Through homologous recombination, a Rhizobium leguminosarum ohrB mutant was constructed,and its antioxidation and symbiotic nitrogen fixation phenotypes were studied.The growth ability of the ohrB mutant was the same as the wild type under free-living condition.However,cells of the ohrB mutant were sensitive to CuOOH,H2O2 and NaClO.The ohrB mutant was highly sensitive after the short exposure to CuOOH.The results of quantitative real-time PCR showed that the expression of ohrB was not stimulated by exogenous H2O2.The transcription of other antioxidant genes in the ohrB mutant strain was not changed.The ohrB mutant had normal nitrogenase activity and the same colonization ability in the pea rhizosphere compared to the wild type.However,the ohrB gene in 7-day-old pea bacteroids was expressed significantly higher than that in free-living cells.The competition nodule ability of ohrB mutant was reduced compared to that of the wild type.It is indicated that ohrB is essential in antioxidation and competition of nodule ability,playing an important role in antioxidation and symbiosis.