Many plants produce constitutive antifungal molecules belonging to the
saponin family of secondary metabolites, which have been implicated in plant defense. Successful pathogens of these plants must presumably have some means of combating the chemical defenses of their hosts. In the oat root pathogen Gaeumannomyces graminis, the
saponin-detoxifying
enzyme avenacinase has been shown to be essential for pathogenicity. A number of other phytopathogenic fungi also produce
saponin-degrading
enzymes, although the significance of these for
saponin resistance and pathogenicity has not yet been established. The tomato leaf spot pathogen Septoria lycopersici secretes the
enzyme tomatinase, which degrades the tomato steroidal glycoalkaloid
alpha-tomatine. Here we report the isolation and characterization of
tomatinase-deficient mutants of S. lycopersici following targeted gene disruption.
Tomatinase-minus mutants were more sensitive to
alpha-tomatine than the wild-type strain. They could, however, still grow in the presence of 1 mM
alpha-tomatine, suggesting that nondegradative mechanisms of tolerance are also important. There were no obvious effects of loss of
tomatinase on macroscopic lesion formation on tomato leaves, but
trypan blue staining of infected tissue during the early stages of
infection revealed more dying mesophyll cells in leaves that had been inoculated with
tomatinase-minus mutants. Expression of a defense-related basic beta-1,3 glucanase gene was also enhanced in these leaves. These differences in plant response may be associated with subtle differences in the growth of the wild-type and mutant strains during
infection. Alternatively,
tomatinase may be involved in suppression of plant defense mechanisms.