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Plant Growth and Development II "It is at the edge of a petal that love awaits.” ...William Carlos Williams apical/basal, axial Patterns of Development SAM embryogenesis Zygote Embryo RAM primary growth SAM ? ? ? primary growth Cotyledons Hypocotyl Root Leaf Primordia Cell Differentiation Stem Tissues ? 1o Growth 2o Growth Structure/Function RAM Root Tissues 1o Growth ? 2o Growth Today • How do plant organs, tissues and cells develop? – Examine Plant Growth, • • • • primary growth, secondary growth, cell elongation molecular development. Lateral Meristems... …provide for secondary growth by producing secondary vascular tissue and periderm (secondary dermal tissue). Stem Tissues Vascular Bundles Buttercup (dicot) Corn (monocot) Secondary Growth of Stems • Two Lateral Meristems, – Vascular cambium; produces secondary vascular tissue, – Cork cambium; produces tissue (periderm) that replaces the epidermis, • secondary phloem and periderm comprise bark. Vascular Cambium Secondary Growth Year 1 Lateral Meristem Cells Fusiform initials: meristematic cells that give rise to xylem and phloem. Ray initials: meristematic cells that give rise to (primarily) parenchyma cells that serve as radial connections. Tangential Section Secondary Growth Year 2 See Fig. 36.28 Secondary Growth Secondary Growth Assignment …same w/ root, w/o periderm. Be able to construct a tree from a seedling using these meristems, - at the tissue level. Growth / Differentiation • Growth, • the irreversible increase in size that (in plants) almost always results from both cell division and cell enlargement, • Differentiation, • the process by which a cell acquires metabolic, structural and functional properties distinct from those of its progenitor. Cell Division / Cell Walls / Cell Growth Plane(s) of Division Plant Cell Walls Cell Morphology Water Relations Plant Morphology Pathogen Defense Mechanical and Structural Bulk Flow Biochemistry Cellulose / Cell Walls Polysaccharide and Protein Links Cell Wall Synthesis Microtubules Cell Expansion Biased Microfibril Distribution allows for directional growth. Secondary Walls: More ordered, restricts general enlargement, often lignified (wood). Primary Walls: Less ordered, allows general enlargement. Turgor: water potential is lowered in the cell, allowing water uptake. The force of the water pressure drives cell expansion. Acid-Growth Hypothesis 1. Plasma Membrane H+-ATPases acidify the apoplast (cell wall). 2. Cell wall loosening enzymes are activated. 3. Electrochemical gradient drives solutes into the cell, - lowers osmotic potential, H2O? 4. Vacuolar ATPase provides membrane potential for transport of solutes into the vacuole, etc. etc. ATP hydrolases (ATPases) know this Developmental Biology Modern approach is driven primarily by the study of genetics, – primarily through the study of mutants, organisms blocked in specific developmental pathways, – Model Organisms. Model Organisms • • • • • • • Ease of cultivation, Rapid Reproduction, Small size, Fecund (large brood size), Mutants are available and easy to identify, Scientifically relevant (ecologically, organ system, etc.) Extant Literature, co-ordinated research emphasis. 60 - 70 % similarity in all eukaryotes. flowering plants Arabisopsis thaliana Thale cress/Mouse Ear Cress • Arabidopsis is a plant belonging to the Mustard family, Cruciferae. Arabidopsis' agronomic value is as a Model Organism, • • • • • • weedy: world-wide distribution and easily grown in the lab. self-fertilizing: it is easy to generate and maintain genetic stocks. lifecycle: about 42 days at 200 C and continuous light. fecundity: up to 50,000 seeds per plant. mutable: yes, lots of ways. literature: 9718 journal articles (PubMed) – • - ~ 1000 devoted labs. Arabidopsis is THE plant model organism with over 7000 full-time scientists devoted to understanding the growth and development of this organism, and the extension of this knowledge to other plants and organisms. Arabisopsis thaliana + 26,000 Genes Genetics: analysis of mutant phenotypes, Reverse Genetics: analysis of mutant genotypes, Genomics: use of DNA sequence for all aspects of plant growth, development, evolution, ecology... Cell Growth fass Mutant: cortical microtubules do not organize. Pattern Formation lacks apical-basal axis GNOM: signal transduction gene Homeotic Gene KNOTTED gene expression results in the differentiation of cells into vasculature. wt glabra-2 mutant Wednesday • Transport I – Ch 37: 813 – 819, • Quiz includes Wednesday’s reading assignment! – Last 20 minutes