2. Development of seed and seedlingSeed germination • Period of quiescence: stage between embryo...
Transcript of 2. Development of seed and seedlingSeed germination • Period of quiescence: stage between embryo...
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Development of seed and pseedling
Seeds, Seed germination and development,
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SeedsSeeds
• Seeds for propagation and for survivalSeeds for propagation and for survival under adverse environmental condition
• Seed coat presence of phenolics lectins• Seed coat, presence of phenolics, lectins, toxic glycosides, enzyme inhibitor discourage bacteria fungi and predatorsdiscourage bacteria, fungi and predators
• With every thing needed for germination d i t b t dliseeds germinate- embryo grow to seedling
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Stages in seed development• Early embryogenesis: basic embryo plan within female
gametophyte- root shoot axis, cotyledon, demarcation of primary tissuesprimary tissues
• Midembryogenesis: Cell enlargement and accumulation of food material. Albuminous and exalbuminous seeds. Endosperm continue to grow also by cell divisionp g y
• Late embryogenesis: lose water, desiccate and mature. Seed covering hardens, accumulate phenolics and toxins against pathogens, vascular supply with mother break and g p g , pp ydesiccate (10 to 15%)-metabolic process highly reduced. Dehydration protection mechanism
• Seed quescence and Seed dormancy: Resting period. y gseed grow under favourable condition of water, oxygen and temperature. Some seed become dormant due to some physiological activity and do not germinate.Ph t h l i l l i th h h f• Phytohormones play crucial role in the phase change of seed development
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Embryo development and maturation in flowering plantflowering plant
Body plan within female gametophyte
Cell enlargement Desiccate, toxin accumulation
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Dormancy• High level of genetic expression during
earlier stages of embryogenesis• As the embryo reaches maturity, thereAs the embryo reaches maturity, there
is a shift from embryo development to food storage. High levels of specific storage protein mRNAs are present atstorage protein mRNAs are present at different times in embryonic development.
• Connection with mother tissue breaks and desiccates
• Integument to seed coat and• Integument to seed coat and embryogenesis ends seed enter dormancy –for weeks or years Survival valuevalue
• Viviparous mutants with blocked dormancy-embryo continue to grow
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Dormany and hormone• Role of hormone: ABA maintain dormancy and GA break
dormancy• Germination only after breaking of dormancy• Some dormant embryo show after ripening- low metabolic
activities leads to embryo maturation prepare foractivities leads to embryo maturation prepare for germination
• Mature seeds Germinates by interacting with environment-needs favourable environmentneeds favourable environment.
• Temperate seeds need startification- ensure germination only after hard winter months.
• Some germinate in light to ensure photosynthesis before reserve food ends
• Scarification in seeds with hard seed coat-frugivoresScarification in seeds with hard seed coat frugivores naturally help
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Seed germination• Period of quiescence: stage between embryo
development and embryo growth• Germination: Events starting from hydration of seed and
ending after emergence of embryonic axis generally radicle f d tfrom seed coatImbibition of water: metabolism begins, membrane leakyMetabolic activation of imbibed seeds: Membrane restore liquid crystalline structure and semipermeable character- restore functional organelles, Protein synthesis begins, rise in respn
A i b i i l h i iActivate embryo to receive signal: hormone activation and new synthesis Radicle growth and penetration of testa:
• Seed germination may be modulated by relative ratios of ABA and GA
• Scotomrphogenesis when germinate in dark g g• Sporophytic body plan develops from merestem after
germination
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Events associated with seed germination and seedling growth
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GA and reserve food mobilizationGA synthesised by embryo transported to aleurone layer where hydrolytic enzymes are produced that break reserve foodthat break reserve food
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SeedlingS dli i th t iti t t b t b i l t• Seedling is the transition state between embryonic plant development and post-embryonic development of mature plantB bili ti f f d b li t t b li• By mobilization of reserve food: anabolic to catabolic (quiescence in between), may remain dormant
• Seedling respond to environment: light, gravity and waterE b i i i h l l i l• Embryogenesis to germination: hormonal control mainly GA (promote) and ABA (inhibit). Gene for the synthesis of these hormones or signal transduction pathway affect the transitiontransition.
