Detoxification of the potato phytoalexin rishitin by Gibberella pulicaris

The potato phytoalexin lubimin displayed a complex pattern of metabolism by strains of the potato pathogen Gibberella pulicaris (anamorph: Fusarium sambucinum). The metabolites 15-dihydrolubimin and isolubimin were common to both lubimin-sensitive and lubimin-tolerant strains. In one lubimin-tolerant strain, several unique metabolites, including the tricyclic compounds cyclodehydroisolubimin and cyclolubimin (2-dihydrocyclodehydroisolubimin) and their epoxides, accumulated at the expense of 15-dihydrolubimin and isolubimin. These tricyclic compounds were not toxic to the lubimin-sensitive strain. These results indicate that a likely pathway for lubimin detoxification in some strains of G. pulicaris involves cyclization of isolubimin to cyclodehydroisolubimin via an unsaturated intermediate. All highly virulent strains tested were tolerant of lubimin and were able to convert lubimin to apparently nontoxic products which is indirect evidence that lubimin detoxification contributes to virulence of G. pulicaris on potato tubers.