Seed Germination HORT 301 – Plant Physiology September 26, 2008 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 Growth and Development Lectures Seed development, maturation and germination Vegetative development Flowering Seed development and dormancy – embryogenesis, embryo maturation and acquisition of dormancy Seed dormancy release and germination – dormancy facilitates overwintering and germination when environmental conditions are favorable for plant development

Hartmann and Kester et al. Plant Propagation 2002 Pollination, fertilization and seed development Stylized mature angiosperm flower (lily) stamen (pollen) and ovary (ovule) development

Pollen associates with the stigmatic surface and germinates, positive interaction between pollen and stigmatic surfaces Tube traverses the style (chemotropic response) and deposits two generative into the ovule, micropylar end One nucleus fertilizes the egg (becomes the zygote) and other fuses with the polar nuclei to form the endosperm (nutritive tissue) Hartmann & Kester et al. Plant Propagation 2002 Double Fertiliziation

Graham et al. Plant Biology 2006 Seed development – embryogenesis and embryogeny are processes of differentiation and development of the zygote into a mature embryo Endosperm - develops contiguously with the 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), nuclellus/perisperm (beet) and solid endosperm (monocot/wheat)

Hartmann & Kester et al. Plant Propagation 2002 Seed development – seed developmental stages are embryogenesis (histodifferentation), embryogeny (cell expansion) and maturation (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 and quiescence – state in embryo development that occurs during seed maturation, adaptive processes that prevent germination Ensure embryo maturation, and environmental and ecological fitness, i.e. facilitates germination of mature embryos in favorable climatic environments and ecosystem competitiveness

Finkelstein et al. (2008) Annu Rev Plant Biol Primary dormancy – seeds do not germinate in spite of environmental conditions that are appropriate for germination Quiescence – competent to germinate but germination does not occur because environmental conditions are not appropriate quiescence

Seeds typically are dormant while associated with the plant and removal transitions seeds from dormancy to quiescence Seeds of crops are selected for uniform germination to enhance crop production Premature germination reduces product quality and yield, and ecological fitness

Finkelstein et al. (2008) Annu Rev Plant Biol Secondary dormancy – another adaptive process that is a response to unfavorable environmental conditions after germination has been initiated, e.g. drought episode that occurs shortly after rain

Regulation of primary seed dormancy – exogenous and endogenous factors ensure that seeds germinate in favorable environmental and ecological conditions Exogenous dormancy – caused by factors such as: Chemicals in the fruit that prevent premature germination while seeds are associated with the fruit Impermeable and impervious seed coats – alleviated by scarification Seed coat pigments (e.g. flavanoids) accumulate in the seed coat and cross-link into the cell walls increasing mechanical resistance and reduce permeability Inhibitors – usually in the seed coat, which are leeched during imbibition

Finch-Savage & Leubner-Metzger New Phytol 2006 Endogenous dormancy – release requires physiological responses to environmental stimuli such as stratification (moisture and low temperature), light or dark or periods of dry storage to alleviate dormancy Abscisic acid (ABA) and gibberellin content increase and decrease, and signaling responses interplay to regulate seed dormancy and germination ABA causes dormancy - ABA content and ABA sensitivity increase during dormancy GA releases dormancy and causes germination

Precocious germination (vivipary) in the ABA-deficient vivipary 14 (vp14) mutant of maize, VP14 encodes NCED Mutation that blocks ABA biosynthesis results in premature seed germination in maize (and other species) ABA induces seed dormancy, preventing premature germination

ABA biosynthetic enzymes are “activated” during embryo maturation and as seeds acquire desiccation tolerance NCED (encodes 9-cis-epoxycarotenoid dioxygenase) expression is induced during embryo maturation as a response to dehydration Finch-Savage & Leubner-Metzger New Phytol 2006

ABA perception and signaling determinants are linked to seed dormancy, including putative ABA receptors, transcription factors, and protein kinases and phosphatases that regulate the activity of transcription factors ABA increases desiccation tolerance – induction of genes that encode proteins involved in sugar (osmotic adjustment) and structural protein biosynthesis (e.g. LEA) Finkelstein et al. (2008) Annu Rev Plant Biol ABA→ABA receptor (ABAR/GCR2)→transcription factors (e.g. ABI3)→dormancy

Finch-Savage & Leubner-Metzger New Phytol 2006 Seed Dormancy Release and Germination – ABA catabolism and increased gibberellin synthesis occur coincident with dormancy release, reduced ABA and increased GA levels

After ripening (cool & dry storage), nitrate and nitric oxide (NO) initiate decline in ABA levels, and ethylene inhibits ABA signaling Stratification and light cause increased GA levels by inducing expression of GA synthetic genes and reducing expression of GA catabolic genes Secondary dormancy is initiated by accumulation of ABA in response to seed dehydration caused by drought

Finkelstein et al. Annu Rev Plant Biol 2008 Germination GAs induce hydrolytic enzymes that degrade storage product reserves, e.g. α-amylase for starch breakdown And, breakdown the cell wall components of the seed coat, which facilitates cell expansion Components of the GA signaling pathway regulate germination: GA→SLY1 (ubiquitin E3 ligase) facilitates degradation of DELLA proteins→negatively regulates expression of genes encoding hydrolytic enzymes→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 – due mainly to cell expansion driven by the water potential gradient caused by osmotic adjustment (more negative solute/osmotic potential) Then cell division of the root meristem Physiologists consider radical protrusion from the seed coat as the indicator of germination ↓s↓s

Wilson et all Botany 1971 Germination patterns of dicots and monocots illustrating radicle and plumule development

Finch-Savage & Leubner-Metzger New Phytol 2006

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Ohto et al. Annu Plant Rev 2007