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Gene Expression Is Not Systematically Linked to Phytoalexin Production During Alfalfa Leaf Interaction with Pathogenic Bacteria

March 1997 , Volume 10 , Number  2
Pages  257 - 267

Christophe Sallaud , 1 Jose Zuanazzi , 2 Joumana El-Turk , 1 Juliette Leymarie , 1 Colette Breda , 1 Dominique Buffard , 1 , 3 Isabelle de Kozak , 1 Pascal Ratet , 1 Philippe Husson , 2 Adam Kondorosi , 1 and Robert Esnault 1 , 3

1Institut des Sciences Végétales, C.N.R.S., 91198 Gif-sur-Yvette Cedex, France; 2Institut de Chimie des Substances Naturelles, C.N.R.S, 91198 Gif-sur-Yvette Cedex, France; 3Université Paris 7, Paris, France

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Accepted 12 November 1996.

During an incompatible interaction between alfalfa leaves and Pseudomonas syringae pv. pisi, flavonoids accumulated between 6 and 24 h, whereas they could not be detected during the first 96 h of a compatible interaction 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 phytoalexin medicarpin was found only at a very low level. Analysis of both the infected and noninfected zones revealed that these flavonoids were detectable only in the infiltrated tissue. Northern hybridizations showed that transcripts encoding for chalcone synthase (CHS), chalcone reductase, chalcone isomerase, and isoflavone reductase (IFR) accumulated in both infiltrated and noninfiltrated zones. Measurements of the CHS and IFR activities in the infiltrated and noninfiltrated zones indicated that the levels of CHS activity were highly increased only in the infiltrated zones, whereas the levels of IFR were very slightly stimulated. These results suggested that an apparently coordinated expression of genes, involved in both the early and late steps of isoflavonoid biosynthesis, is not a sufficient condition for phytoalexin accumulation and that the fundamental regulatory steps might act at the post-transcriptional level.

Additional keywords: CHS and IFR activity , flavonoid biosynthesis , incompatible/compatible interaction , infiltrated/noninfiltrated tissues.

© 1997 The American Phytopathological Society