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 ofPhaseolus 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 phaseollin 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 hostpathogen interaction.Plant disease resistance involves not only static protection but also active defense mechanisms, prominent among which is the induced accumulation of host-synthesized phytoalexin antibiotics (1). Phytoalexins can be induced by glycan, glycoprotein, and lipid elicitors present in fungal and bacterial cell walls and culture filtrates (2) and by a variety of structurally unrelated, artificial inducers (3). Accumulation is largely a result of increased synthesis from remote precursors (4, 5) and increases in the levels of appropriate biosynthetic enzymes are observed at the onset of the defense response (6).In cell suspension cultures of dwarf French bean (Phaseolus vulgaris L.) elicitor treatment causes marked but transient increases in the rate of synthesis of phenylalanine ammonia-lyase (EC 4.3.1.5), the first enzyme in general phenylpropanoid metabolism, and chalcone synthase, the first enzyme specific to the flavonoid/isoflavonoid branch pathway of phenylpropanoid biosynthesis, concomitant with the onset of accumulation of phaseollin and related isoflavonoid phytoalexins (7)(8)(9)(10). The transient increases in enzyme synthesis reflect increases in the levels of the corresponding mRNA activities (11), which, together with changes in the apparent stability of the enzymes in vivo, are responsible for the marked increases in enzyme activity that control phytoalexin accumulation in response to elicitor (9,12 (19). Hypocotyls, a natural site of the disease, can be reproducibly infected, without prior mechanical damage, by surface inoculation with conidia, the natural infective propagule. Detailed physiological studies have established the biological importance of phytoalexin accumulation in this system (20), which, therefore, provides a model for biochemical analysis of disease resistance. In the present paper we report clear spatial and temporal differences in the pattern of induction of chalcone synthase mRNA activi...
