The rkpU gene of Sinorhizobium fredii HH103 is required for bacterial K-antigen polysaccharide production and for efficient nodulation with soybean but not with cowpea

摘要

In this work, the role of the rkpU and rkpJ genes in the production of the K-antigenpolysaccharides (KPS) and in the symbiotic capacity of Sinorhizobium fredii HH103, a broadhost-range rhizobial strain able to nodulate soybean and many other legumes, was studied. TherkpJ- and rkpU-encoded products are orthologous to Escherichia coli proteins involved incapsule export. S. fredii HH103 mutant derivatives were contructed in both genes. To ourknowledge, this is the first time that the role of rkpU in KPS production has been studied inrhizobia. Both rkpJ and rkpU mutants were unable to produce KPS. The rkpU derivative alsoshowed alterations in its lipopolysaccharide (LPS). Neither KPS production nor rkpJ and rkpUexpression was affected by the presence of the flavonoid genistein. Soybean (Glycine max) plantsinoculated with the S. fredii HH103 rkpU and rkpJ mutants showed reduced nodulation and clearsymptoms of nitrogen starvation. However, neither the rkpJ nor the rkpU mutants weresignificantly impaired in their symbiotic interaction with cowpea (Vigna unguiculata). Thus, wedemonstrate for the first time to our knowledge the involvement of the rkpU gene in rhizobial KPSproduction and also show that the symbiotic relevance of the S. fredii HH103 KPS depends onthe specific bacterium–legume interaction.