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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