Plant Growth and Development II "It is at the edge of a petal that love awaits.” ...William Carlos Williams
apical/basal, axial SAM embryogenesis RAM primary growth Leaf Primordia ? ? ? Stem Tissues 2o Growth ? primary growth Structure/Function 2o Growth ? Patterns of Development Zygote Embryo Cotyledons Hypocotyl SAM Cell Differentiation 1o Growth RAM Root Tissues 1o Growth
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.
Lateral Meristems... …provide for secondary growth by producing secondary vascular tissue and periderm (secondary dermal tissue).
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 Fig. 35.20
Secondary Growth Year 1 Fig. 35.21
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 Fig. 35.21
Secondary Growth Fig. 35.21
Assignment • 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 Fig. 35.10c Fig. 12.8
Plane of Division Fig 35.28 Fig 35.27
Plant Cell Walls Cell Morphology Water Relations Bulk Flow Biochemistry Plant Morphology Cell Morphology Mechanical and Structural Pathogen Defense
Cellulose / Cell Walls Fig. 5.8
Microtubules (pp. 127, Fig. 7.21) Cell Wall Synthesis Fig 35.29
Cell Expansion 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. Secondary Walls: More ordered, restricts general enlargement, often lignified (wood). Primary Walls: Less ordered, allows general enlargement.
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)
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 thalianaThale 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 to all aspects of plant growth, development, evolution, ecology...
FASS gene: not cloned Cell Growth fass Mutant: cortical microtubules do not organize.
Homeotic Gene KNOTTED gene expression results in the differentiation of cells into vasculature. Pattern Formation lacks apical-basal axis GNOM: guanine nucleotide exchange factor
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
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.
Clavata Phenotypes Clavata phenotype mutant wt phenotype CLAVATA turned off during development results in determinant growth, I.e. a leaf.
Transport Friday Quiz: Through Chapter 36, 748 - 754