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Transcript
Chapter 3 Degradation of cell walls by plant pathogens Cell walls Middle lamella Primary cell wall Secondary cell wall Cell Primary cell walls from onion parenchyma EM of the outer epidermal cell wall of a bean hypocotyl (胚軸) Components of cell walls Cellulose Pectic substances Hemicellulose Lignin Proteins Sugars commonly found in plant cell walls Synthesis and delivery of matrix polysaccharides to the cell wall Wall matrix Structural components of the primary cell wall Structural model of a cellulose microfibril Cellulose synthesis by the cell Particle Rosettes (terminal complexes) Celluloses are synthesized by a multisubunit complex containing cellulose synthase. Pectins (果膠質) Pectins are the most soluble of the wall polysaccharides. Pectins are gel-forming components containing Galacturonic acid Rhamnose Galactose Arabinose Pectin is a linear chain of (1-4)-linked D-galacturonic acid that forms the pectin-backbone, a homogalacturonan. pectins can form a complex mixture of acidic and neutral polymers composed of (1,4)-linked homogalacturonan residues and interspersed with 2-linked rhamnosyl residues. Pectic polysaccharides Pectic polysaccharides The carboxyl groups of some of the galacturonosyl residues are methyl esterified. Esterification is more than 75% => pectins (果膠質) Esterification is less than 75% => pectinic acid (果 膠質酸) No esterification => pectate (果膠酸) (RG I) Pectic polysaccharides Hemicellulose A hemicellulose can be any of several heteropolymers (matrix polysaccharides) present in almost all plant cell walls along with cellulose. Cellulose contains only anhydrous glucose, while hemicellulose contains many different sugar monomers. For instance, besides glucose, sugar monomers in hemicellulose can include xylose, mannose, galactose, rhamnose, galacturonic acid, mannuronic acid and arabinose. Hemicelluloses contain most of the D-pentose sugars, and occasionally small amounts of L-sugars as well. Xylose is always the sugar monomer present in the largest amount. Structure of hemicullulose Components of cell walls Cellulose Pectic substances Hemicellulose Lignin Proteins Lignification and disease resistance The lignin monomers arise from the aromatic amino acid phenylalanine and tyrosine via the phenylpropanoid pathway Deposited in secondary walls of most plant cells Extremely resistant to microbial degradation Increased lignification in cell wall in response to pathogen infection, especially in the incompatible host-parasite interaction. Fungal cell wall and plant cell wall are elicitors. Lignin Coumaryl alcohol Phenylalanine Coniferyl alcohol Sinapyl alcohol Lignin biosynthesis 黃酮 異黃酮 黃酮醇 Structural proteins linked to cell wall Structural proteins linked to cell wall Extensin of tomato Extensin contains repeated hydroxyproline-rich motifs, extensive glycosylation, and an intramolecular isodityrosine bond. Plant pathogens secrete cell-wall-degrading enzymes Elicitors Enzymes that degrade the cell wall are most useful for necrotrophic pathogens with wide host range. For examples Botrytis species (灰霉菌) attack more than 1000 plant species. Erwinia species (軟腐病菌) produce pectinases that cause tissue maceration Cell-wall-degrading enzymes produced by plant pathogens Enzymes that degrade pectic substances Pectin methylesterase Polygalacturonases Pectin lyase Pectate lyase Rhamnogalacturonanases a-L-arabinofuranosidase Enzymes that degrade hemicelluloses Enzymes that degrade cellulose Enzymes that degrade lignin Enzymes that degrade proteins Enzymes that degrade pectic substances Pectin lyase Pectin methylesterase Pectate lyase Polygalacturonases Pectin lyase (transeliminase) Pectin lyase splits a-1,4-linkage between methylgalacturonides (pectin). Pectin lyase Methylgalacturonide Pectin methylesterase Galacturonan Pectate lyase Polygalacturonase Pectin methylesterase (PME) Degrade pectin and pectinic acid to form pectate. Pectate is more soluble and easily be degraded by other pectate degrading enzymes. Pectin lyase Pectin methylesterase Pectate lyase Polygalacturonase Pectate lyase Pectate lyases split the a-(1-4) linkage between galacturonosyl residues in pectate (homogalacturonan, HGA). Pectin lyase Pectin methylesterase Pectate lyase Polygalacturonase Polygalacturonase (pectin glycosidase, PG) Polygalacturonase attacks a-1,4-glycosidic bonds of pectate. Pectin lyase Pectin methylesterase Pectate lyase Polygalacturonase Fusarium oxysporum f.sp. lycopersici (蕃茄萎凋病) The culture from Fusarium oxysporum f.sp. lycopersici can cause the same disease symptom – wilting of shoot and leaves, chlorosis of vascular system, decrease of transpiration. The substance responsible for the cause of the symptom was found to be pectate degrading enzymes. Cell-wall degrading enzymes and plant pathogenesis In order for a cell-wall degrading enzyme to be important in plant pathogenesis, the enzyme should meet several criteria: It should be detected at the infection sites and in infected tissues. The enzyme should reproduce disease symptoms. Enzyme production should be positively correlate with virulence of the pathogen. Cell-wall-degrading enzymes produced by plant pathogens Enzymes that degrade pectic substances Pectin methylesterase Polygalacturonases Pectin lyase Pectate lyase Rhamnogalacturonanases a-L-arabinofuranosidase Enzymes that degrade hemicelluloses Xylanase Glucanase Galactanase Enzymes that degrade hemicelluloses Xylanase b-(1,4)-linkage xylans are degraded by endo- and exoxylase to oligomer or xylose. Glucanase b-(1, 3)-glucan is a minor component of plant tissue, but it is important in plant disease resistance because it occurs primarily in cell wall appositions and papillae in the form of callose in response to fungal penetration. Many pathogens produce b-(1, 3)-glucanase to degrade b(1, 3)-glucane. Galactanase Degrading arabinogalactan Cell-wall-degrading enzymes produced by plant pathogens Enzymes that degrade cellulose Complete degradation of native cellulose to glucose requires three enzymes. b-1,4-glucanase Cellobiohydrolase b-glucosidase Enzymes that degrade lignin Laccase Lignin peroxidase Manganese-dependent peroxidase Tyrosinase Enzymes that degrade proteins Protease END