Agriculture Reference
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(1998) inactivation of the endopolygalacturonase gene Bcpg1 has no effect on
penetration efficiency, but results in a reduction of the spread of the pathogen into
the host tissue. Similar results have been obtained for Fusarium oxysporum f.sp.
lycopersici . Neither endopolygalacturonases nor exopolygalacturonases are essential
for pathogenicity on tomato plants (Di Pietro and Roncero, 1998; Garcia-Maceira
et al. , 2000).
Gene disruption experiments constructing specific mutants lacking one or more
CWDEs have been used in order to better understand the role of these enzymes in
pathogenicity. The first targeted mutation of a CWDE gene in a fungus was reported
for an endopolygalacturonase from C. carbonum (Scott-Craig et al. , 1990). The
mutant displayed no reduction in virulence on maize, its natural host.
Detoxifying enzymes . Pathogenic fungi are confronted with toxic secondary
metabolites of their host plants. These substances may be preformed, or as in the
case of phytoalexins, induced by pathogen attack. Some of these metabolites have
been shown to induce the synthesis of fungal enzymes required for their
detoxification (for review, see Morrissey and Osbourn, 1999).
In Nectria haematococca , the causal agent of stem and root rot disease of the
common pea, pisatin demethylase - a cytochrome P450 monooxygenase - is
induced to break down the pea antimicrobial isoflavonoid compound pisatin
(Matthews and VanEtten, 1983; Hirschi and VanEtten, 1996). Disruption of a pisatin
demethylase gene showed that lack of the encoded enzyme reduces but does not
eliminate the virulence of N. haematococca (Wasmann and VanEtten, 1996).
Consequently, this enzyme is not essential for pathogenicity.
Another detoxifying enzyme is avenacinase produced by the oat root-infecting
pathogen Gaeumannomyces graminis var. avenae which detoxifies the triterpenoid
avenacin. Fungal defect mutants that have lost the ability to produce avenacinase have
been shown to be unable to infect oats, indicating that this enzyme is an essential
determinant of host range for G. graminis var. avenae (Bowyer et al. , 1995).
Different pathogens of tomato, including Septoria lycopersici , Botrytis cinerea ,
Fusarium oxysporum f. sp. lycopersici , produce tomatinases that detoxify the
steroidal glycoalkaloid tomatine (Sandrock and VanEtten, 1998; Osbourn, 1996). As
in the case of pisatin demethylase, the tomatinase of S. lycopersici was not essential
for pathogenicity (Osbourn et al. , 1995). Nevertheless, recently it was shown that
the degradation product of the S. lycopersici tomatinase suppresses the host plant
defense responses, indicating a dual function for this enzyme: the degradation of the
tomatine and suppression of disease resistance (Bouarab et al. , 2002).
(b) Colonization supported by toxins
One successful strategy adopted by many necrotrophic pathogens is the production
of low-molecular weight secondary metabolites with phytotoxic activity. There is
great diversity of fungal toxins in structure as well as in their modes of action. The
non-host selective toxins typically affecting fundamental processes potentially have
activity on the host as well as on non-host plants. Consequently, evidence for their
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