ and ‘Sehar’
demonstrated remarkable development, yield and
biochemical matter and subsequently could be utilized
as parental material for achieving better yield under
saline conditions (Ashraf et al., 2013).
1.3.2 effects of salinity stress on morphological attributes of wheatMorphologically, salt stress causes reduction in overall
plant growth but shoots are seen to be more affected
than roots and the ratio between them is increased
(Allen, 1995). Salt stress also reduces the total dry
matter yield and number plus appearance of wheat tiller
(Mass & Poss, 1989). Salt stress also causes stunting of
shoots (Walker et al., 1981). Two reasons why increased
salt levels found in soil water cause reduction of plant
growth are (1) plants’ ability to take up water from soil
is decreased due to ionic imbalance which decelerates
natural growth processes, and (2) when salts get into a
plant through any possible route, such as transpiration
stream into the transpiring leaves, this eventually
injures cells which also restricts growth (Munns, 2005).
Initial contact with salinity causes water stress in
plants, resulting in reduction of leaf expansion ability.
Another associated problem, osmotic stress, also arises
in plants on initial exposure to salt stress and can lead to
inhibition of cell division, expansion and function of
stomata (Munns, 2002).
Soil salinity causes formation of Na+ and Cl– that
affects the ionic composition taken up by plants
(Rengasamy, 2006). Salt stress directly affects plant
growth through osmotic stress and ionic toxicity caused
by Na+ and Cl– ions which promote imbalance in plant
nutrient metabolism (Rowell, 1994). Adverse effects of
salt stress on cell morphology include accumulation of
toxic ions that disrupts intra‐ and extracellular compo
nents like DNA, enzymes, membranes, mitochondria,
chloroplasts and many more by the development of
reactive oxygen species (ROS) (Allen, 1995; Saqib
et al., 2012).
1.3.3 effects of salinity stress on physiological attributes of wheatSalt stress has numerous consequences for germination
methods. Germination is delayed due to high salt con
centrations in soil which create high osmotic pressure,
thus reducing water intake by seeds (Khan & Weber,
2008), that may cause the metabolism of nucleic acid
digestion (Gomes‐Filho et al., 2008), changes in metab
olism of protein (Dantas et al., 2007) and aggravation of
hormonal offset (Khan & Rizvi, 1994), as well as less
ening the ability to utilize seed stores (Othman et al.,
2006). It might likewise influence the fine structure of
cell, tissues and organs (Al‐Maskri et al., 2014; Rasheed,
2009). However, there are different intramural (plant)
as well as external (natural) aspects that influence seed
germination under saline conditions which incorporate
nature of seed layer, seed torpidity, seedling power, seed
polymorphism, seed age, water, gases (Mguis et al.,
2013), light and temperature (Wahid et al., 2011).
Death of the plant occurs at higher concentrations of
the salt as a result of hyperionic and hyperosmotic
stress. The result of these impacts may cause membrane
layer harm, nutrient unevenness, distorted levels of
enzymatic hindrance, developmental regulators and
metabolic abnormality, including photosynthesis which
at last prompts plant demise (Hasanuzzaman et al., 2012;
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Mahajan & Tuteja, 2005). Similar consequences have
been observed in various varieties globally, including
Triticum aestivum (Akbarimoghaddam et al., 2011),
Brassica spp. (Ulfat et al., 2007), Zea mays (Khodarahmpour
et al., 2012), Oryza sativa (Xu et al., 2011), Vigna spp.
(Jabeen et al., 2003), Helianthus annuus (Mutlu &
Buzcuk, 2007) and Glycine max (Essa, 2002). It was
observed that germination of seeds has a negative rela
tionship with salinity (Rehman et al., 2000). Higher
salinity represses the germination of seeds while seed
dormancy is incited by lower levels of salt stress (Khan &
Weber, 2008).
The most harmful ions which cause several
physiological impacts on plants are Na+ and Cl–
(Tavakkoli et al., 2010). Many stresses, abiotic and biotic,
affect photosynthesis which is a complex process
involving many components such as photosynthetic
pigments, photosystems, the electron transport system,
CO2 reduction pathways, etc. Stress of any kind can
affect any of these components, reducing the photosyn
thetic capacity of plants. To combat this harm, harvested
plants typically utilize protein kinases, for example,
MAPKs and transcription factors (Ashraf et al., 2013;
Saad et al., 2013; Zhang L et al., 2012).
Research undertaken by Rajendran et al. (2009)
showed the impact of ion exchange under salt stress in
advanced stages of plant development. They observed
that hazardous ions accumulated after 2–4 weeks of
exposure of salt stress. The stress caused by ions (Na+
and/or Cl–) overlaps with the osmotic impacts and dem
onstrates more hereditary variety than osmotic impacts
(Munns et al., 2002).
1.3.4 effects of salinity stress on biochemical attributes of wheatFrom the biochemical point of view, life‐sustaining
pathways like respiration (Shekoofa et al., 2013) and
photosynthesis as well as their associated enzymes are
affected by high salt levels while responses to these
salts by cellular machinery mostly use enzymes (Walker
et al., 1981). In such conditions, salt stress triggers
further phenomena such as hyperosmotic shocks, cell
turgidity is lost, ROS are formed and stomatal size is
minimized (Price & Hendry, 1991). Eventually, these
conditions collectively or individually may restrict
plant growth.
As NaCl is an actual component of saline soil, plants
gather Na+ and Cl– particles up to levels that are
detrimental (Cuddy et al., 2013). Shoot Na+ poisonous
quality is connected with the decline of stomatal con
ductance while high shoot Cl– levels immediately
influence chlorophyll and repress photosystem II
(Ashraf & Ashraf, 2012; Tavakkoli et al., 2011).
Higher Na+ and Cl– in plant cells are seen as the key
components of ionic damage (Cuin et al., 2009; Munns &
Tester, 2008; Rajendran et al., 2009). There are various
studies and evaluations that discuss the connection of
Na+, K+ and K+/Na+ homeostasis with salt stress toler
ance in harvest plants (Horie et al., 2009). The effect of
Cl– homeostasis in salt tolerance is little understood
(Teakle & Tyerman, 2010). The elevated levels of Cl–
that congregate in the plant leaves developed under
saline conditions will affect the whole plant (White &
Broadley, 2001). Hence, it is striking that minimal
investigation has been undertaken into the impacts of
Cl– content in connection to salt tolerance; however,
there are no reports on the hereditary control of this
attribute. Just a few studies showed that treatment of
Cl– may be essential for salt tolerance in a few products
including grain (Tavakkoli et al., 2010a; Tavakkoli et al.,
2010b; Teakle & Tyerman, 2010).
Metabolically harmful quantities of Na+ to a greater
extent are a consequence of its capability to contend
with K+ for binding components vital for cell capacity.
High Na+/K+ proportions can disturb different enzymatic
courses of action in the cytoplasm (Tester & Davenport,
2003). Stress caused by ions is connected with a decline
in chlorophyll content and restrains photosynthesis,
impelling leaf senescence and early leaf fall. Ionic stress
consequently diminishes photosynthesis limit, biomass
and yield (Tester & Davenport, 2003).
In one study, the impact of salicylic acid or indole
acetic acid (IAA) was tested by spraying them on
Triticum aestivum genotypes which were subjected to
different saline levels in order to check their impact on
growth of different plant organs. It was concluded
that under such circumstances, cell enzymes having
antioxidant properties like reducing sugars, catalase,
peroxidase, ascorbate peroxidase, photosynthetic
shades, amino acid, proline in shoot and root were
enhanced (Nassar et al., 2013).
