DIFFERENTIAL EXPRESSION OF CDNA CODING FOR CHALCONE REDUCTASE, A KEY ENZYME OF THE 5-DEOXYFLAVONOID PATHWAY, UNDER VARIOUS STRESS CONDITIONS IN MEDICAGO-SATIVA

In legumes, two classes of flavonoids (5-hydroxy- or 5-deoxyflavonoids ) are produced and the key enzyme involved in the 5-deoxyflavonoid syn thesis is chalcone reductase (CHR), co-acting with chalcone synthase ( CHS). We have isolated seven clones encoding CHR from a cDNA library c onstructed with transcripts extracted from Medicago saliva ssp. saliva leaves infiltrated with a suspension of Pseudomonas syringae pv. pisi (incompatible interaction). Analysis of the 3' UTR (untranslated regi on) as well as Southern blot hybridizations indicated that CHR is enco ded by a small multigene family. Due to the pivotal roles played by CH S (involved in the synthesis of both 5-hydroxy- and 5-deoxyflavonoids) we compared the expression of CHR and CHS genes in various organs and under different physiological stresses. Both CHR and CHS were highly expressed in roots and nodules induced by Rhizobium meliloti, but not in leaves, while during flower development only CHS expression was obs erved. CHR was induced in various organs by UV-B treatment. After leaf wounding, CHR, was induced at a higher level than CHS, whereas the ki netics and intensity of their expression during incompatible interacti on with P. syringae pv. pisi were similar. Our results indicate that C HR and CHS expression are under the control of different developmental mechanisms.