Development of seed and pseedling

Seeds, Seed germination and development,

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

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

Embryo development and maturation in flowering plantflowering plant

Body plan within female gametophyte

Cell enlargement Desiccate, toxin accumulation

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

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

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

Events associated with seed germination and seedling growth

GA and reserve food mobilizationGA synthesised by embryo transported to aleurone layer where hydrolytic enzymes are produced that break reserve foodthat break reserve food

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

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

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.

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.

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

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

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