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

paul.m.hasegawa.1@purdue.edu

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

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Hartmann and Kester et al (2002) Plant Propagation