VIRULENCE OF A PISATIN DEMETHYLASE-DEFICIENT NECTRIA-HAEMATOCOCCA MPVI ISOLATE IS INCREASED BY TRANSFORMATION WITH A PISATIN DEMETHYLASE GENE

Previous studies have shown that all isolates of the fungus Nectria ha ematococca mating population (MP) VI pathogenic on pea are able to det oxify the pea phytoalexin pisatin by demethylation, This demethylation is catalyzed by pisatin demethylase (pdm), a cytochrome P450, and gen es (PDA) encoding this cytochrome P450 have been cloned, Naturally occ urring isolates of N. haematococca that lack the ability to demethylat e pisatin (Pda(-)) normally lack PDA genes and are not pathogenic on p ea, In this study, the effect of Pda on virulence was evaluated by int roducing a pdm-encoding gene, PDAT9, into a Pda(-) isolate, Of the 146 Pda(+) transformants obtained, 52 were significantly more virulent on pea than the Pda(-) recipient and control transformants but none was highly virulent on pea, An analysis of three Pda(+) transformants reve aled that all three transformants, though differing in virulence, were equally tolerant of pisatin and produced similar pdm activity, The re sults suggest that ability to detoxify pisatin can increase the virule nce of N. haematococca MPVI but that for high virulence on pea additio nal genes are needed.