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Structure, Biogenesis, and Expansion
BIOL3745 Plant Physiology Unit 3 Chapter 15 (14 in 6th) Cell Walls Structure, Biogenesis, and Expansion
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Figure 15.1 Cross-section of a stem of a buttercup (Ranuculus repens)
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Determines the mechanical strength of plant structures
Cell walls Determines the mechanical strength of plant structures Glue cells together, preventing them from sliding past one another Plant morphogenesis depends on the control of cell wall properties The wall acts as a cellular “exoskeleton” that controls cell shape and allows high turgor pressure to develop Transpirational water flow in the xylem requires a strong wall A major barrier to pathogen invasion
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Figure 15.2 Three views of primary cell walls
Surface view Surface view by SEM Cellulose microfibrils PP5e-Fig jpg Outer epidermal cell wall
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Cell wall Thickness and composition of cell walls vary depending on the tissue (cell types) and species, even variation exists in individual side of a cell. Wall composition: cellulose, pectin, lignin, cutin, suberin, waxes, silica, structural proteins Primary walls: formed by growing cells. Usually thin and architecturally simple. Secondary walls: formed after cell enlargement stops, between the plasma membrane and the primary cell wall. Middle lamella: a thin layer of material at the interface where the walls of neighboring cells come into contact. Pectic polysaccharides.
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Figure 15.3 Diversity of cell wall structure
Thickened secondary walls of a vascular bundle of fern stem PP5e-Fig jpg The fibers of Tetramerista xylem Thin wall of rice stem
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Figure 15.4 Major structural components of the primary cell wall and their likely arrangement
PP5e-Fig jpg The basic model of the primary cell wall is of a network of cellulose microfibrils embedded in a matrix of hemicelluloses, pectins, and structural proteins
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Figure 15.5 Conformational structures of sugars commonly found in plant cell walls
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Figure 15.6 Structural model of a cellulose microfibril
PP5e-Fig jpg The individual glucan chains are composed of 2000 – >25,000 glucose residues.
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Figure 15.7 Cellulose synthesis by the cell
Cellulose microfibrils are synthesized by large protein complexes “rosettes” – made up of six subunits, each of them contains multiple units of cellulose synthase (or cellulose synthase-like enzymes) PP5e-Fig jpg
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Figure 15.8 Cellulose synthesis by a multisubunit complex containing cellulose synthase
PP5e-Fig jpg Cellulose synthase: sugar-nucleotide polysaccharide glycosyltransferases – transfer monosaccharides from sugar nucleotides to the growing end of the polysaccharide chain
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Figure 15.9 Structure of a sterol glucoside
PP5e-Fig jpg An initial primer for cellulose synthesis
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Figure 15.10 Partial structures of common hemicelluloses
PP5e-Fig jpg Matrix polymers are synthesized in the Golgi apparatus and secreted via vesicles; hemicelluloses bind to celluloses
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Figure 15.11 Partial structures of the most common pectins
PP5e-Fig jpg Pectins are hydrophilic gel-forming components of the matrix.
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Figure 15.12 Triple-fluorescence-labeled section of tobacco stem
PP5e-Fig jpg Blue: cellulose Red and green: pectic homogalacturonan
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Figure 15.13 Linear arrangement of various pectin domains to each other
PP5e-Fig jpg Formation of a pectin network involves ionic bridging of the nonesterified carboxyl groups (COO-) by calcium ions
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PP5e-Table jpg They are cell wall structural proteins that may be involved in plant defense responses
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Figure 15.14 A repeated hydroxyproline-rich motif from a molecule of HRGP from tomato
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Figure 15.15 A highly branched arabinogalactan molecule
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New primary cell walls are assembled during cytokinesis
synthesis deposition assembly modification Self-assembly: wall polymers have inherent tendencies to aggregate into somewhat ordered structure (a physical process) Enzyme-mediated assembly: xyloglucan endotransglucosylase (XET)
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Figure 15.16 Action of xyloglucan endotransglucosylase (XET)
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Secondary cell walls are synthesized after wall expansion ceases.
Secondary walls in woody tissues contain a higher percentage of cellulose and xylans than do primary walls and become highly impregnated with lignin Lignin is a complex polymer made up of phenyl propanoid subunits (monolignols) that are oxidatively cross-linked in the wall and that lock the matrix and cellulose into a hydrophobic and indigestible material
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Figure 15.17 Phenolic subunits of lignin infiltrate the space between cellulose microfibrils
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Figure 15.18 Secondary cell walls
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Root hairs, pollen tubes
Figure The cell surface expands differently during tip growth and diffuse growth Root hairs, pollen tubes PP5e-Fig jpg
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Figure The orientation of newly deposited cellulose microfibrils determines the direction of cell expansion (isotropic) (anisotropic) PP5e-Fig jpg
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Figure 15.21 Interdigitating cell growth of leaf pavement cells
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Figure Orientation of microtubules in the cortical cytoplasm mirrors the orientation of newly deposited cellulose microfibrils in the wall of cells that are elongating PP5e-Fig jpg
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Figure Disruption of cortical microtubules results in a dramatic increase in radial cell expansion and a concomitant decrease in elongation PP5e-Fig jpg CesA protein microfibrils
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Patterns of cell expansion
Wall expansion may be highly localized (tip growth) or evenly distributed over the wall surface (diffuse growth) In diffuse-growing cells, the orientation of cell growth is determined by the orientation of cellulose microfibrils, which is determined by the orientation of microtubules in the cytoplasm Complicated cell growth patterns found in the “jigsaw” pattern of the leaf epidermis of many plants involve G-protein signals that organize the cytoskeletal elements, thereby directing the local pattern of wall synthesis and growth
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Rate of cell elongation
Biochemical loosening of the cell wall lead to wall stress relaxation, which dynamically links water uptake with cell wall expansion in the growing cell; The actions of hormones (such as auxin) and environmental conditions (light and water availability) modulate cell expansion by altering wall extensibility and / or yield threshold;
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Figure 15.25 Reduction of cell turgor pressure (water potential) by stress relaxation
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Figure 15.26 Acid-induced extension of isolated cell walls
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Figure 15.27 Scheme for the reconstitution of extensibility of isolated cell walls (A)
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Figure 15.27 Scheme for the reconstitution of extensibility of isolated cell walls (B)
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Expansin Acid-induced cell wall extension is characteristic of primary walls and is mediated by the protein expansin, which loosens the linkage between microfibrils. Cessation of cell growth during cell maturation involves multiple mechanisms of cell wall cross-linking and rigidification. Alteration of newly secreted matrix polysaccharides; De-esterification of pectins, leading to more rigid pectin gels Cross-linking of phenolic groups Removal of (13; 14) –D-glucan in grass cell walls in coincident with growth cessation in the wall and may cause wall rigidification.
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