Skotomorphogenesis Seed germination Genes and enzymes Embryo and Seed development Plant life cycle Photomorphogenesis Photoreceptors Phytochrome Cells and cell growth Phytochrome: regulation of light responses Photosynthesis: light reaction Photosynthesis: carbon fixation Photorespiration Seed development Respiration Primary metabolism Secondary metabolism Nitrogen fixation Mineral nutrition Water transport Phloem translocation Abiotic stress Biotic stress Flowering Fertilization and embryogenesis

Seeds

Seed – embryo, storage tissue and seed coat Storage tissue/organ - cotyledons (bean), endosperm (castor bean), nuclellus/perisperm (beet) and solid endosperm (monocot/wheat) Storage material -carbohydrates (starch), lipids and proteins

Lipid biosynthesis Storage lipids in oil seeds: soybean sunflower seeds oil palms canola castor bean

Different types of vacuoles are found in plant cells Protein bodies Lytic vacuoles Storage of proteins in specialized protein bodies Activation of proteins by lytic digestion in vacuoles

Plant life cycle

Fertilization of the egg cell

Stages of embryo development embryo suspensor

Stages of embryo development apical-basal radial symetry zygote 1-cell 2-cell 8-cell dermatogen globular early heart late heart torpedo walking stick as, hor ver horperic

Stages of embryo development apical-basal polarity defined with first cell division Identification of ground tissue and epidermis Formation of meristems and cotyledons

Stages of embryo development ca. 20,000 cells

Gene expression during seed development

Stages of embryo development Germinating seed testa radicle hypocotyl

Stages of embryo development seedling (cotyledon stage)

Plant Biology (2010) Smith et al Seed development/dormancy and germination is regulated by ABA and GA Smith et al. (2010) Plant Biology

Seed germination: mobilization of storage compounds Campbell, 2003

Seed germination: mobilization of storage compounds 100 ppb GA3 1 ppb GA3 H2OH2O

Hartmann & Kester et al. Plant Propagation 2002 Seed development : embryogenesis (histodifferentation), embryogeny (cell expansion) maturation (drying, 5 to 20% moisture content) - Seed desiccation allows storage and tolerance of environmental extremes - acquire capacity for germination before drying - usually dormant/quiescent until after drying

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

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-deficient maize mutants (viviparous) show pre-mature germination

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

Svalbard Global seed vault, Norway