Analysis of the major inducers of the Rhizobium nodA promoter from Vicia sativa root exudate and their activity with different nodD genes.

Root exudate of Vicia sativa contains 7 inducers for the nodA promoter of Rhizobium leguminosarum biovar viciae. Six of these inducers are flavanones. One inducer was identified as 3,5,7,3'-tetrahydroxy-4'-methoxyflavanone, and a second inducer most likely is 7,3'-dihydroxy-4'-methoxyflavanone. The inducing activity of these compounds and the other inducers depends on the nodD gene present in the test strains, which originated either from R. leguminosarum biovars viciae or trifolii, or from R. meliloti. Three inducers are 'common', three others almost exclusively induce the nodA promoter in the presence of the R. leguminosarum biovar viciae nodD gene, and the last one is active with the noD genes of either R. leguminosarum biovar viciae or that of R. meliloti. Testing of a large number of flavonoids revealed two classes of structural features required for inducing ability: (i) features required for induction in general, and (ii), features restricting the inducing ability to (a) specific nodD gene(s). These features are discussed in relation to current models of the process of nodD-mediated transcription activation of the inducible nod genes.