ABSTRACT
Mesorhizobium huakuii strain 7653R,isolated from nodules of A.sinicus L,contains two indige-nous plasmids,p7653Ra and p7653Rb,the latter being the symbiotic plasmid.We eliminated the plasmids via Tn5-sacB insertion and obtained its symbiotic plasmid-cured derivative 7653RD.Then,we transferred the symbiotic plasmid pJB5JI of Rhizobium leguminosarum bv.Viciae T83K3 into 7653R and 7653RD.The pot plant test showed an increase in competitive ability and symbiotic nitrogen fixation of transconjugant 7653R-197(pJB5JI) compared to 7653R.pJB5JI could not restore the ability of 7653RD to nodulate Astra-galus sinicus.7653RD(pJB5JI) could form ineffective nodules on peas,implying that the symbiotic plasmid pJB5JI could express its function at the chromosomal background of Mesorhizobium huakuii 7653R.We checked the stability of plasmid in transconjugants under free-living and during symbiosis.The results indi-cated pJB5JI could not be detected in some nodule isolates.We amplified kan resistance gene from all transconjugants and nodule isolates which suggested that pJB5JI might fully or partially integrated into the chromosome of recipients.