Effects of targeted replacement of the tomatinase gene on the interaction of Septoria lycopersici with tomato plants

Many plants produce constitutive antifungal molecules belonging to the sapo nin family of secondary metabolites, which have been implicated in plant de fense. Successful pathogens of these plants must presumably have some means of combating the chemical defenses of their hosts. In the oat root pathoge n Gaeumannomyces graminis, the saponin-detoxifying enzyme avenacinase has b een shown to be essential for pathogenicity. A number of other phytopathoge nic fungi also produce saponin-degrading enzymes, although the significance of these for saponin resistance and pathogenicity has not yet been establi shed. The tomato leaf spot pathogen Septoria lycopersici secretes the enzym e tomatinase, which degrades the tomato steroidal glycoalkaloid alpha -toma tine. Here we report the isolation and characterization of tomatinase-defic ient mutants of S. lycopersici following targeted gene disruption. Tomatina se-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 -toma tine, suggesting that nondegradative mechanisms of tolerance are also impor tant. There were no obvious effects of loss of tomatinase on macroscopic le sion formation on tomato leaves, but trypan blue staining of infected tissu e during the early stages of infection revealed more dying mesophyll cells in leaves that had been inoculated with tomatinase-minus mutants. Expressio n 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 su btle differences in the growth of the wild-type and mutant strains during i nfection. Alternatively, tomatinase may be involved in suppression of plant defense mechanisms.