Salinity has been reported to induce oxidative stress
as the ROS are enhanced by producing superoxide (O2),
hydorxyl (OH) and hydrogen peroxide (H2O
2) radi
cals. As the accumulation of ROS increases, scavengers
initiated protection mechanisms in plants from
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Biotechnological applications to improve salinity stress in wheat 7
salinity‐induced damage (Al‐Quraan et al., 2013; Hayat
et al., 2014). Plants harbour an antioxidative defence
mechanism, activated in salinity to overcome oxidative
stress mediated by ROS, which is composed of ions
(Asgari et al., 2012), antioxidant enzymes and osmotic
homeostatic conditions. Ionic homeostasis under salinity
stress is to be maintained for physiological and
biochemical conditions of the plant because such ionic
balance regulates the essential ion concentration rather
than toxic ions (Gupta & Huang, 2014; Hajiboland,
2012). Certain plants possess a potential to maximize
the cellular level of osmotically compatible solute
concentration. Such osmotically compatible solutes can
mediate ionic homeostasis related to water concentration,
eventually leading to minimized impact of ionic concen
trations upon cell proteome including enzymes, protein
complexes plus membrane proteins and other proteins
required in cell stability under stress (Ma et al., 2012).
The role of antioxidative enzymes, for instance APX
(ascorbate peroxidase), SOD (superoxide dismutase),
CAT (catalase) and POD (peroxidase), was important in
minimizing oxidative stress/damage induced during
salinity stress (Ahmad et al., 2010a).
Under saline conditions, the activities of the cell anti
oxidant system, for example, SOD (superoxide dismutase)
and CAT (catalase), in susceptible cultivars were lower as
compared to controls. With respect to APX, there was no
significant difference between saline and control condi
tions. Under salt stress, the MSI (membrane stability
index) of two tested cultivars was adversely affected.
Hydrogen peroxide (H2O
2) content of salinity susceptible
cultivars was higher than controls. Salt‐tolerant mixtures
had more K+ levels and Na+ ratio; relative water sub
stance, yield and chlorophyll under saline condition and
susceptible cultivars accumulate higher Na+ content at
the tillering stage. The method of salt stress may be
accomplished because of low lipid peroxidation, presum
ably, fewer changes in MSI, evasion of Na+ quantity and
release of antioxidant enzymes (Rao et al., 2013).
Salt tolerance in plants is dependent on their ability to
alter osmotically to decreased soil water potential as
well as keeping intracellular levels of ROS under con
trol. Both of these techniques are accepted to depend on
accumulation of natural osmolytes. In one study,
outside NaCl complex was connected by the dynamic
congregation in leaf Na+. This aggregation was much
higher in old leaves and juvenile ones. In old leaves,
three inorganic ions (Cl–, K+ and Na+) contributed 70.4%
and 67.7% of leaf osmotic potential in wheat and grain
individually when presented to 200 mm NaCl treatment,
while in junior leaves their assertion was just
46.8%and 43.9% separately. It was suggested that
salinity prompted increase of natural osmolytes in
wheat grain and leaves corresponding with exaggerated
oxidative stress tolerance and provides the confirmation
of a system of cross‐tolerance between these two stresses
(Puniran‐Hartley et al., 2014).
So as to overcome perverse intercellular ROS, plants
have created cell antioxidant guard frameworks assist
ing them in management of ROS levels. Plants utilize
different components to protect themselves from the
distant impacts of salinity (Zhang Y et al., 2012). Plants
react to excessive salinity by accretion of osmoprotec
tants including proline and sugar (Gurmani et al., 2013).
Plant hormones have outstanding effects on the
framework of cell division and development of a whole
plant. Salt stress‐induced decline in general development
signifies endogenous irregular hormonal levels (Iqbal &
Ashraf, 2013).
1.4 Wheat natural tolerance and defence against salinity
Wheat also possesses particular mechanisms that help it
to combat harmful effects of primary as well as secondary
stress with the accretion of osmolytes and antioxidants
production (Ahmad et al., 2010a; Ahmad et al., 2010b;
Ashraf & Foolad, 2007; Devi & Prasad, 1998; Foyer et al.,
1994). The principal antioxidant enzymes that assist
plants to withstand numerous environmental stresses
include peroxidases, superoxide dismutases, catalases
and glutathione reductases. These enzymes co‐ordinate
in orchestra; for instance, superoxide dismutase con
verts superoxide anions to H2O
2 (Alscher et al., 2002).
Similarly, catalase stands as a second line of defence
against varied stresses by converting H2O
2 to molecular
oxygen and water. With reference to their potential of
free radical quenching, quantization and other patterns
of analysing antioxidant enzymes and non‐enzymatic
antioxidants levels are utilized to determine the effec
tiveness of oxidative defence mechanisms in plants
(Geebelen et al., 2002).
Tolerance to salinity stress is generally evaluated from
the biomass produced under saline compared to con
trolled conditions grown over a convenient time period
0002580878.indd 7 10/3/2015 6:20:17 PM
8 Chapter 1
(Munns, 2002). Tolerance levels in plants also vary
depending upon genetic potential plus biochemical and
physiological characteristics. A halophytic salt‐tolerant
plant upon abrupt exposure to prevailing salinity or
salinity shock will develop diverse strategies that could
lead to gradual acclimation, depending on develop
mental stages and sensitivity of plants. Conversely,
tolerance mechanism is determined by interaction bet
ween environment and particular plant species. Some
plant species may demonstrate stage‐specific sensitivity,
exhibiting greater susceptibility to salt in the germina
tion phase while others may be highly sensitive in the
reproduction stage (Mahmoodzadeh et al., 2013).
Evolution is also important in this regard and numerous
mechanisms are evolved in plants to defend against
salinity stress.
1.4.1 Mechanisms of salt tolerance in wheatSalinity in plants is counteracted by diverse components
(Maas, 1986). Agricultural productivity of certain plants
appears to be more sensitive to high levels of salt con
centrations, such as glycophytes. While halophytic
species are exceedingly salt tolerant and can survive and
maintain development even at saline levels much
higher than that of seawater, none of the radical exam
ples can tolerate more than 25% salt levels without
yield loss and development impairment. The high salt
tolerance of halophytes is ascribed to rare anatomical
and morphological adjustments, or avoidance tools
(Greenway & Munns, 1980). However, halophytes are
exceptional among the 250,000 types of promising
plants (Flowers & Flowers, 2005). Past studies have
grouped plants into two classifications: salt includers
and salt excluders. Sodium ions (Na+) are transported to
shoots with the help of salt includers where it is used in
vacuole‐mediated osmoticum‐intended tolerance fol
lowed by salt excluders which adjust these ions to saline
conditions by eluding uptake of Na+ ions (Mian et al.,
2011a). Overall, plant response to salinity is categorized
in three classes: (1) osmotic stress tolerance; (2)
excluding Na+ from leaves; and (3) tolerance at tissue
level (Munns & Tester, 2008).
Halophytic species form systems like proficient Na+
sequestration into the vacuole, which preserve low
cytosolic Na+, as well as allowing these plants to utilize
Na+ as an osmoticum to maintain cell turgor and
development. Accordingly higher Na+ uptake can lead
to rapid development of halophytes (Blumwald, 2000).
Change of salt tolerance in harvested glycophytes like
rice and durum wheat has been attained by the advance
ment of cultivars with low Na+ in shoot or high K+/Na+
proportion (Munns et al., 2012; Thomson et al., 2010).
