BROAD-HOST-RANGE EFFECTIVE MUTANTS OF RHIZOBIUM-FREDII STRAINS

Rhizobium fredii strains USDA 192 and USDA 205 form nitrogen-fixing no dules on Asiatic and different unbred soybean cultivars but are ineffe ctive on American-adapted soybeans. Further to our report on the isola tion of spontaneous mutant derivatives of Rh. fredii strains USDA 192 and USDA 205 that were able to form nitrogen-fixing nodules on the Ame rican soybean cultivars Wells and Fred, we show that the symbiotic pro perties of these mutants are very different from those of their parent al strains. All the mutants effectively nodulated other American soybe an cultivars different from that (Wells or Fred) used to select the mu tants and they had also acquired the capacity to form nitrogen-fixing nodules on Cajanus cajan and Macroptilium atropurpureum. These results demonstrated that although Rh. fredii strains USDA 192 and USDA 205 a re either completely ineffective or poorly effective on American soybe an cultivars, C. cajan and M. atropurpureum, they have the inherent ca pacity to establish effective nitrogen-fixing symbioses with these leg umes. Transfer of the symbiotic plasmid of strain AB-7 (a broad host-r ange mutant derivative of USDA 192) to Rh.fredii strain SVQ-127 (USDA1 92-background) produced transconjugants that formed nitrogen-fixing no dules on the American soybean cultivar Williams and M. atropurpureum. These results indicate that the mutation that produces the broad host- range phenotype of mutant strain AB-7 is located in its symblotic plas mid. Rhizobium fredii strain USDA 192 was more competitive than its de rivative AB-7 to nodulate on the unbred soybean cultivar Malayan. More over, the presence of USDA 192 in the inocula severely inhibited nodul ation on soybean cultivar Williams by the effective derivative AB-7. S train SVQ-127 was able to nodulate effectively on the Asiatic soybean cultivar Peking although its symbiotic plasmid was no longer visible b y agarose gel electrophoresis. DNA-DNA hybridization experiments showe d that the symbiotic plasmid, or part of it, was still present in the bacterial genome.