dark development photosynthesis nutrient uptake respiration phytohormones enzymes gene regulation...
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Darkdevelopment
Photosynthesis
Nutrient uptake
Respiration
PhytohormonesEnzymes
Gene regulation
Long distance transport
Membranes
Cells and cell walls
Photoreceptors
Phytochrome
Water and transpiration
Secondary metabolism
Biotic and abiotic stress
Flowering
Fertilization and embryogenesis
Seed and fruit development
Dormancy and senescence
Germination
Seed GerminationHORT 301 – Plant Physiology
August 31, 2009Finkelstein et al. (2008) Annu Rev Plant Biol 59:387-415
Finch-Savage and Leubner-Merzger (2006) New Phytol 171:521-523Hartmann & Kester et al. (2002) Plant Propagation, pp. 199-220
Plant Life Cycle
Seed development and dormancy – embryogenesis, embryo maturation and acquisition of dormancy
Seed dormancy release and germination – mechanisms and processesDormancy is an adaptation to promote germination when environmental conditions are favorable for plant development
Hartmann and Kester et al. Plant Propagation 2002
Seed Development and Dormancy
Stylized mature angiosperm flower (lily) stamen (pollen) and ovary(ovule) development
Pollination and FertilizationPollen associates with stigma and germinates, interaction between pollen and stigmatic surfacesTube moves through style (chemotropic response), two generative cells deposited into ovule (micropylar end)One nucleus fertilizes egg (becomes the zygote) and other fuses with polar nuclei (endosperm)
Hartmann & Kester et al. Plant Propagation 2002
Graham et al. 2006, Plant Biology
Graham et al. (2006) Plant Biology
Asymmetric and symmetric cell divisions in embryogenesis
Graham et al. Plant Biology 2006
Seed DevelopmentEmbryogenesis and embryogeny - differentiation and development of the zygote into a mature embryo
Endosperm – develops with embryo, nutritive tissue for embryo development and seed germination
Seed coat – develops from integuments of the ovule
Hartmann & Kester et al. Plant Propagation 2002
Seed – embryo, storage tissue and seed coat
Storage material -carbohydrates (starch), lipids and proteins
Storage tissue/organ - cotyledons (bean), endosperm (castor bean), nucellus/perisperm (beet) and solid endosperm (monocot/wheat)
Hartmann & Kester et al. Plant Propagation 2002
Seed Maturation and Dormancy – maturation involves seed drying, (5 to 20% moisture content)
Seed desiccation facilitates storage time and tolerance of environmental extremes
Seeds acquire the capacity for germination prior to drying but usually are dormant/quiescent until after drying
embryogenesis embryogeny
Seed dormancy occurs during seed maturation, processes that prevent germinationEnsures embryo maturation, environmental and ecological fitness, uniform seed productionPrimary dormancy – seed does not germinate in spite of favorable environmental conditionsQuiescence – competent to germinate, germination does not occur due to inappropriate environmental conditions
Finkelstein et al. (2008) Annu Rev Plant Biol
Seeds typically are dormant on the plant, removal transitions seeds from dormancy to quiescence
Primary Seed Dormancy Regulation Exogenous and endogenous factors ensure germination in favorable environmental/ecological conditions
Exogenous dormancy: Chemicals in fruit that prevent premature germination
Impermeable and impervious seed coat – alleviated by scarification
Seed coat pigments (e.g. flavanoids) - cross-link cell walls and increasing mechanical resistance and reduce permeability
Inhibitors – in seed coat, which are leeched during imbibition
Finch-Savage & Leubner-Metzger New Phytol 2006
Endogenous dormancyAbscisic acid (ABA) - synthesis and accumulation occurs during dormancy induction/maintenance and decrease during dormancy release
Gibberellin (GA) levels are low during dormancy but increase during dormancy release
ABA induces dormancy and GA causes dormancy release and germination
Mutation that blocks ABA biosynthesis results in premature seed germination in maize (and other species)
Precocious germination (vivipary) in the ABA-deficient vivipary 14 (vp14) mutant of maizeVP14 encodes NCED (9-cis-epoxycarotenoid dioxygenase), catalyzes rate limiting step in ABA biosynthesis
ABA treatment of seed prevents germination
ABA prevents premature seed germination by inducing and maintaining seed dormancy
ABA biosynthesis and signaling are induced by seed dehydration during dormancy induction and maintenance
Seed drying → ABA→ABA receptor (ABAR/GCR2)→signaling intermediates (kinases/phosphatases) → transcription factors (e.g. ABI3)→dormancy
Seeds become desiccation tolerant during embryo drying/maturation
ABA induces expression of genes in response to dehydration that encode proteins involved in sugar biosynthesis (osmotic adjustment) and desiccation tolerance (e.g. LEA)
ABA causes seed desiccation tolerance during embryo drying
Finkelstein et al. (2008) Annu Rev Plant Biol
Seed Dormancy Release and GerminationRelease occurs in response to environmental stimuli, e.g. stratification (low temperature w/moisture), light and dark, periods of dry storage After ripening (cool & dry storage) initiate decline in ABA levels, and ethylene inhibits ABA signaling
Stratification and light increase GA levels by inducing expression of GA biosynthetic genes and reducing expression of GA catabolic genes
Finch-Savage & Leubner-Metzger New Phytol 2006
Finkelstein et al. Annu Rev Plant Biol 2008
GerminationGAs induce hydrolytic enzymes that degrade storage product reserves, e.g. expression of the α-AMYLASE , enzyme for starch breakdownGenes that encode hydrolytic enzymes that degrade seed coat cell wall, facilitates cell expansion
Components of the GA signaling pathway regulate germination: GA SLY1 (ubiquitin E3 ligase) DELLA (degraded) hydrolytic enzyme genes germination
Hartmann & Kester et al. Plant Propagation 2002
Three phases of germination: imbibition, lag and radicle emergence from the seed coat
Primarily due to the matrix potential of dry seed (water potential gradient) after seed coat becomes water permeable
Imbibition – period of rapid water uptake
Lag phase – period of intense metabolic activity with minimal water uptake
Mitochondrial activation for energy production
Synthesis of proteins for pre-existing mRNAs
Gene expression and production of additional proteins
Hydrolysis of cell walls, wall loosening
Breakdown of storage products (proteins, carbohydrates (starch), lipids (oils)) and metabolism of amino acids, sugars and fatty acids for energy production
Osmotic adjustment
Radical emergence from the seed coat – cell expansion driven by turgor pressure (water potential gradient) and water uptake
Osmotic adjustment (e.g. conversion of starch to sugars) - more negative symplastic solute/osmotic potential and water potential gradient (symplast more negative)
Water (imbibition) moves from the apoplast to the symplast
Then, root meristematic cells divide, initiation of root development
↓s
Hartmann and Kester et al (2002) Plant Propagation