Plants use three normal instruments of salt tolerance
(Rajendran et al., 2009): osmotic change; suitable con
firmation for Na+ uptake via roots and restricting its
entry into susceptible tissue; and tissue tolerance (Na+
incorporation, Na+ compartmentation). These systems
are controlled by integrated physiological, biochemical
pathways (Zhou et al., 2012). Osmotic alteration
includes the combination and aggregation of perfect
solutes inside the cytoplasm. Compatible solutes are
smaller water‐solvent particles that contain nitrogen‐
holding blends, for example, betains, amino acids,
additionally natural sugars, polyols and acids (Chen
et al., 2007). The ability of the compatible solutes is not
constrained to maintaining osmotic balance. Compatible
solutes are usually hydrophilic as well as they may have
the capacity to displace water at the protein surface
advancing towards low subatomic weight chaperones
(Carillo et al., 2011). Furthermore, these solutes have
the ability to maintain cell structure through ROS
scavenging (Hasegawa et al., 2000).
High quantities of Na+ and Cl– are lethal to all plant
cells. The ability of plants to keep up a high K+/Na+
proportion in the cytosol is a contributory element of
plant salt resistance. A few genes and transporters that
plants utilize to maintain high K+/Na+ proportion have
been described (Jamil et al., 2011). These include the
following.
1 Na+/H+ antiporters in plasma layers that expel Na+
from the cytosol as a major aspect of the administrative
SOS pathway (Zhu, 2001). Three obviously subtle
(SOS) proteins (SOS 1, 2 and 3) suggest an organiza
tional fraction in the communication of particle trans
porters to maintain small cytoplasmic amounts of Na+
under salt stress (Lu et al., 2014). Zhu (2003) proposed
that a protein kinase complex composed of calcium‐
bound protein (SOS3) and serine/threonin protein
kinase (SOS2) is activated by stress induced in salinity‐
mediated calcium signals. Subsequently, transporter
particles such as Na+/H+ plasma layer and SOS1 anti
porters are phosphorylated by protein kinases, as
shown in Figure 1.1.
2 Vacuolar Na+/H+ antiporters (NHXs) and energy
suppliers of these NHXs (like H+ pumps: HVA/68 and
0002580878.indd 8 10/3/2015 6:20:17 PM
Biotechnological applications to improve salinity stress in wheat 9
Hvp1) (Blumwald et al., 2000; Ligaba & Katsuhara,
2010). NHX proteins sequester Na+ in the vacuoles
and provide an effective component to avoid the
harmful impacts of Na+ in the cytosol and sustain
osmotic equivalence (Glenn et al., 1999). Thus, Cl– is
likely transported into the vacuole by anion trans
porters, for example, CLC proteins (Teakle & Tyerman,
2010; Zifarelli & Pusch, 2010).
3 High‐ and low‐partiality K+ transporters (HKT).
The HKT family comprises two classes which work
either as particular Na+ transporters or Na+ and K+
co‐transporters (Hauser & Horie, 2010; Shabala et al.,
2010). HKT21 was demonstrated to improve Na+
uptake and higher Na+ levels in xylem sap (salt
including conduct) which are associated with pro
longed salt tolerance (Mian et al., 2011a). Numerous
researchers proposed that Na+ avoidance from the
shoot is connected with salt tolerance and that genes
from the HKT1 subfamily, for example, HKT1;4 and
HKT1;5, are included (James et al., 2011; Munns et al.,
2012). Shabala et al. (2010) indicated that both salt
exclusion and deliberation are vital for grain salt toler
ance. In fact, grain is an upright illustration of a harvest
that links halophytic and glycophytic properties, and
accordingly may be an outstanding model to study
both the glycophytic and halophytic components that
might be used to adapt to salt stress (Mian et al., 2011b).
1.4.2 Conventional strategies to enhance salinity stress toleranceVarious tools and techniques have been devised by
many researchers around the world to support plants
in acquiring tolerance against salinity and producing
higher yields. The most common strategy to enhance
induction of salinity tolerance in plants is seed priming
which requires less energy and is quite successful in
minimizing the harmful effects of salinity on plants
(Fercha et al., 2014; Hou et al., 2013). Speed and rate
of germination in equipped seeds under salt stress
have reportedly increased. This pre‐sowing priming
technique with different growth regulators, osmoprotec
tants and halotolerant non‐pathogenic micro‐organisms
Na
Na
ADP
ATP
Vacuole
NaNa
Ca
CaCa
H+
H+
H+
H+H+
SOS1
SOS2
SOS1
AtHKT
AtNHX1
AtHKT
Plasma membrane
SOS3
Figure 1.1 SOS pathway indicating Na+/H+ antiporters in plasma layers that expel Na+ from the cytosol.
0002580878.indd 9 10/3/2015 6:20:17 PM
10 Chapter 1
(Ramadoss et al., 2013) as well as water has also
proved supportive in developing field‐level salinity
tolerance in plants because it establishes an aid to
germination (Qiu et al., 2014). Seeds initially primed
with CaCl2 followed by KCl and NaCl remained effec
tive in minimizing adverse effects of salinity of wheat
plants as they change levels of numerous phytohor
mones. Physiological attributes of plants have also shown
improve ment after priming seeds at both laboratory and
field level. This technique also illustrates the complete
representation of salinity tolerance mechanisms in plants
(Cantliffe, 2003).
Some other strategies utilized to minimize salinity‐
induced effects on plants, especially in wheat, include
removal of excessive salts from soil surface or harvesting
aerial parts containing accumulated salt in areas where
low rainfall occurs, and utilizing practices that aid in
saline soil amelioration (Bacilio et al., 2004).
1.5 Biotechnological applications to improve salinity stress in wheat
Advancements in agricultural biotechnology are
employed to overcome several problems related to
plants, including salinity. The primary goals of agricul
tural biotechnology include minimizing production
cost by derogating inputs like pesticides and nutrient
requirements grown in diverse climates. The main goal
is to enhance plant quality by producing strains which
have better yield with low inputs, can also yield good
crop rotation for conservation of natural resources and
also obtain more nutritious products that are suitable
for long‐term storage or travel and low purchase costs
for consumers.
After years of expensive and intensive studies, today
agricultural biotechnology has made it possible to pro
duce transgenic plants commercially. In 1990, it was
speculated that 40 million hectares of land had been uti
lized in the growth of 20 important species including
corn, rapeseed, soybean and cotton (ISAAA 1999). But
due to widespread criticism and opposition at the
beginning of the 21st century, transgenic wheat was not
accepted to be cultivated and commercialized. However,
in 2009 the NAWG (National Association of Wheat
Growers) conducted a survey which revealed that 76%
of the growers supported the cultivation of transgenic
wheat for large commercial‐scale purposes.
Over the previous decades, diverse methodologies
have been used to enhance salt resistance in plant har
vests (Munns et al., 2006). New salt‐tolerant hybrids,
including tobacco (Hu et al., 2012), rice and wheat, have
been utilized in nations far and wide like Pakistan, India
and the Philippines (Bennett & Khush, 2003). Screening
of a substantial accumulation (~5000 increases) of
common wheat in Australia and 400 Iranian wheat
mixed bags in California for salt tolerance has recog
nized lines that delivered seeds under high salt focus
(half seawater) or provided exceptional profits on saline
soil. So far no new cultivar has been developed from
these tolerant lines (Munns et al., 2006).