• aba mutant: with low ABA level show viviparous or precocious germinationabi m tant insensiti e to normal le el of ABA and era• abi mutant: insensitive to normal level of ABA and eramutant: inhanced response to ABA show opposite phenotype (both control signal transduction pathway)
• GA synthesis or GA signalling mutants showed delayed• GA synthesis or GA signalling mutants showed delayed germination
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Gene involved in transition of embryonic to seedling development (A thaliana)seedling development (A. thaliana)Process Responsible genes
ABA synthesis ABA class: ABA1, ABA2, ABA3, ABA4, ABA5
ABA (perception or response mutant)
ABI class (insensitive) : ABI1, ABI2ERA class (enhanced response)ERA1
GA signalling GA class: GA1, GA2, GA3
Dormancy RDO class (reduced dormancy): RDO1Dormancy RDO class (reduced dormancy): RDO1, RDO2
Specification of post- LEC class (leafy cotyledon): LEC1, p pgermination character
( y y ) ,LEC2, LEC3
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Gene affecting germination other than h th i d tihormone synthesis and perception
• Affect embryo specific or seedling specific gene expression
• Mutation cause trichome and anthocynine in cotyledon (l f t l d LEC l )(leafy cotyledon, LEC class)
• LEC1 and FUSCA3 mutant: show viviparous and leaf like cotyledon, reduction in storage protein
• ABA1, LEC1 and FUS3 genes encode TF during embryogenesis- upregulate gene for embryogenesis and down regulate gene for post embryonic development.b 3 l 1 d f 3 i i d i• aba3, lec1 and fus3 mutant: genes inactive during
embryonic development but expressed during seedling development are de-repressed. O i f LEC1 i i ti l t i• Overexpression of LEC1 in germinating plant give embryo like plant.
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Light dependent morphogenesismorphogenesis• Skotomorphogeneis: dark
controlled etiolated seedlingg• Photomorphogenesis: light
controlled chlorophyll synthesis and leaf developmentand leaf development
• Light perception mutants of Arabidopsis p(skotomorphogenesis in light and photomorphogenesis in dark) helped to find the genetic basis ofhelped to find the genetic basis of seedling development.
• Light insensitive continuously etiolated seedling called long hypocotyle (hy) mutant.
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Light perception mutantA bid i ith di ti t t t h t• Arabidopsis with distinct mutant phenotype
• Skotomorphogenesis in light- etiolated-continuously or long hypocotyle (hy mutant constitutive etiolated)
• Photoremorphogenesis in dark: -DET (deetiolated) or constitutive photomorphogenesis COP mutant. DET and COP genes are identical to FUSCA genesPh d f i di• Photoreceptor mutants: defect in encoding prhotoreceptors responding specific wave length of light
• Double mutant analysis: HY5 Gene works down stream f h t t Th h t t ff t th i lof photoreceptor, The photoreceptors affect the signal
transduction pathway through HY5 gene product, a bZIPtranscription factor.St d of det/cop/f s m tants combined ith h 5 sho ed• Study of det/cop/fus mutants combined with hy5 showed that DET/COP/FUS function down stream of HY5
• Photomorphogenesis is default pathway actively inhibited in dark by various DET/COP/FUS genesinhibited in dark by various DET/COP/FUS genes.
• det or cop: mutant with photomorphogenesis in dark
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Light activate HY5 Transcription factor and initiate light dependent g pdevelopmental pathways
• COP1 repressor of HY5 are found in cytoplasm in light p y p gbut in nucleus in dark and interact with HY5. In light COP1 transported to cytoplasm releasing HY5-activate light activated genes
• Exogenous application of brassinosteroid in det/cop/fus mutants show etioleted phenotype also in dark
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ReferencesReferences
• R M Twyman: Developmental BiologyR.M. Twyman: Developmental Biology• Lalit M Shrivastav: Plant growth and
developmentdevelopment• Bhattarai, Tribikram: Plant Physiology• Gilbert , Scott F: Developmental biology