Biochemical and Molecular Analyses of Rhizobial Responses to Legume Flavonoids

In addition to inducing the expression of nod genes and the subsequent synthesis of Nod factors in rhizobia, legume flavonoids also elicit a number of other responses in the free-living form of these microsymbionts. These include degradation of the flavonoid itself (Rao and Cooper, 1994), expression of genes which share no homology with nodulation gene promoters (Sadowsky et al., 1988; Perret et al., 1994) and exudation of new proteins into the surrounding environment (Krishnan and Pueppke, 1993). We have used an array of biochemical, chemical analytical and molecular techniques to investigate mechanisms of flavonoid degradation exhibited in rhizobia, changes in flavonoid content of legume root exudates during incubation with rhizobia, the fate of a nod gene-inducing flavonoid during Nod factor synthesis and the nature of gene expression in rhizobia during exposure to flavonoids. This paper summarises the principal findings to date from our research on these four facets of the interaction between flavonoids and rhizobia.