GENE-EXPRESSION IS NOT SYSTEMATICALLY LINKED TO PHYTOALEXIN PRODUCTION DURING ALFALFA LEAF INTERACTION WITH PATHOGENIC BACTERIA

During an incompatible interaction between alfalfa leaves and Pseudomo nas syringae pv, pisi, flavonoids accumulated between 6 and 24 h, wher eas they could not be detected during the first 96 h of a compatible i nteraction with Xanthomonas campestris pv, alfalfae. Three flavonoids accumulated which were identified as 4',7-dihydroxyflavanone and 4',7- dihydroxyflavone and 2',4,4'-trihydroxychalcone. Surprisingly, the phy toalexin medicarpin was found only at a very low level, Analysis of bo th the infected and noninfected zones revealed that these flavonoids w ere detectable only in the infiltrated tissue. Northern hybridizations showed that transcripts encoding for chalcone synthase (CHS), chalcon e reductase, chalcone isomerase, and isoflavone reductase (IFR) accumu lated in both infiltrated and noninfiltrated zones. Measurements of th e CHS and IFR activities in the infiltrated and noninfiltrated zones i ndicated that the levels of CHS activity were highly increased only in the infiltrated zones, whereas the levels of IFR were very slightly s timulated. These results suggested that an apparently coordinated expr ession of genes, involved in both the early and late steps of isoflavo noid biosynthesis, is not a sufficient condition for phytoalexin accum ulation and that the fundamental regulatory steps might act at the pos t-transcriptional level.