Two broad types of approaches can be employed to
overcome the problems caused by salt stress. Soil
affected with salt can be managed and brought under
cultivation by reclamation, or alternatively, modern bio
technological techniques have been utilized to exploit
the germplasm for the development of salinity tolerant
varieties (Farshadfar et al., 2008).
Among conventional germplasms, an Indian cultivar/
landrace named Kharchia 65 was tested against salt stress
and was found to be salt resistant up to a certain level
(Mujeeb‐Kazi et al., 2008), making plant breeders select
this variety over others for cultivation. Numerous
examples/landraces of wheat with the potential for salt
tolerance have been reported from several areas of
Pakistan, Iran and Nepal (Martin et al., 1994). In
addition to these, several other wheat varieties, for in
stance KRL1‐4 and KRL 19 (India), LU26 S and SARC‐1
(Pakistan) and Sakha 8 (Egypt), are also considered as
salt‐tolerant cultivars (Munns et al., 2006).
Numerous genes control the characteristics that may
be included in salt tolerance. These genes are communi
cated diversely throughout the plant lifespan and are
affected by numerous ecological variables (Roy et al.,
2011). Plant agriculturists are searching for more
authentic methodologies with the assistance of molec
ular markers or transgenic methodologies (Arzani, 2008).
The vast majority of the genes that may help salt tol
erance still remain a mystery, even in model plants like
Arabidopsis and rice. Likewise, salt tolerance is a multi
genic attribute; consequently, extensive change focused
around alteration of a single gene is not likely to occur
(Colmer et al., 2005). Distinctive evidence of new attrib
utes aiding in salt tolerance is possible through immediate
traditional selection in stress situations or focused
around mapping investigations of QTL (Holland, 2007).
0002580878.indd 10 10/3/2015 6:20:17 PM
Biotechnological applications to improve salinity stress in wheat 11
At present, association mapping seems an alluring and
effective methodology to distinguish extra genes serving
the regular occurrence of changes for salt tolerance in
varieties, landraces and wild relatives of yields.
Previously the molecular foundation of the characters
that aid in salt tolerance has been determined: marker‐
aided selection (Munns et al., 2012). This might be
utilized to productively exploit the new qualities and
genes or to induce hereditary adjustment which could
produce genetically engineered crops with new genes
exhibiting enhanced levels of salt tolerance.
The relevant selection approach to screen expansive
mapping populations and produce precise data on
attributes is crucial for recognizing the characteristics
and genes for assisting salt tolerance (Ramezani et al.,
2013). This will provide understanding on the vicinity/
extent of the heritable variety for tolerance attributes,
their inheritance and the magnitude of genotype and
environmental co‐operation.
1.5.1 plant phenotypingTo reveal the hereditary premise of complex qualities
like salt tolerance, it is important to assist genotypic
marker data with the relating phenotypic information.
The precise phenotyping is a basis to ascertain and
present new genes for salt tolerance into productive
plants (Munns et al., 2006). Recently, advancement in
DNA marker and sequencing advances has allowed
high‐throughput genotyping of numerous individual
plants with moderately minimal effort. Rapid strategies
to assess huge amounts of genotypes are critical to
completely exploit the immediate improvement of bio
technological systems and to encourage hereditary
analysis of complex qualities.
Traditional selection for execution and yield under
saline field conditions has different impediments identi
fied by natural variables, for example, soil heterogeneity
and climate conditions (Chen et al., 2005; Isla et al.,
1998; Munns et al., 2006). The supportive physiological
characteristics serving salt tolerance and the genes
underlying these qualities could be distinguished more
proficiently under natural conditions (Cuin et al., 2008).
Effective screening systems that were used recently to
assess the response of grains to salinity were plasticized
on hydroponics (Chen et al., 2005; Munns & James,
2003) or on sand as well as soil‐based substrates (Munns
et al., 2002; Tavakkoli et al., 2010b). The shoot Na+ (Cl–)
content and K+/Na+ degree have been recommended as
dependable characteristics for salt tolerance determina
tion in products (Munns & Tester, 2008; Munns et al.,
2002; Tester & Davenport 2003). Hereditary investiga
tions employing traits that influence particle homeostasis
have distinguished QTLs characterized by Na+ and K+
transporters which facilitate salt tolerance in rice
(Bonilla et al., 2002; Ren et al., 2005) and in wheat
(Munns et al., 2012). Comparative studies in barley
have not yet unravelled genes for salt tolerance, despite
the fact that it is the most salt‐tolerant oat crop.
Shabala et al. (2010) and Mian et al. (2011a) demon
strated that both particle exclusion and deliberation
assist grain salt tolerance. Further precise and appro
priate screening strategies may be required that permit
numerous stage estimations of salt stress throughout
the life cycle of barley. Also, the processes ought to
allow examination of the synthesis and collaborative
impacts between distinctive qualities and incorporate
Cl– risk as Cl– is an ‘overlooked adversary’ for salt
tolerance research (Munns & Tester, 2008; Teakle &
Tyerman, 2010).
1.5.2 QtL mappingQTL mapping has been a key apparatus in studying
the genetic structure for engineering of complex
characteristics in plants (Kearsey, 1998). Most agro
nomically significant characteristicss, for example
yield, grain quality and tolerance to biotic and abiotic
stresses, are indistinct (Azadi et al., 2014). Genetic
assembly modelling alludes to various genome areas
with genes that influence the attributes, the magni
tude of the impact, and the relative assurance,
prevailing and epistatic impacts (Holland, 2007). The
discovery of QTLs of agronomical significance and the
underlying genes has significantly extended our under
standing of the intricacy of characteristicss (Salvi &
Tuberosa, 2005).
Advancement in distinguished QTLs that trigger the
characteristics will fundamentally assist breeding through
marker‐aided selection (Collard & Mackill, 2008) and
pyramiding of numerous suitable alleles (Yang et al.,
2012). Biparental (customary) QTL mapping focused
around a single dividing population inferred from two
homozygous parental genotypes has been the basic
methodology for genetic investigation of salt tolerance in
rice (Lee et al., 2006), wheat (Genc et al., 2010) and grain
(Xue et al., 2009). A few loci were found to encode parts
of the HKT group of particle transporters which together
0002580878.indd 11 10/3/2015 6:20:17 PM
12 Chapter 1
enhance salt tolerance like the Kna1 locus in common
wheat (Dubcovsky et al., 1996) as well as Nax1 plus Nax2
in durum wheat (Munns et al., 2012). In the meantime,
biparental QTL mapping has constraints identified with
the reduced investigation of allelic variety exhibited in
the gene pool for each of the loci influencing the qual
ities, absence of isolation for some characteristics, and
poor determination (Rock Garcia, 2003). Biparental QTL
mapping discovers genomic areas connected with char
acteristics with precision successively in normal range
from 10 to 30 centimorgan (cm) (Bernardo, 2008). Such
chromosomal locales could harbour up to a few thou
sand genes (Ingvarsson et al., 2010), thus proving the fact
that effective QTLs which are currently being cloned are
underlined by more than one gene (Mackay & Powell,
2007). Further research is required to determine the
mapping requisite in order to overcome the utilization
of the hereditary variety for salt tolerance in barley
germplasm.
1.5.3 association mappingAssociation mapping, also known as linkage disequi
librium mapping, is a system utilized for mapping QTLs
which interprets outstanding disequilibrium linkage
with phenotype connection (perceptible characters) to
genotypes. Recently, association mapping has been
supported as the technique for distinguishing loci
included in the inheritance of complex characters in
genetics. This technique includes distinctive markers
connected with the phenotypes of interest found
among a group of irrelevant individuals (Pritchard
et al., 2000). Association mapping has recently been
presented in plant genetics (Kloth et al., 2012) and
demonstrated promise to utilize the exact capacity of
novel molecular markers and sequencing progress
(Zhu et al., 2008).
