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

paul.m.hasegawa.1@purdue.edu

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