Virulence of a Pisatin Demethylase-Deficient Nectria haematococca MPVI Isolate Is Increased by Transformation with a Pisatin Demethylase Gene. Lynda M. Ciuffetti. Plant Pathology Department, Cornell University, Ithaca, NY 14853 U.S.A. Hans D. VanEtten. Plant Pathology Department, Cornell University, Ithaca, NY 14853 U.S.A. MPMI 9:787-792. Accepted 5 September 1996. Copyright 1996 The American Phytopathological Society.

Previous studies have shown that all isolates of the fungus Nectria haematococca mating population (MP) VI pathogenic on pea are able to detoxify the pea phytoalexin pisatin by demethylation. This demethylation is catalyzed by pisatin demethylase (pdm), a cytochrome P450, and genes (PDA) encoding this cytochrome P450 have been cloned. Naturally occurring isolates of N. haematococca that lack the ability to demethylate pisatin (Pda^-) normally lack PDA genes and are not pathogenic on pea. In this study, the effect of Pda on virulence was evaluated by introducing 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 revealed that all three transformants, though differing in virulence, were equally tolerant of pisatin and produced similar pdm activity. The results suggest that ability to detoxify pisatin can increase the virulence of N. haematococca MPVI but that for high virulence on pea additional genes are needed.