Summary: Chemotaxis of biovar RP8002 towards a range of carbohydrates, phenolic compounds and flavonoids was assayed. Xylose (peak response 10 ), sucrose (peak response 10 ) and raffinose (peak response 10 ) 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, -hydroxybenzoic acid (both peak responses 10 ) and 3,4-dihydroxybenzoic acid (peak response 10 ) all evoked strong chemotactic responses. Amongst the -inducing flavonoids, apigenin and luteolin were both strong chemoattractants (peaks at 10 ) 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 -induction, umbelliferone and acetosyringone, both produced strong chemotactic responses, with peaks at 10 and 10 respectively. This evidence is indicative of a role for chemotaxis towards -inducing flavonoids in the initiation of root nodule formation by rhizobia, and also suggests that chemotaxis may influence the host range of the interaction.


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