Expression of the Pisatin Detoxifying Genes (PDA) of Nectria haematococca in Vitro and in Planta. Kendal Hirschi. Department of Plant Pathology, University of Arizona, Tucson 85721 U.S.A. Hans VanEtten Department of Plant Pathology, University of Arizona, Tucson 85721 U.S.A. MPMI 9:483-491. Accepted 1 April 1996. Copyright 1996 The American Phytopathological Society.

The phytopathogenic fungus Nectria haematococca detoxifies pisatin, a phytoalexin produced by pea. Pisatin de-methylating ability (a phenotype called Pda) is due to pisatin demethylase (pdm) and the genes encoding this enzyme are called PDA. Some isolates rapidly acquire a high to moderate rate of pisatin demethylating activity in culture in response to pisatin (phenotypes Pda^SHand Pda^SM), while other isolates only slowly demethylate pisatin (phenotype Pda^LL). Here we report that PDA -specific RNA levels increased more quickly in response to pisatin in isolates with PDA genes confering a Pda^SH or Pda^SM phenotype than in isolates with genes confering a Pda^LL phenotype. In addition, the pdm activity of transformants of N. haematococca containing chimeric constructs of PDA^SH and PDA^LL genes in which the 5' regulatory regions of these genes had been switched supports the conclusion that differential expression of PDA genes is responsible for the different Pda phenotypes detected in vitro. Northern analysis of pea tissue infected with isolates carrying PDA^SH or PDA^LL genes indicated that differential induction of these genes also occurred in planta. Only PDA^SH-specific RNA is readily detected in tissue infected with isolates containing PDA^SH and PDA^LL genes. Recently a pisatin biosyn-thetic gene, isoflavone reductase (IFR), has been identified. Using the polymerase chain reaction, qualitative detection of IFR and PDA^SH transcripts in infected tissue were made to assess the relative timing of these genes' expression. No transcripts were detected 6 h after inoculation, but transcripts of both genes were detected at 12 h, suggesting an interplay between the regulatory systems controlling the plants's defense response and the pathogen's counter response.