The LPS O-antigen in photosynthetic Bradyrhizobium strains is dispensable for the establishment of a successful symbiosis with Aeschynomene legumes

The photosynthetic bradyrhizobia are able to use a Nod-factor independent process to induce nitrogen-fixing nodules on some semi-aquatic Aeschynomene species. These bacteria display a unique LPS O-antigen composed of a new sugar, the bradyrhizose that is regarded as a key symbiotic factor due to its non-immunogenic character. In this study, to check this hypothesis, we isolated mutants affected in the O-antigen synthesis by screening a transposon mutant library of the ORS285 strain for clones altered in colony morphology. Over the 10,000 mutants screened, five were selected and found to be mutated in two genes, rfaL, encoding for a putative O-antigen ligase and gdh encoding for a putative dTDP-glucose 4,6-dehydratase. Biochemical analysis confirmed that the LPS of these mutants completely lack the O-antigen region. However, no effect of the mutations could be detected on the symbiotic properties of the mutants indicating that the O-antigen region of photosynthetic Bradyrhizobium strains is not required for the establishment of symbiosis with Aeschynomene