Association mapping depends on the local character
istics related to linkage disequilibria in accrual of
normally different germplasms (Mackay & Powell,
2007). It provides dynamic utilization of all the recom
bination incidents that have occurred throughout the
long evolutionary history of a crop species, distributing
meagre linkage obstructions than those found in bipa
rental QTL mapping studies (Nordborg & Tavare, 2002).
Furthermore, association mapping deals with all actual
allelic variants of QTLs, influencing the attributes of
study when performed with an adequate association
mapping group dialogue to the majority of the crop
gene pool (Figure 1.2).
In association mapping, linkage disequilibrium (LD)
is a central element. LD is a population detail for
non‐arbitrary relationship between alleles of distinc
tive polymorphic loci. The destruction in LD among
adjacent markers determines the marker thickness
and trial outline required to perform association map
ping effectively. Linkage, choice, transformation and
assortment all influence the level of LD. Furthermore,
LD depends upon the mating framework and hence
fluctuates from species to species among populations
(Rostoks et al., 2006).
An association mapping group includes vast land
regions, areas of adjustment with an upright represen
tation of its evolutionary history typically non‐arbitrary
because of familial relatedness and may indicate dis
tinctive sorts of structure (Pritchard et al., 2000). This
may suggest counterfeit marker quality affiliations
(Zhao et al., 2007). In this way it is essential to have
appropriate statistical procedures and methods to eluci
date such complexities (Patterson et al., 2006). The
most prevalent course is to assemble the constituents
affiliated with mapping section and consolidate the
data in measurable models in which markers are being
discovered within the familiar subpopulations (Balding
et al., 2006).
Other important approaches to control population
structure are the utilization of intermingled models
to document contrast in genetic relatedness between
section representatives (kinship matrix) (Malosetti
et al., 2007). While evaluating, population structure is
an important computational demand; Patterson et al.
(2006) presented a methodology utilizing principal
component analysis (PCA) to manage the issue of coun
terfeit associations. This is quick and simple and works
well with extensive information sets.
1.5.4 proteomic approachProteomics is the most advanced approach to categori
zation of diverse proteins that are included in unique
and/or distorted structures (Maleki et al., 2014).
Particular genes or families of genes might regulate a
few protein types to control specific characteristics.
Advancement in proteomics has been used to focus
information on instruments that manage complex inherent
characteristics (Capriotti et al., 2014). Peng et al. (2009)
0002580878.indd 12 10/3/2015 6:20:17 PM
Biotechnological applications to improve salinity stress in wheat 13
assessed that cultivars of wheat Shanrong 3 and progenitor
cultivar Jinan 177 are utilized in two‐dimensional gel
electrophoresis and mass spectroscopy for protein pro
filing. Consequences revealed that 6 and 34 protein
conserved differentially in leaves (Maleki et al., 2014)
and roots, respectively.
A few uniquely conserved proteins could further be
described in terms of their capacities in metabolomics or
other indicators for transduction cross‐communication
in salt tolerance systems in plants. This was also the
case with seedlings of wheat when treated with sali
cylic acid (0.5 mM) and sodium chloride NaCl (250 mM)
for 3 days. In both salicylic acid and salt analysis, 39
proteins are demonstrated by 2d PAGE and MALDI‐
TOF/TOF‐MS is used to control 38 protein (Kang
et al., 2012). The research proposed that communi
cated proteins are being included in diverse cells
along with metabolic methodologies, for example
metabolism, stress safeguard, signal transduction and
photosynthesis.
Selection of germplasm with genetic diversity
Phenotypic measurementin multiple replication
trials in differentconditions
Genotypingwith molecular markers
SNPs, SSRs,and AFLPs
Quanti�cationof LDusing
molecular markersdata
Measurement ofcharacteristics
ofpopulation
Marker traitcorrelation with suitable method
Marker tags are identi�ed that areassociated with trait of interest
Cloning and annotation of tag loci forpotential biological functions
Figure 1.2 Schematic representation of association mapping.
0002580878.indd 13 10/3/2015 6:20:17 PM
14 Chapter 1
1.5.5 Salt tolerance‐related genesEnhanced salt tolerance is controlled by genes (Table 1.1)
that can be classified into three categories (Munns, 2005)
as discussed below.
1.5.5.1 Genes for salt uptake and transportA considerable amount of research is dedicated globally
to investigating the processes and understanding the
interactions occurring among the genome and pro
teome within a plant cell during salt stress (Hirayama &
Shinozaki, 2010). One of these processes is the primary
phase of gene expression, transcription, which is a vital
process because the production of a vigorous transcrip
tome is a promising feature for better protein yield and
stabilized cellular activities (Chew & Halliday, 2011;
Christov et al., 2014). Salt stress influences the genome
and gene expression patterns, thus resulting in a diverse
proteome (Nakashima et al., 2009).
Researchers from the Commonwealth Scientific and
Industrial Research Organization (CSIRO) isolated two
salt‐tolerant wheat genes, Nax1 and Nax2, evolved
from an old relative of wheat, Triticum monococcum. Both
genes were responsible for limiting toxic sodium passage
from root to shoot, thus causing inhibition of toxic
sodium. In one study evaluating the Nax2 gene in field
trials, a variety with the Nax2 gene showed a 25%
greater yield than without Nax2 under saline environ
ment (James et al., 2006).
Proteins implanted within the membrane lipid bilayer
are involved in controlling Na+ ion uptake from soil and
in transport across the whole plant body. The Na+ ion
may be regulated by the K+ ion transporter and chan
nels directly since they are not completely selective for
K+ ions as it buffers the cell for uptake of Na+ ions by
rigorous K+ homeostasis (Munns, 2005). Ion selective
channels are involved in transportation of ions passively
under electrochemical gradient. Non‐selective channels
allow transport of Na+/K+ (Demidchik et al., 2002).
Active particle transport occurs through symporters and
antiporters. Transport occurs under conditions of elec
trochemical potential distinction of a conjugated solute,
generally H+. Numerous genes significantly maintain
Na+ or K+ homeostasis in higher plants while possibly
being promising for hereditary controls (Munns, 2005).
Different genes like AKTI, AKT2 and KATI encode for
K+ ion channels such as AKTI, AKT2, KATI (which encode
for shaker‐type inward channels having a single pore),
KCOI (two‐pore channel of KCO family): K+ antiporters
like KEA, SKOR (shaker‐type outward channel), CPA
(KVlf antiporter). K+ transporters like HAKI IO, KUPI
4 and SOS I, HKTI; Na+ antiporters like NHXI‐5 and
proton pumps like AVPI code for H+‐ATPase. AIIA2 plus
H+‐PPase (Mäser et al., 2002) can be employed to
enhance salt tolerance.
1.5.5.2 Genes for osmotic functionSolutes that show a protective or osmotic effect are clas
sified into four categories: (1) N‐containing compounds
like glycine betaine and proline; (2) sugars including
sucrose and raffinose; (3) straight‐chain polyhydric
alcohols like mannitol and sorbitol; and (4) cyclic poly
hydric alcohols such as myoinositol, pinitol and ononitol.
Numerous genes have been recognized that code for
such osmotically important compounds, such as P5CS
gene coding for proline (Hong et al., 2000), mtID for
mannitol, otsA and S6PDH for sorbitol (Gao et al., 2001),
codA for glycine betaine, ots8 for tetrahalose (Gong
et al., 2002) and imtl for myoinositol. These genes are
Table 1.1 Genes for salinity tolerance in wheat.
