PLASMIDS PJP4 AND R68.45 CAN BE TRANSFERRED BETWEEN POPULATIONS OF BRADYRHIZOBIA IN NONSTERILE SOIL

IncP plasmid r68.45, which carries several antibiotic resistance genes , and IncP plasmid pJP4, which contains genes for mercury resistance a nd 2,4-dichlorophenoxyacetic acid degradation, were evaluated for thei r ability to transfer to soil populations of rhizobia. Transfer of r68 .45 was detected in nonsterile soil by using Bradyrhizobium japonicum USDA 123 as the plasmid donor and several Bradyrhizobium sp. strains a s recipients. Plasmid transfer frequencies ranged up to 9.1 x 10(-5) i n soil amended with 0.1% soybean meal and were highest after 7 days wi th strain 3G4b4-RS as the recipient. Transconjugants were detected in 7 of 500 soybean nodules tested, but the absence of both parental stra ins in these nodules suggests that plasmid transfer had occurred in th e soil, in the rhizosphere, or on the root surface. Transfer of degrad ative plasmid pJP4 was also evaluated in nonsterile soil by using B. j aponicum USDA 438 as the plasmid donor and several Bradyrhizobium sp. strains as recipients. Plasmid pJP4 was transferred only when strains USDA 110-ARS and 3G4b4-RS were the recipients. The plasmid transfer fr equency was highest for strain 3G4b4-RS (up to 7.4 x 10(-6)). Mercury additions to soil, ranging from 10 to 50 mug/g of soil, did not affect population levels of parental strains or the plasmid transfer frequen cy.