Based on analyzing the transcriptome of Mesorhizobium huakuii 7653R, MCHK_8182 was identified to be upregulated significantly during the stage of symbiotic nitrogen fixation.SOEPCR primers were designed and an MCHK_8182 replacement vector was constructed.Through three parental matting,the replacement vector was introduced into M.huakuii 7653R.The transconjugants were screened under a pressure of sucrose culture.The MCHK_8182 replacement mutant was finally obtained and its correctness was confirmed by PCR and sequencing.The result of the pot experiment showed that the phenotype of host plants induced by MCHK_8182 mut had a remarkably reduced number of root nodules compared with wildtype 7653R,but nodules nitrogenase activity remained unchanged.