Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
92 Postharvest Diseases of Fruits and Vegetables
G. PATHOGENESIS-RELATED PROTEINS
Several glucanohydrolases found in plants, such as chitinase and
P-l,3-glucanase, have received considerable attention as they are
considered to play a major role in constitutive and inducible resistance
against pathogens (El Ghaouth, 1994). These enzymes are lowmolecular-weight
proteins, frequently referred to as pathogenesis-related
(PR) proteins. They hydrolyze the major components of fungal cell walls
which results in the inhibition of fungal growth (Schlumbaum et al., 1986).
The chitinases, which are ubiquitous enzymes of bacteria, fungi,
plants and animals, hydrolyze the P-l,4-linkage between the
N-acetylglucosamine residues of chitin, a polysaccharide of the cell wall
of many fungi (Neuhaus, 1999). The glucanases, which are abundant,
highly regulated enzymes, widely distributed in seed-plant species, are
able to catalyze endo-type hydrolytic cleavage of glucosidic linkages in
P-l,3-glucans (Leubner-Metzger and Meins, 1999).
The chitinases and P-l,3-glucanases are stimulated by infection and in
response to elicitors (Bowles, 1990). Chitosan, a P-l,4-glucosamine
polymer found as a natural constituent in cell walls of many fungi, is
capable of both directly interfering with fungal growth and eliciting
defense mechanisms in the plant tissue (El Ghaouth, 1994). Postharvest
treatment with this elicitor has been found to activate antifungal
hydrolases in several fruits: treatment of strawberries, bell peppers and
tomato fruits with chitosan induced the production of hydrolases, which
remained elevated for up to 14 days after treatment and reduced lesion
development by Botrytis cinerea (El Ghaouth and Arul, 1992; El Ghaouth
et al., 1997). When applied as a stem scar treatment to bell peppers,
chitosan stimulated the activities of chitinase, chitosanase and
p-l,3-glucanase. Being capable of degrading fungal cell walls, these
antifungal hydrolases are considered to play a major role in disease
resistance (Schlumbaum et al., 1986; El Ghaouth, 1994).
In chitosan-treated bell peppers the production of lytic enzymes was
followed by a substantial reduction of chitin content of the cell walls of
invading fungal hyphae, expressed as a reduction of chitin labeling in the
walls (El Ghaouth and Arul, 1992). The hypothesis of fungal infection
suppression was also supported by the finding that invading fungal hyphae
were mainly restricted to the epidermal cells ruptured during wounding (El
Ghaouth et al., 1994). It was thus suggested that the deliberate stimulation
and activation of PR proteins in the fruit tissue might lead to disease
suppression by enhancing host resistance to infection.
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