Conserved genes in Sinorhizobium contribute to efficient symbiosis with legumes

For efficient plant growth, legumes form symbiosis with a group of nitrogen-fixing bacteria, Rhizobia. Recent results in Sinorhizobium meliloti Rml 021 have shown that jspA, IppA, and podJ are involved in the production of succinoglycan (EPS-I), an essential exopolysaccharide for symbiosis. However, Rml021 contains a mutant expR allele, which down-regulates galactoglucan (EPS-II), another exopolysaccharide that contributes to symbiosis. To confirm the results seen in Rml021,1 evaluated the functions of orthologous genes in a related rhizobium, S. medicae WSM419, which does not contain a mutant expR allele. EPS-II is known to rescue EPS-I mutants during symbiosis, but jspA or IppA mutations in WSM419 still cause less efficient symbiosis during co-infections. Here I demonstrate that jspA and IppA in WSM419 are important in EPS-I production. EPS-II production in WSM419 does not rescue the symbiosis defect of jspA and IppA mutants. Surprisingly, WSM419 requires podJ for survival in low-nutrient media. Confirming the roles of these genes in S. medicae WSM419 moves us closer to determining the overall effects of these genes on symbiosis in Rhizobia and the possibility of conserved gene functions in other related Alphaproteobacteria.