Regulation of pisatin biosynthesis in naked mesophyll tissue of pea leaves by suppressor (F5) and elicitor isolated from Mycosphaerella pinodes was studied using a radioactive precursor of pisatin synthesis, 14C-phenylalanine. Incorporation of radioactivity became detectable 4.5-6 hr after elicitor treatment and increased thereafter. Concomitant presence of F5 inhibited the pisatin inducing activity of the elicitor completely. Treatment of pea leaves with F5 after elicitor-activation caused a reduction of pisatin synthesis and an increased accumulation of cinnamic acid, an intermediate of pisatin synthesis. F5 was found to inhibit in vitro the activity of phenylalanine ammonia-lyase and cinnamate 4-hydroxylase. The suppressing ability of F5 on pisatin synthesis seemed to be reversible.
