Organization and differential activation of a gene family encoding the plant defense enzyme chalcone synthase in Phaseolus vulgaris

Abstract

Chalcone synthase (CHS) catalyzes the first and key regulatory step in the branch pathway of phenylpropanoid biosynthesis specific for synthesis of ubiquitous flavonoid pigments and UV protectants. In bean (Phaseolus vulgaris L.) and other members of the Leguminoseae, chalcone synthase is also involved in the synthesis of the isoflavonoid-derived phytoalexin antibiotics characteristic of this family. We have demonstrated that the haploid genome of bean contains a family of about six to eight CHS genes, some of which are tightly clustered. Treatment of bean cells with fungal elicitor activates several of these genes leading to the accumulation of at least five and probably as many as nine distinct CHS transcripts encoding a set of CHS isopolypeptides of Mr 42-43 kDa but with differing pI in the range pH 6-7. In elicited cells specific transcripts and encoded polypeptides are differentially induced with respect to both the extent and kinetics of accumulation. Wounding or infection of hypocotyl tissue also activates several CHS genes with marked differences in the pattern of accumulation of specific transcripts and encoded polypeptides in wounded compared to infected tissue or elicited cells, indicating operation of more than one cue for defense gene activation. Illumination induces accumulation of a different set of CHS transcripts including only one of the set hitherto demonstrated to be induced by biological stress. The organization and differential regulation of the CHS gene family in bean are discussed in relation to the functions of this enzyme in adaptative and protective responses to diverse environmental stresses.