INDUCTION OF CHALCONE SYNTHASE EXPRESSION BY RHIZOBIA AND NOD FACTORSIN ROOT HAIRS AND ROOTS

Chalcone synthase (CHS) of Vigna unguiculata is encoded by a gene fami ly that is abundantly transcribed in leaves and nodules, Inoculation w ith Rhizobium sp. NGR234, which nodulates V. unguiculata, or with NGR Delta nodABC, a mutant deficient in Nod factor production, induced rap id accumulation of CHS mRNAs in roots and root hairs. As both Nod(+) a nd Nod(-) bacteria provoke responses, induction of CHS gene expression may involve symbiotic or defense responses. Four days after inoculati on with the wild-type Rhizobium sp,, the transcript levels increased i n roots but decreased in root hairs. Use of a region unique to the 5' end of a specific CHS gene (VuCHS1) showed that increases of transcrip t levels in root hairs 24 h after inoculation with both rhizobia were specific to this gene. Transcripts of this gene in roots were only det ectable 4 days after treatment with NGR234, it is possible therefore t hat accumulation of VuCHS1 follows the infection pathway of rhizobia e ntering legume roots. Purified Nod factors induced accumulation of tra nscripts, showing that they might be part of the signal transduction p athway leading to CHS expression.