Serial no. Gene category Genes References
1 Genes encoding
for K+ channels
AKTI, AKT2, KATI (encoding for shaker‐type single‐pore inward channels), SKOR
(shaker‐type outward channel), KCOI (two‐pore channel of KCO family), K+ antiporters
like KEA, CPA (KVlf antiporter), K+ transporters like HAKI‐IO, KUPI‐4 and HKTI
Mäser et al. 2002
2 Na+ antiporters NHXI‐5 and SOS I and proton pumps like AIIA2 and AVPI encoding for H+‐ATPase and
H+‐PPase, genes for Na+ exclusion, named Nax1 and Nax2
Mäser et al. 2002
Munns and James
2003
3 HKT gene family HKT7 (HKT1;4), HKT8 (HKT1;5), TmHKT7 (TmHKT1;4‐A2), TmHKT8 (TmHKT1;5‐A)
(origin in T. monococcum)
Huang et al. 2006
Byrt et al. 2007
4 HKT gene family TaHKT8 (TaHKT1;5‐D), AtHKT1;1 (origin in T. aestivum) Byrt et al. 2007
0002580878.indd 14 10/3/2015 6:20:18 PM
Biotechnological applications to improve salinity stress in wheat 15
mainly present in model cash crops as well as other
plants required to be transformed followed by field trials
to produce and commercialize easy, cost‐effective and
high‐yield varieties (Yamaguchi & Blumwald, 2005).
1.5.5.3 Genes for cell growthCertain genes have also been identified that play vital
roles in plant growth such as development of new roots
or leaves, or may also be involved in life‐sustaining
biochemical pathways like photosynthesis. Once such
genes are well characterized and transformed efficiently,
this can lead to mediating cell division, growth rate
and other measurements under varied environmental
conditions. Similarly rate of photosynthesis, stomatal
closure and opening or measuring mesophyll cells, sig
nalling pathways (Choi et al., 2014; Schmidt et al., 2014)
and co‐workers like hormones, a variety of proteins and
enzymes (Kahrizi et al., 2012) like kinases and phospha
tases are easily studied and regulated via detailed
assessment of such genes (Zhang et al., 2004). It is pro
posed that such genes can simultaneously also be
instructive in water stress (Chaves et al., 2003). Some
other factors like CBFs (C‐repeat binding factors) and
ABFs (ABRE binding factors) have also proved enhance
ment in tolerance against varied abiotic stresses (Oztur
et al., 2002).
1.5.5.4 Genes for reducing Na+ accumulationCereal crops like durum wheat (Cuin et al., 2010; Munns &
James, 2003), rice (Haq et al., 2010), pearl millet
(Krishnamurthy et al., 2007), Medicago sativa (Castroluna
et al., 2014) and grain (Shavrukov et al., 2010) all pos
sess the pattern of salinity tolerance with exclusion of
Na+ from plant leaves. On the other hand, wheat wild
progenitors like Triticum tauschii (Schachtman et al.,
1991), durum (pasta) wheat (Triticum turgidum), tall
wheatgrass (Colmer et al., 2006) and hordeum species
are more susceptible to salinity stress as compared to
Triticum aestivum (bread wheat) due to its low Na+
exclusion potential (Munns & James, 2003).
In order to obtain salinity‐tolerant durum wheat,
durum or durum‐associated wheat, genotypes were
gathered across the world. Initial studies revealed ’Line
149 genotype’ among gathered genotypes which sur
prisingly possessed the property to avoid Na+. Following
detailed investigations, it was found that two genes,
Nax1 and Nax2 from Line 149, are both responsible for
Na+ exclusion (James et al., 2011). QTL analysis revealed
that Nax1 is located on chromosome 2A, distinguished
by mapping as Na+ ion transporter of HKT7 (HKT1:4)
HKT gene family (Huang et al., 2006). Nax2 is located on
chromosome 5A with HKT8 (HKT1:5) (Byrt et al., 2007).
Currently wheat does not exhibit Nax genes but both
Nax1 and Nax2 genes were coincidentally shifted into
Line 149 after crossing Triticum monococcum (C68 101)
with durum wheat with the intention to exchange rust
resistance genes (James et al., 2006). Following that,
these genes were named as TnHKT8 (TmHKT1: 5‐A)
and TmHKT7 (TmHKT1: 4‐A2) in Triticum monococcum,
respectively.
These genes in durum wheat play their role in
exclusion of Na+ from xylem so that leaves may receive
lesser amounts of Na+ ions (James et al., 2006). More
specifically, Nax1 ejects Na+ ions from roots, lower parts
of leaves and xylem, while Nax2 plays the same role in
root xylems. Nax2 bears a phenotype for expulsion of
Na+ and enhances K+/Na+ selection in Triticum aestivum
(bread wheat) while Nax1 has a phenotype of high
sheath‐blade proportion of Na+ ion concentration
(Dvořak et al., 2004). It was demonstrated that, in
Triticum aestivum, Nax2 is homologous to Kna1, espe
cially Tahkt8 (Tahkt1:5‐D) (Byrt et al., 2007). The HKT
gene family also encodes transporters in plasma mem
brane which mediate Na+ or K+ uptake from apoplast
(Hauser & Horie, 2010). These are crucial for cell
homeostasis in terms of Na+ and K+, and if carried to
stele, more specifically the parenchymatous lining in
xylem, they recover Na+ ions from the transpiration
path and thus protect leaves from Na+ ions (Hauser &
Horie, 2010; Munns & Tester, 2008). Transportation of
Na+ to leaves is decreased if stele‐specific supporting
genes are upregulated, thus increasing salt tolerance to
Arabidopsis (Moller et al., 2009). These Nax genes are
associated with Triticum monococcum, a diploid wheat
progenitor, while they have vanished from advanced
wheat cultivars (Huang et al., 2008).
These genes are brought into bread wheat by the
usual combination of tetraploid wheat, i.e. durum being
crossed with hexaploid wheat and resultant F1 genera
tion, i.e. pentaploid was again backcrossed to bread
wheat. Offspring of hexaploid wheat that contain one or
both NaX genes were selected as four Australian culti
vars of wheat. Genes near Nax genes were examined for
their ability to promote Na+ exclusion and division of
Na+ ions between the sharpened and sheath stele as well
as their photosynthetic implementation in 150 mM
0002580878.indd 15 10/3/2015 6:20:18 PM
16 Chapter 1
NaCl. Saline soils were frequently wet throughout the
time in advance of planned vegetative development
(Colmer et al., 2005).
1.5.6 Molecular markersMolecular markers are principally related to evaluating
polymorphisms in DNA arrangements (i.e. base pair
cancellations, substitutions, augmentations or patterns).
Molecular markers are amongst the most effective
machinery for the assessment of genomes and allow the
relationship of heritable qualities with underlying
genomic diversity to be determined (Table 1.2).
The most widely used DNA marker systems for
assessment of genetic diversity in wheat are SNPs (single
nucleotide polymorphisms), SSRs (simple sequence
repeats), ISSRs (inter‐simple sequence repeats), AFLP
(amplified fragment length polymorphism), RFLP (restric
tion fragment length polymorphism), RAPD (random
amplified polymorphic DNA), ESTs (expressed sequence
tags) and microarray technology. All molecular marker
methods can be utilized for diverse applications inclu
ding germplasm characterization, hereditary diagnostics,
characterization of transformants, investigation of
genome association, marker‐assisted selection (MAS)
and phylogenic dissection (Mishra et al., 2014).
