Chemotaxis of Rhizobium leguminosarum biovar phaseoli towards flavonoid inducers of the symbiotic nodulation genes

Department of Botany, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK ICI Biological Products Business, PO Box 1, Billingham, Cleveland, TS23 1LB, UK ABSTRACT Summary: Chemotaxis of Rhizobium leguminosarum biovar phaseoli RP8002 towards a range of carbohydrates, phenolic compounds and flavonoids was assayed. Xylose (peak response 10 –4 M ), sucrose (peak response 10 –6 M ) and raffinose (peak response 10 –5 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 10 –6 M ) and 3,4-dihydroxybenzoic acid (peak response 10 –4 M ) all evoked strong chemotactic responses. Amongst the nod -inducing flavonoids, apigenin and luteolin were both strong chemoattractants (peaks at 10 –5 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 10 –3 M and 10 –2 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.