Differential induction of chalcone synthase mRNA activity at the onset of phytoalexin accumulation in compatible and incompatible plant-pathogen interactions.

Changes in the mRNA activity of chalcone synthase, the first enzyme of phenylpropanoid metabolism specific to flavonoid/isoflavonoid biosynthesis, have been investigated in relation to expression of the phytoalexin defense response in race-cultivar specific interactions between hypocotyls of Phaseolus vulgaris and the partially biotrophic fungus Colletotrichum lindemuthianum, causal agent of anthracnose. In an incompatible interaction (host resistant) there is an early but localized increase in chalcone synthase mRNA activity prior to the onset of accumulation of the phenylpropanoid-derived phytoalexin phaseoflin and expression of hypersensitive resistance. In contrast, in a compatible interaction (host susceptible) there is no induction of mRNA activity in the early stages of infection but rather a delayed, widespread increase during attempted lesion limitation at the onset of symptom development. The data indicate that control of phytoalexin gene expression is a key early component in the defense responses of biologically stressed cells during a race-cultivar specific host-pathogen interaction.