Conditional Requirement for Exopolysaccharide in the Mesorhizobium-Lotus Symbiosis

Rhizobial surface polysaccharides are required for; nodule formation on the roots of at least some legumes but the mechanisms 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 characterized their symbiotic phenotypes on two Lotus spp. The wild-type M. loti R7A produced both high molecular weight EPS. and lower molecular weight (LMW) polysaccharide fractions whereas most mutant strains produced only LMW fractions. Mutant's affected in predicted early biosynthetic steps (e.g., exoB) formed nitrogen-fuzing nodules on Lotus corniculatus and L. japonicus 'Gifu', whereas mutants affected in mid or late biosynthetic steps (e.g., exoU) induced uninfected nodule primordia and, occasionally, a few infected nodules following a lengthy delay. The Se 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 IT 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 that 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 signaling role at the stages' of both IT initiation and bacteria release.