Dark development Photosynthesis Nutrient uptake Respiration Phytohormones Enzymes 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 Germination HORT 301 – Plant Physiology August 31, 2009 Finkelstein et al. (2008) Annu Rev Plant Biol 59: Finch-Savage and Leubner-Merzger (2006) New Phytol 171: Hartmann & Kester et al. (2002) Plant Propagation, pp Plant Life Cycle

Seed development and dormancy – embryogenesis, embryo maturation and acquisition of dormancy Seed dormancy release and germination – mechanisms and processes Dormancy 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 Fertilization Pollen associates with stigma and germinates, interaction between pollen and stigmatic surfaces Tube 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 Development Embryogenesis 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 germination Ensures embryo maturation, environmental and ecological fitness, uniform seed production Primary dormancy – seed does not germinate in spite of favorable environmental conditions Quiescence – 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 dormancy Abscisic 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 maize VP14 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 Germination Release 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 Germination GAs induce hydrolytic enzymes that degrade storage product reserves, e.g. expression of the α-AMYLASE, enzyme for starch breakdown Genes 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↓s Hartmann and Kester et al (2002) Plant Propagation