Seed Germination HORT 301 – Plant Physiology September 26, 2008 Finkelstein et al. (2008) Annu Rev...
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Transcript of Seed Germination HORT 301 – Plant Physiology September 26, 2008 Finkelstein et al. (2008) Annu Rev...
Seed GerminationHORT 301 – Plant Physiology
September 26, 2008Finkelstein 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 Growth and Development LecturesSeed 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 processesof differentiation and development of the zygote into a mature embryo
Endosperm - develops contiguously with the embryo, nutritive tissue forembryo 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
GerminationGAs 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
Wilson et all Botany 1971
Germination patterns of dicots and monocots illustrating radicle and plumule development
Finch-Savage & Leubner-Metzger New Phytol 2006
Hartmann & Kester et al. 2002
Ohto et al. Annu Plant Rev 2007