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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
CHAPTER 5
ATTACK MECHANISMS OF THE PATHOGEN
A. ENZYMATIC ACTIVITY
Many plant pathogens produce extracellular products that may
influence their growth, be determinant factors in their pathogenic
capability, or contribute to their pathogenicity or virulence. Such
products may include enzymes capable of hydrolyzing the cuticle layer of
the epidermic plant cells (Kolattukudy, 1985), cell wall degrading
enzymes, toxins, hormones, siderophores, DNA and signaling molecules
(Salmond, 1994), or factors capable of degrading host chemical defense
agents (Staples and Mayer, 1995). We will concentrate on the secretion of
extracellular enzymes and toxins, the two major attack means with
which the pathogen may be equipped, and the ability of the pathogen to
detoxify host defense compounds.
Cutinase
The enzyme cutinase, which hydrolyzes the primary alcohol ester
linkages of the polymer, cutin (Kolattukudy, 1985), has been found in
many plant pathogenic fungi, including Botrytis cinerea (Shishiyama et
al., 1970). However, the importance of this enzyme for penetration of the
cuticle seems to be a matter of debate (Stahl and Schafer, 1992). Many
lines of evidence suggest that cuticular penetration of Colletotrichum
gloeosporioides into the host is associated with the induction of
extracellular cutinase production, and that insertion of the cutinase gene
into this pathogen facilitates infection of an intact host (Dickman et al.,
1989). Following the finding that cutinase secretion was also directed
towards the region that penetrates the host, the infection peg of an
appressorium produced by Colletotrichum spores (Podilia et al., 1995), it
was suggested that the inhibition of this enzyme by active inhibitors
could result in delay in the penetration of germinating appressoria into
the host (Prusky, 1996).
Secretion of extracellular cutinase has been reported for Aspergillus
flavus (Guo et al., 1996) when grown on purified cutin as the sole carbon
source. Pretreatment of corn kernels with A flavus culture filtrate or
with bacterial cutinase promoted increased levels of aflatoxin production
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