RHIZOBIUM INOCULATION AND PHYSICAL WOUNDING RESULT IN THE RAPID INDUCTION OF THE SAME CHALCONE SYNTHASE COPY IN TRIFOLIUM-SUBTERRANEUM

The gene of genes encoding chalcone synthase (CHS) in the legume Trifo lium subterraneum (subterranean clover) were induced within 6 hr after inoculation with Rhizobium leguminosarum bv. trifolii strain ANU843. No induction was found in uninoculated controls or plants inoculated w ith either the nodulation-deficient R. l. bv. trifolii strain ANU845 ( pSym(-)) or R. meliloti strain 1021, which is capable of nodulating al falfa but not clover. Morphological examination of the interaction bet ween the legume and bacteria in this system showed that root hair dist ortion (a marker of the early events in the interaction) was apparent within 10 hr after inoculation. This indicated that CHS induction coul d occur before any detectable sign of rhizobial penetration of root ha irs. The addition of a crude preparation of R. l. bv. trifolii lipooli gosaccharide signals (Nod metabolites) to the plant growth medium had no effect on the expression of CHS over 36 hr, although root hair dist ortion was apparent over this time. These treatments were then contras ted with physical wounding. Wounding the plants led to a rapid inducti on of CHS, occurring within 2 hr. Sequence analysis of cloned CHS cDNA from pools sampled after Rhizobium inoculation or wounding treatments showed the gene designated CHS5 was the major CHS species in both tre atments. Conserved sequences were found in promoters of CHS5 and soybe an Gmchs7, a gene which has overlapping expression patterns. These fin dings support the view that the induction of the phenylpropanoid pathw ay is involved in the very early events of the Rhizobium infection of legumes.