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Plant Growth and Development II "It is at the edge of a petal that love awaits.”...William Carlos Williams
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Patterns of Development Zygote Embryo Cotyledons Hypocotyl Cotyledons Hypocotyl SAM RAM apical/basal, axial embryogenesis primary growth SAM Cell Differentiation Cell Differentiation Leaf Primordia Leaf Primordia Stem Tissues Stem Tissues 1 o Growth primary growth RAM Root Tissues Root Tissues ? ?? Structure/Function ? ? 1 o Growth 2 o Growth
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Today (continue) How do plant organs, tissues and cells develop? –Examine Plant Growth, primary growth, secondary growth, cell elongation –How is Plant Cell Differentiation Studied, discovering the process by which a cell acquires metabolic, structural and functional properties.
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Lateral Meristems... …provide for secondary growth by producing secondary vascular tissue and periderm (secondary dermal tissue).
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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.
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Vascular Cambium Fig. 35.20
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Secondary Growth Year 1 Fig. 35.21
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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
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Secondary Growth Year 2 Fig. 35.21
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Secondary Growth Fig. 35.21
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Secondary Growth
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Assignment Be able to construct a tree from a seedling using these meristems, - at the tissue level.
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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.
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Cell Division / Cell Walls / Cell Growth Fig. 12.8 Fig. 35.10c
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Plane of Division Fig 35.27 Fig 35.28
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Plant Cell Walls Cell Morphology Mechanical and Structural Water Relations Plant MorphologyCell Morphology Pathogen Defense Bulk Flow Biochemistry
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Cellulose / Cell Walls Fig. 5.8
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Cell Wall Synthesis Microtubules (pp. 127, Fig. 7.21) Fig 35.29
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Cell Expansion Primary Walls: Less ordered, allows general enlargement. Secondary Walls: More ordered, restricts general enlargement, often lignified (wood). Biased Microfibril Distribution allows for directional growth. Turgor: water potential is lowered in the cell, allowing water uptake. The force of the water pressure drives cell expansion.
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Acid-Growth Hypothesis ATP hydrolases (ATPases) 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.
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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.
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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. Model Organisms
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60 - 70 % similarity in all eukaryotes. flowering plants
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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 20 0 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.
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Arabisopsis thaliana + 26,000 Genes Genetics: analysis of mutant phenotypes, Reverse Genetics: analysis of mutant genotypes, Genomics: use of DNA sequence to all aspects of plant growth, development, evolution, ecology...
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Cell Growth fass Mutant: cortical microtubules do not organize. FASS gene: not cloned
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Pattern Formation GNOM: guanine nucleotide exchange factor lacks apical-basal axis Homeotic Gene KNOTTED gene expression results in the differentiation of cells into vasculature.
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Shoot Development Begins at the Shoot Apical Meristem
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Maintenance of the Meristem CLAVATA and WUSCHEL protein interactions constitute a tightly regulated control mechanism to maintain and delimit the meristem, wuschel Mutant = Meristem Disappears clavata Mutant = Enlarges Meristem WUSCHEL Gene = Maintains the Meristem CLAVATA Mutant = Delimits the Meristem
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Clavata’s Molecular Mechanism clavata 1, 2 and 3 mutants have identical phenotypes of enlarged meristems CLAVATA3 protein acts as a signal molecule on the two-component clavata receptor, constituted by CLAVATA 1 and 2 The clavata receptor is a leucine rich repeat (lrr) serine/threonine kinase receptor.
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Clavata phenotype mutant wt phenotype CLAVATA turned off during development results in determinant growth, I.e. a leaf. Clavata Phenotypes
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Transport Friday Quiz: Through Chapter 36, 748 - 754
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