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Embryogenesis. Meristems

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Embryogenesis. Meristems BLGY1115 Plant cell & tissue biology Prof. Paul Knox [email protected] Books • BIOLOGY th Campbell et al., 10 Global edition, Pearson Publishers • Chapter 35 (p 816). Plant Structure & Growth Also Chapters 36-39 Plants………... major source of biomass on Earth ….. a source of food……….. …….. and drink! • Special features of plant growth & development • Embryogenesis. Meristems • Primary and secondary growth. Wood formation • Plant cell walls • Plant tissues and cell types • Plasticity and plant development Plants are complex multi-cellular organisms Distinctive features of plant development • Post-embryonic development • Plant cells do not migrate • Plant cells are totipotent & have developmental plasticity Post-embryonic development • The plant embryo has a simplified body plan • Most of the plant body is produced post- embryonically by the activity of meristems • Morphogenesis occurs continuously throughout the life of a plant • Plants can respond to the environment with changes in growth and development Development of a human embryo Late stage plant embryo future SAM future RAM SAM – shoot apical meristem RAM – root apical meristem Plant cells cannot migrate - because of the cell wall • The relative positions of cells in the plant body is always important • Cellular processes must be highly co-ordinated in both space and time • CELL DIVISION: timing and orientation • CELL EXPANSION: extent and direction • CELL DIFFERENTIATION: spatial relationships of cell types important The plant kingdom & plant diversity • The flowering plants (Angiosperms) are an extremely widespread, successful & diverse group of organsims • They are structurally the most complex plants & have evolved from structurally less complex plants • The table shows a summary of the major extant groups of the plant kingdom Division/Class # species Angiosperms Dicotyledons 300,000 Monocotyledons 65,000 Gymnosperms Seed Conifers 550 Cycads 100 Ginkgo 1 Vascular Ferns (Pteridophytes) 12,000 Horsetails & club mosses 1,000 Seedless Non- Mosses & liverworts 16,000 (Bryophytes) vasc. Green algae 7,000 Red algae 4,000 Algae Brown algae 1,500 Dicotyledon leaf Monocotyledon leaves Monocots & Dicots Characteristic DICOTYLEDONS MONOCOTYLEDONS • Number of cotyledons Two One • Leaf structure Petiole Sheath • Leaf venation Network Parallel • Stem vascular bundles Circular pattern Scattered • Root system Tap root Fibrous • Root protoxylem poles Small number Large number • Secondary growth Present Absent • Adventitious roots Absent Present • Flower parts Multiples of 2 or 5 Multiples of 3 Arabidopsis thaliana Thale cress Arabidopsis • Arabidopsis thaliana • Angiosperm belonging to the cabbage/mustard family - Brassicaceae • A very important plant in modern plant biology • It has become the preeminent model system for plant molecular genetics and the study of the mechanisms of growth and development Arabidopsis • It is small enough to be grown in sterile containers in the lab. • Prolific seed production • Has a short life cycle (c. 6 - 8 weeks) • Easy transformation (introduction of DNA) • It has a small genome (115 Mb DNA - 5 chromosomes) • Large numbers of mutant lines - available from stock centers • Arabidopsis - first plant genome to be fully sequenced (2000) in a programme comparable to the human genome sequencing project • Indicating that it has c. 25,000 genes • Arabidopsis has considerable genetic resources/stock centres to support its study. Development of plant structures Embryogenesis Embryogenesis • Fertilisation of the egg cell induces cell divisions, cell expansions and cell differentiation resulting in the formation of an embryo • Development of three tissue systems & the root and shoot apical meristems • Highly regulated cell processes leading to defined cell patterns Dicotyledon embryogenesis Egg zygote proembryo suspensor Further precisely determined divisions Protoderm is now defined - anticlinal divisions Globular stage embryo suspensor cotyledons forming Heart-shaped stage embryo procambium future shoot apical meristem Mature embryo future root apical meristem Monocot embryo development • Similar to dicot - a single cell zygote develops into a multicellular embryo • One cotyledon (scutellum)- a notch forms at globular stage and embryonic axis forms at one side • Meristems more developed • Shoot meristem enclosed in a sheath - coleoptile • Several root meristems may form directly in embryo Embryogenesis Embryo sac in context of Endosperm nucleus developing seeds Zygote Endosperm Cotyledon Pea Seed coat Carrot (testa) Flowering plant (Angiosperm) life cycle Double fertilisation event What is known of the cellular & molecular mechanisms that underpin development events? Plant embryogenesis initiates with the establishment of an apical-basal axis Molecular mechanisms of this event unclear. Recent work has shown that a small cysteine-rich peptide is required for formation of the zygotic basal cell lineage and proembryo patterning in Arabidopsis. Peptides accumulate before fertilization in central cell gametes and thereafter in the endosperm cells. Costa et al. (2014) Science 344, 168-172 Seed germination Grass sprouting on left (a monocot), showing hypogeal germination (the cotyledon remains underground and is not visible) Compare to a dicot (right) with epigeal germination Fig. 38-9 Two common types Foliage leaves of seed germination Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Hypocotyl Hypocotyl Radicle Seed coat (a) Common garden bean Foliage leaves Coleoptile Coleoptile Radicle (b) Maize 3 tissue systems • Protoderm dermal tissue – EPIDERMIS • Procambium vascular tissues – XYLEM, PHLOEM • in between ground (or fundamental) tissues – PITH, CORTEX PARENCHYMA Three tissue systems set up in the embryo are elaborated into distinct spatial arrangements by the activity of meristems DERMAL VASCULAR GROUND Root Shoot Leaf Development of plant structures Meristems • Meristem development is different to embryogenesis • Embryogenesis: whole group of cells undergoes changes in form e.g. transition from globular to heart-shaped embryo • Meristem: a set of cells undergoing repetitive processes –iterative development • Meristems: regions of plants that remain embryonic with the capacity to produce new cells • Cells that are continually dividing and adding to the population of cells are known as initials • When an initial divides the two daughter cells have differing fates: – One cell: divides again – One cell: differentiates • In this way an initial can produce a file of cells – a line of attached cells • As a cell moves relatively further away from an initial it differentiates The development of a file of cells x x x Meristem x x initial x x x The basis of Cell that leaves meristem meristem and activity differentiates • Most of the plant body is produced by the iterative activity of meristems • Meristems are complex with 50 to 100 initials. (Ferns have one initial cell) • Meristem activity can be – determinate: limited development to give rise to a specific structure e.g. flower - has an endpoint – indeterminate: continuous production of organs e.g. leaves - no developmental endpoint In plants, a meristem gives rise to all cell types - no separate stem cells for each cell type Root and shoot apical meristems have very different structures: different spatial organisations of cell divisions and expansions Meristems develop from embryos . SAM – shoot apical meristem RAM – root apical meristem SAM RAM These are not the only meristems: during development new meristems arise in shoot and root structures .
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