Rhizobial surface polysaccharides are required for nodule formation on the roots of at least some legumes but the mechanism(s) by which they act are yet to be determined. As a first step to investigate the function of exopolysaccharide (EPS) in the formation of determinate nodules, we isolated Mesorhizobium loti mutants affected in various steps of EPS biosynthesis and characterised their symbiotic phenotypes on two Lotus species. The wild-type M. loti strain R7A produced both high-molecular-weight EPS and lower-molecular-weight (LMW) polysaccharide fractions whilst most mutant strains produced only LMW fractions. Mutants affected in predicted early biosynthetic steps (e.g. exoB) formed nitrogen-fixing nodules on L. corniculatus and L. japonicus cv. Gifu, whereas mutants affected in mid/late biosynthetic steps (e.g. exoU) induced uninfected nodule primordia, and occasionally a few infected nodules following a lengthy delay. These mutants were disrupted at the stage of infection thread (IT) development. Symbiotically-defective EPS and Nod factor mutants functionally complemented each other in co-inoculation experiments. The majority of full-length ITs observed harbored only the EPS mutant strain and did not show bacterial release, whereas the nitrogen-fixing nodules contained both mutants. Examination of the symbiotic proficiency of the exoU mutant on various L. japonicus ecotypes revealed both host and environmental factors were linked to the requirement for EPS. These results reveal a complex function for M. loti EPS in determinate nodule formation and suggest that EPS plays a signalling role at both the stages of IT initiation and bacterial release.
Kelly S, Muszyński A, Kawaharada Y, Hubber AM, Sullivan J, Sandal N, Carlson R, Stougaard J, Ronson C. (2012). Mol Plant Microbe Interact. Nov 7. [Epub ahead of print]