EFFECT OF GENE DISRUPTION OF TRICHODIENE SYNTHASE ON THE VIRULENCE OF GIBBERELLA-PULICARIS

Gibberella pulicaris (anamorph: Fusarium sambucinum) accumulates the trichothecene, 4,15-diacetoxyscirpenol (DAS), a potent, host-nonspecific phytotoxin and eukaryotic protein synthesis inhibitor. To investigate the role of DAS in plant pathogenesis, we tested the virulence of five hygromycin-resistant (Hyg(r)) transformants that carry a disrupted gene for trichodiene synthase (Tox5) and no longer produce DAS in vitro or in planta. Virulence levels of all five DAS- transformants were reduced on parsnip root slices, but high levels of virulence were retained on potato tuber slices. Tetrad analysis of one transformant, strain BC51, which carried a single copy of the transforming plasmid, resulted in either cosegregation of the Hyg(r) DAS-, parsnip reduced-virulence phenotypes or in the simultaneous loss of all three traits. In contrast, all progeny from these tetrads, whether DAS+ or DAS-, were highly virulent on potato tubers. Moreover, the Hyg(r) DAS- phenotype was recovered at high frequency from mixed populations after one 3-wk cycle in potato tubers, which suggested that loss of DAS production did not reduce competitiveness of the pathogen. We conclude that DAS accumulation increases the virulence of G. pulicaris on parsnip root but does not affect virulence on potato tubers. The apparent effect of the host on the importance of DAS in virulence is still unexplained, but it suggests that when assessing the role of trichothecenes in plant disease, one should be cautious in generalizing results from one plant species to another.