1.5.6.1 Single nucleotide polymorphismsSingle nucleotide polymorphisms (SNPs) are a single
base‐change or small insertions or terminations in
homologous sections of DNA. In human genome
sequencing, 10–30 million SNPs were discovered and
were the greatest source of polymorphisms (Collins
et al., 1998), present both in coding and non‐coding
locales (Aerts et al., 2002). As markers, SNPs are
favoured over other marker frameworks owing to their
more continuous, co‐dominant nature and occasional
connection with morphological progressions (Lindblad‐
Toh et al., 2000). Genomes of higher plants such as
barley (Kanazin et al., 2002), soybean (Choi et al., 2007),
maize (Tenaillon et al., 2001), sunflower (Lai et al.,
2005), sugar beet (Schneider et al., 2001), rye (Varshney
et al., 2007) and cotton (Ahmad et al., 2007; Lu et al.,
2005; Shaheen et al., 2006) have furthermore been
studied for SNP revelation and characterization. Since
SNPs are exceptionally polymorphic, each gene must
contain few SNPs even among strains (Cho et al., 1999).
Table 1.2 Advantages and disadvantages of some commonly used DNA markers.
Serial no. Molecular marker Advantages Disadvantages References
1 RFLP Robust
Reliable
Transferable across population
Time consuming
Laborious
Expensive
Large amount of DNA required
Limited polymorphism
Beckman and Soller
1986
Kockert 1991
2 RAPD Quick and simple
Inexpensive
Multiple loci from single primer
possible
Small amount of DNA required
Problems with reproducibility
Generally non‐transferable
Penner 1996
Williams et al. 1990
3 SSR Technically simple
Robust and reliable
Transferable between populations
Large amount of time and labour required
for production of primers
Usually require PAGE
McCouch et al. 1997
Rodcr et al. 1995
4 AFLP Multiple loci
High level of polymorphism generated
Large amount of DNA required
Complicated methodology
Vos et al. 1995
5 SNP High abundance
Cross‐study comparisons are easy
Low mutation rate
Easy to type
Expensive to isolate
Low information content of the single SNP
Substantial rate of heterogeneity among
sites
Christian 2004
AFLP, amplified fragment length polymorphism; RAPD, random amplified polymorphic DNA; RFLP, restriction fragment length polymorphism;
SNP, single nucleotide polymorphism; SSR, simple sequence repeat.
0002580878.indd 16 10/3/2015 6:20:18 PM
Biotechnological applications to improve salinity stress in wheat 17
The MT‐shsp gene is a vital gene which serves to protect
against high temperature shock. MT‐shsp ensures
NADH; ubiquinone oxidoreductase of the electron
transport chain throughout temperature stress in plants
(Herman et al., 1994). SNP markers, joined with QTL
information for phenotypic character, can provide
another arrangement of breeding, i.e. gene‐interceded
background rather than marker‐aided determination
(Lange & Whittacker, 2001). Genetic advances in crops
and farming profits will be improved by the accessibility
of rapidly developing genetic assets and tools, including
high‐density genetic maps (Lacape et al., 2005).
Polyploid genomes are more useful to investigate for
SNPs than diploids. The proportion of SNP alleles shifts
up in polyploid genomes (Adams et al., 2003).
1.5.6.2 Simple sequence repeatsSimple sequence repeats (SSRs), also called microsatel
lites, are 1–6 nucleotide repeats of DNA which provide
characterization of genetic contents for marker‐assisted
selection to detect deletions, insertions, duplication of
genes and allelic variations (Sehgal et al., 2012). SSRs
are more significant compared to other molecular
markers used for detection of polymorphic levels
because SSRs exhibit more potential co‐dominance in
inheritance. Simple detection of SSRs shows another
advantageous attribute. In wheat, many studies have
been conducted using SSRs both for wheat germplasm
improvement (Borner et al., 2000; Huang et al., 2002)
and the assessment of genetic diversity (Hammer et al.,
2000). One such research report that determined the
level of genetic diversity among 12 wheat cultivars with
the aid of four SSRs was conducted by Islam & Begum
(2012). They reported 10 alleles while per locus aver
aged 2.5 with a range from 2 to 4. Among these wheat
cultivars grouped in clusters, ‘Gourab’ and ‘Akbar’ plus
‘Gourab’ and ‘BAW‐1064’, the value of genetic distance
obtained was 0.4697 while the groups ‘Balaka’ and
‘Aghrani’ plus ‘Triticale’ and ‘BAW‐1036’ were the
lowest. The genetic diversity, allelic number, size and
type of repeats were all positively correlated, suggesting
that the maximum number of SSRs can be applied to
find more characterized genetic diversity (Islam &
Begum, 2012). Similar studies have also suggested the
same persuasive explanation and numerous SSR
markers have been applied (Song et al., 2005).
These SSR‐mediated genomic analyses show a prom
ising future to combat natural problems like abiotic
stress, more specifically salt stress. In polymorphism
identification, the allelic variations can also be detected
which during salt stress can lead us to detect the effects
of salt stress on genetic level based on polymorphism.
Salinity‐induced genetic alterations and identification
of salt‐tolerant genes are the likely outcomes for this
technique which can be further utilized in genetic engi
neering to develop salt‐tolerant varieties.
1.5.6.3 Inter‐simple sequence repeatsInter‐simple sequence repeats (ISSRs) are a technique that
joins the SSR/microsatellite primers coupled with poly
merase chain reaction which yields multi‐locus markers
which contain advantages of SSRs and AFLP as well as
RAPD (Reddy et al., 2002). Compared to others, ISSR
markers are more polymorphic and are used in phylogenic
analysis, genetic diversity, genome mapping, gene tagging
as well as evolutionary biology. Like other molecular
markers, ISSRs have been utilized in evaluating genetic
diversity and gene mapping as well as crop breeding
studies (Gupta & Varshney, 2000; Staub et al., 1996).
ISSRs have minimized major limitations in various
plant improvement studies in terms of cost, reproduc
ibility and expertise required as compared to RAPD and
AFLP (Godwin et al., 1997). They are applied in areas of
phylogenetic analysis, gene/genome mapping, gene tag
ging and related evolutionary studies. ISSR lies within
the SSR or microsatellite repeats, variable number of
tandem repeats (VNTRs) or short tandem repeats (STRs)
(Tautz & Renz, 1984).
Although wheat possesses a huge genome, and usu
ally such huge genomes pose problems in genome‐based
studies, yet these ISSRs could be a promising solution
for salt resistance studies in wheat. ISSRs serve the same
function as SSRs but with its advantages like gene tag
ging, it provides a more developed tool for further
analysis. For instance, in order to obtain insight into a
particular gene under salinity stress, ISSR is a good tool
to use. Once the gene is mapped, followed by tagging
technique to analyse that gene during salt stress, it will
be more convenient to figure out and procure salt‐
tolerant genes or related genes that influence cellular
mechanisms during such harsh conditions.
1.5.6.4 Amplified fragment length polymorphismAmplified fragment length polymorphism (AFLP) serves
as a precise tool for identification and estimation of
genetic diversity in plant germplasms (Martos et al., 2005).
0002580878.indd 17 10/3/2015 6:20:18 PM
18 Chapter 1
Widely studied genetic diversity of common crops, sig
nificantly wheat, reveals that with the passage of time,
practices like domestication and other breeding tech
niques have caused a reduction in genetic composition
of germplasm, thus neglecting genome‐wide recombi
nation (Donini et al., 2000). In a study by Martos et al.
