Chemotaxis of Rhizobium leguminosarum biovar phaseoli towards Flavonoid Inducers of the Symbiotic Nodulation Genes

Chemotaxis of Rhizobium leguminosarum biovar phaseoli RP8002 towards a range of carbohydrates, phenolic compounds and flavonoids was assayed. Xylose (peak response M) were strong chemoattractants amongst the carbohydrates, whilst glucose, fructose, galactose and maltose produced little or no detectable response. Of the monocyclic phenolic compounds, vanillyl alcohol, p-hydroxybenzoic acid (both peak responses M) and 3,4-dihydroxybenzoic acid (peak response 1 0-4 M) all evoked strong chemotactic responses. Amongst the nod-inducing flavonoids, apigenin and luteolin were both strong chemoattractants (peaks at M) while naringenin produced a very low response. Competition experiments suggest that apigenin and luteolin are recognized by a common receptor, but that there exists a separate receptor for luteolin alone. The inhibitors of nod-induction, umbelliferone and acetosyringone, both produced strong chemotactic responses, with peaks at loW3 M and M respectively. This evidence is indicative of a role for chemotaxis towards nod-inducing flavonoids in the initiation of root nodule formation by rhizobia, and also suggests that chemotaxis may influence the host range of the interaction. M), sucrose (peak response M) and raffinose (peak response