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Angiosperm Development Embryogenesis Establishes body plan of the plant Apical-based pattern Radial pattern Accompanies seed development 2 Formation of the Embryo Early stages same in all angiosperms Terminal cell == apical cell Basal cell anchors embryo at micropyle Polarity! Divisions embryo proper + suspensor 3 Primary Meristems Protoderm Periclinal divisions (parallel to surface) Procambiun Ground meristem 4 Developmental Stages Globular stage (e-h) – precedes cotyledon development Globular Stage Heart Stage (i) Monocots – cylindrical Torpedo Stage (k-l) Development becomes restricted to meristems 5 Suspensor Supports development of embryo proper Angiosperm suspensors metabolically active Fern and gymnosperm suspensors – push embryo into nutritive tissues Angiosperm suspensor – provides embryo with nutrients and hormones Short-lived Gone by torpedo stage 7 Arabidopsis mutants Source of mutations that explain plant development Expose seed to mutagen Mutants produce seed 8 Arabidopsis mutants B – no apical meristem/cotyledons C – no hypocotyl D – no root E – no apical and no basal portions Which Genes Set Up Body Axes? MONOPTEROS -- sets up apical-basal axis. Codes for MONOPTEROS protein -- transcription factor. Auxin and Monopteros Auxin – hormone produced in meristem Concentration gradient provides positional information Auxin triggers production of regulatory transcription factors specific to cells in the hypocotyl & roots Establishes positional axis Mature Embryo & Seed Embryo continually nourished during development Perisperm (from nucellar tissue) within endosperm Cotyledons Funiculus detaches from ovule nutritionally closed system Seed dessicates Seed coat hardens 12 Mature Embryo Axis bearing cotyledons Opposite ends Shoot apical meristem Root apical meristem Epicotyl – stemlike axis Plumule – embryonic shoot & leaves Hypocotyl – stemlike axis Radicle 13 14 Endosperm & Perisperm Some seeds (e.g., beet) contain both Endosperm – double fertilization Perisperm – proliferation of nucellus When endosperm absorbed – large cotyledons Sunflower, walnut, pea, bean Large amounts of endosperm – membranous cotyledons 15 Dicot Seeds When endosperm absorbed – large cotyledons Sunflower, walnut, pea, bean Large amounts of endosperm – membranous cotyledons 16 Monocot Seeds Grasses – massive cotyledon scutellum Both radicle & plumule sheathed Coleorhiza and coleoptile 17 Seed Germination Embryo growth delayed while seed matures Germination – resumption of embryo growth Many factors o Mature seeds typically dry External – water, oxygen and temperature Imbibation for enzymatic activity Cells that produced reserve materials now digest Cell enlargement & cell division Early germination anaerobic Once seed coat ruptured – process is aerobic! 18 Seed Dormancy Physiologically immature embryo After-ripening Cold periods Digestion Fire Water 19 Embryo to Adult First emergent structure – root Anchors plant Absorbs water Primary root Branch or lateral roots Adventitious roots Shoot-borne roots 20 Epigeal & Hypogeal Germination Epigeal Hypocotyl elongates hook Hook straightens pulls cotyledons & plumule Hypogeal Epicotyl elongates hook Hook straightens pulls plumule 21 Endosperm Present Endosperm Absorbed into Cotyledons Before Germination 1. Corn (Monocot) 2. Pea (Dicot) Hypogeous: Cotyledons Not Emergent Start Seed Development Epigeous: Cotyledons Emergent Germination 4. Onion (Monocot) Castor Bean (Dicot) 3. Bean (Dicot) Type 1 Hypogeous Monocot Highly differentiated embryo Endosperm absorbed during germination Type 2 Hypogeous Dicot Endosperm absorbed well before germination Cotyledons fill seed Type 3 Epigeous Dicot Endosperm absorbed well before germination Cotyledons fill seed Type 4 Epigeous Monocot Endosperm absorbed during germination Type 4 Epigeous Dicot Endosperm absorbed during germination
