One enzyme makes a fungal pathogen, but not a saprophyte, virulent on a new host plant.

Certain genes of Nectria haematococca, a fingal pathogen of pea (Pisum sativum), encode pisatin demethylase (pda), a cytochrome P-450 monoxygenase that detoxifies the phytoalexin pisatin. Because pda is required by N.haematococca for pathogenicity on pea, pisatin helps defend pea against N. haematococca. The possibility that pisatin is a general defense factormicrothat is, that pda can confer pathogenicity to fungi not normally pathogenic on peamicrowas investigated. Genes encoding pda were transformed into and highly expressed in Cochliobolus heterostrophus, a fungal pathogen of maize but not of pea, and in Aspergillus nidulans, a saprophytic fungus, neither of which produces a significant amount of pda. Transformants contained at least as much pda as did wild-type N. haematococca. Recombinant C. heterostrophus was normally virulent on maize, but it also caused symptoms on pea, whereas recombinant A. nidulans did not affect pea. Thus, phytoalexins can function in nonspecific resistance of plants to microbes; saprophytes appear to lack genes for basic pathogenicity.