Synthesis of the phytoalexin pisatin by a methyltransferase from pea

Previous labeling studies in vivo suggest that the terminal step of (+)pisatin biosynthesis in Pisum sativum L. is methylation of the phenol (+)6a-hydroxymaackiain (HMK). We have found that extracts from pea seedlings perform this reaction, using S-adenosylmethionine as the methyl donor. The enzyme activity was induced by microbial infection or treatment with CuCl(2), which elicit pisatin synthesis, though some activity was also present in healthy tissues. It has been reported that CuCl(2)-treated pea tissue provided with (-)HMK or (-)maackiain can synthesize (-)pisatin. Our extract showed no methyltransferase activity dependent on either of these substrates. Methylation of (+)maackiain was detectable, but much slower than that of (+)HMK.