Exopolysaccharide is detrimental for the symbiotic performance of Sinorhizobium fredii HH103 mutants with a truncated lipopolysaccharide core.

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The nitrogen-fixing rhizobia-legume symbiosis relies on a complex interchange of molecular signals between the two partners during the whole interaction. On the bacterial side, different surface polysaccharides, such as lipopolysaccharide (LPS) and exopolysaccharide (EPS), might play important roles for the success of the interaction. In a previous work we studied two Sinorhizobium fredii HH103 mutants affected in the rkpK and lpsL genes, which are responsible for the production of glucuronic acid and galacturonic acid, respectively. Both mutants produced an altered LPS, and the rkpK mutant, in addition, lacked EPS. These mutants were differently affected in symbiosis with Glycine max and Vigna unguiculata, with the lpsL mutant showing a stronger impairment than the rkpK mutant. In the present work we have further investigated the LPS structure and the symbiotic abilities of the HH103 lpsL and rkpK mutants. We demonstrate that both strains produce the same LPS, with a truncated core oligosaccharide devoid of uronic acids. We show that the symbiotic performance of the lpsL mutant with Macroptilium atropurpureum and Glycyrrhiza uralensis is worse than that of the rkpK mutant. Introduction of an exoA mutation (which avoids EPS production) in HH103 lpsL improved its symbiotic performance with G. max, M. atropurpureum, and G. uralensis to the level exhibited by HH103 rkpK, suggesting that the presence of EPS might hide the truncated LPS produced by the former mutant.