Plant Growth and Development II "It is at the edge of a petal that love awaits.”...William Carlos Williams

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

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

Secondary Growth Year 1 Fig

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

Secondary Growth Fig

Secondary Growth

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 Fig c

Plane of Division Fig Fig 35.28

Plant Cell Walls Cell Morphology Mechanical and Structural Water Relations Plant MorphologyCell Morphology Pathogen Defense Bulk Flow Biochemistry

Cellulose / Cell Walls Fig. 5.8

Cell Wall Synthesis Microtubules (pp. 127, Fig. 7.21) Fig 35.29

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.

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.

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.

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

% 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 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.

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...

Cell Growth fass Mutant: cortical microtubules do not organize. FASS gene: not cloned

Pattern Formation GNOM: guanine nucleotide exchange factor lacks apical-basal axis Homeotic Gene KNOTTED gene expression results in the differentiation of cells into vasculature.

Shoot Development Begins at the Shoot Apical Meristem

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 phenotype mutant wt phenotype CLAVATA turned off during development results in determinant growth, I.e. a leaf. Clavata Phenotypes

Transport Friday Quiz: Through Chapter 36,