Multiple copies of rosR and pssA genes enhance exopolysaccharide production, symbiotic competitiveness and clover nodulation in Rhizobium leguminosarum bv. trifolii.

Rhizobium leguminosarum bv. trifolii exopolysaccharide (EPS) plays an important role in determining symbiotic competence. The pssA gene encoding the first glucosyl-IP-transferase and rosR encoding a positive transcriptional regulator are key genes involved in the biosynthesis and regulation of EPS production. Mutation in pssA resulted in deficiency in EPS production and rosR mutation substantially decreased the amount of EPS. Both mutants induced nodules but the bacteria were unable to fix nitrogen. Defective functions of pssA and rosR mutants were fully restored by wild type copies of the respective genes. Introduction of multiple rosR and pssA gene copies on the plasmid vector pBBR1MCS-2 into five R. leguminosarum bv. trifolii nodule isolates resulted in significantly increased growth rates, EPS production and the number of nodules on clover roots. Increase in fresh and dry shoot mass of clovers and nodule occupation was also statistically significant. Interestingly, additional copies of pssA but particularly rosR gene, increased strains' competitiveness in relation to the wild type parental strains nearly twofold. Overall, experimental evidence is provided that increased amount of EPS beneficially affects R. leguminosarum bv. trifolii competitiveness and symbiosis with clover.