(2005), phylogenetic relationship was investigated in
24 wheat (durum) cultivars of Italy and Spain. From the
AFLP‐based results, old Spanish cultivars were found to
have high level of genetic similarity with collected
Italian varieties. Furthermore, genetic variation was
also found to be constant from last century among
overall breeding practices. Another similar study used
seasonal variations in winter and spring wheat cultivars
(Tyrka, 2002). This AFLP analysis was accomplished
using enzyme PstI and capitulated in 111 fragments.
Results demonstrated higher variation in spring‐grown
wheat cultivars as compared to winter wheat.
In addition to identification and phylogeny, polymor
phism among wheat genomes can be calculated based
on landraces, adaptation and environmental conditions
(Bozzini et al., 1998) as well as physiological and mor
phological characterization. The genetic potential and
genetic erosion analysis can be achieved through molec
ular characterization which constricted the genetic
background among the cultivars (Gupta et al., 1999).
AFLP thus supports genetic‐level investigations for
plants in stressed conditions. Genetic responses of plants
to stress (Makhloufi et al., 2014), specifically wheat in
salinity, can be better understood with the advantage of
gene mapping and its activities via PCR‐based high‐
throughput AFLP markers (Manifesto et al., 2001).
1.5.6.5 Restriction fragment length polymorphismRestriction fragment length polymorphism (RFLP) is a
powerful tool for developing a genetic map, commonly
in plants and animals. Genetic maps of many crops
including rice, maize, barley, rye and tomato have been
developed (Bonierbale et al. 1988; Graner et al. 1991;
Helentjaris et al. 1986; McCouch et al. 1988; Wang et al.,
1991). RFLP analysis yields genetic maps based on dis
tance between two markers but it was observed that
distances in genetic maps are different from physical
maps. This is because two genes, for instance, located
closely on a genetic map may be far apart on a physical
map or vice versa. Such problems are encountered using
more advanced techniques with RFLP such as in situ
hybridization (Werner et al., 1992).
A study by Shah et al. (2000) was conducted to check
polymorphism in two wheat varieties, ‘Cheyenne’ and
‘Wichita’, based on their chromosome 3A. For this
purpose, the authors used 52 RFLP, 40 RAPD, 77 STS
and 10 SSR probes upon group 3 homologous
chromosomes of wheat. Detected polymorphisms
included 3.9% STS, 60% SSR and 20% RAPD while the
highest polymorphism was 78.8% detected via RFLP.
This suggested that RFLP is a more suitable tool in poly
morphism studies. These observations indicate that
RFLP can also be informative and a promising tool in
stress assays.
1.5.6.6 Random amplified polymorphic DNARandom amplified polymorphic DNA (RAPD) uses arbi
trary primers to determine genetic diversity. Various
scientific communities have investigated genetic diversity
of numerous wheat cultivars. Bibi et al. (2009) studied 12
hybrid wheat genotypes for their genetic diversity using
RAPD markers. In their investigations, 14 primers were
used to amplify 102 loci and the authors observed that
89.2% (91 loci) showed polymorphism while the remain
ing 10.8% (11 loci) were monomorphic; the size of
fragments ranged between 142 bp and 5.3 kb.
The major difference between RAPD and RFLP is the
use of an arbitrary‐sequence primer in RAPD. However,
both RAPD and RFLP are promising in studying effects
of stress conditions upon plants. Wheat grown under
salinity stress can be assessed using RFLP and RAPD that
can give a better insight into genetic variations and
linkage analysis and thus can be very helpful in allevi
ating salt stress in wheat.
1.5.6.7 Expressed sequence tagsExpressed sequence tags (ESTs) are small fragments of
cDNA which help in tagging those genes from which
mRNA was produced. Sometimes, an unknown EST is
sequenced which results in a 200–700 base pair long
sequence and is used to search for similar genes among
proteomic and genomic databases (Adams et al., 1991).
ESTs can be employed to search for gene‐specific
sequence or motifs or, simply, similar gene sequences in
various organisms. However, EST‐based analysis of
wheat has been limited because only nine ESTs were
available for wheat in May 2000, although this has
increased a great deal recently (Lazo et al., 2004).
In an experiment conducted by Lazo et al. (2004), a
large‐scale map of hexaploid wheat (Triticum aestivum L.)
0002580878.indd 18 10/3/2015 6:20:18 PM
Biotechnological applications to improve salinity stress in wheat 19
genome was exploited based on deletion bin. The EST‐
based chromosome bin map revealed seven homologous
chromosome groups from wheat. In their overall contri
bution, EST sequencing, processing and nomenclature
as well as assembly plus unassembled ESTs were used
for selecting various motifs in genes. They further sub
mitted data from southern hybridization into databases
which has proved supportive in wheat genome‐related
researches. The same findings can also be utilized in
gene identification and finding similarities among wheat
cultivars such as genes for salt tolerance and many other
abiotic as well as biotic stresses.
1.5.6.8 Microarray technologyRecent developments in biological sciences for solving
various problems have focused on more advanced tools
and technologies. One such is microarray technology
that reveals the dynamics of gene expression. Using this
technology, various attributes or vital agricultural traits
in plants can be controlled. However, the judicious use
of this technology can lead to better understanding of
genome (Coram et al., 2008). In wheat under salt stress,
such technology can be more helpful as the expression
pattern of genes under salinity stress can be studied in
detail. The expression patterns during salt stress as well
as other phenomena like co‐expression and related
networks leading to transcriptome plus proteome may
also help in discovering genes that can ensure salinity
tolerance in wheat. Due to its potential, microarray
technology is the most outstanding option for finding
biological solutions to biotic as well as abiotic stress
in plants.
1.6 Conclusion and future perspectives
Environmental stresses threaten plant growth,
development and yield and these threats to wheat are a
major concern because wheat is a daily staple of the
human diet worldwide. With growth in population,
increase in wheat production is mandatory to meet
world requirements but abiotic and biotic stresses con
fine its yield. However, among all stresses, salinity is
most alarming because it is widespread in both arable
and non‐arable lands. The consequences of excessive
salts have been observed to have a disastrous effect
upon wheat. Global scientific communities have been
investigating various aspects to try to resolve this issue.
These range from conventional breeding practices to
modern biotechnological techniques based on molec
ular studies. Conventional breeding practices such as
hybridization and application of supportive chemicals
are helpful to some extent but are limited due to lack of
detailed integrated knowledge about the molecular‐
level processes occurring in cells under such harsh
environmental circumstances.
Hence, it is crucial to understand the cellular mecha
nisms that occur in wheat under saline conditions in the
hope of finding an integrated solution to combat such
problems. Biotechnology holds promise for both aspects:
(1) with the latest tools and techniques, detailed studies
can be carried out to understand the processes within
the cell in various conditions; and (2) the integrated
potential of biotechnology can be employed to combat
such problems by altering the genetic contents or the
expression patterns or by optimizing the biochemical as
well as molecular pathways in many ways.
Despite these advantages, however, some limitations
exist in biotechnological applications although the
future looks promising. Most biotechnological strat
egies utilized for salt tolerance are based on avoidance
mechanisms. But, in accordance with targets, future
endeavours have the potential of optimizing or directing
plants in such a way that a plant will be able to posi
tively utilize available salts with the advantage of
reverting sodic or saline soil to normal soil. Thus, bio
technological applications applied in wheat may be the
solutions to improve salinity stress.
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