TRANSFER OF THE PEA SYMBIOTIC PLASMID PJB5JI IN NONSTERILE SOIL

Transfer of the pea (Pisum sativum L.) symbiotic plasmid pJB5JI between strains of rhizobia was examined in sterile and nonsterile silt loam soil. Sinorhizobium fredii USDA 201 and HH003 were used as plasmid donors, and symbiotic plasmid-cured Rhizobium leguminosarum 6015 was used as the recipient. The plasmid was carried but not expressed in S. fredii strains, whereas transfer of the plasmid to R. leguminosarum 6015 rendered the recipient capable of nodulating pea plants. Confirmation of plasmid transfer was obtained by acquisition of plasmid-encoded antibiotic resistance genes, nodulation of pea plants, and plasmid profiles. Plasmid transfer in nonsterile soil occurred at frequencies of up to 10(-4) per recipient and appeared to be highest at soil temperatures and soil moisture levels optimal for rhizobial growth. Conjugation frequencies were usually higher in sterile soil than in nonsterile soil. In nonsterile soil, transconjugants were recovered only with strain USDA 201 as the plasmid donor. Increasing the inoculum levels of donor and recipient strains up to 10(9) cells of soil-1 increased the number of transconjugants; peak plasmid transfer frequencies, however, were found at the lower inoculum level of 10(7) cells g of soil-1. Plasmid transfer frequencies were raised in the presence of the pea rhizosphere or by additions of plant material. Transconjugants formed by the USDA 201(pJB5JI) x 6015 mating in soil formed effective nodules on peas.