Broad host‐range effective mutants of Rhizobium fredii strains

Abstract

Rhizobium fredii strains USDA 192 and USDA 205 form nitrogen‐fixing nodules on Asiatic and different unbred soybean cultivars but are ineffective on American‐adapted soybeans. Further to our report on the isolation of spontaneous mutant derivatives of Rh. fredii strains USDA 192 and USDA 205 that were able to form nitrogen‐fixing nodules on the American soybean cultivars Wells and Fred, we show that the symbiotic properties of these mutants are very different from those of their parental strains. All the mutants effectively nodulated other American soybean cultivars different from that (Wells or Fred) used to select the mutants and they had also acquired the capacity to form nitrogen‐fixing nodules on Cajanus cajan and Macroptilium atropurpurcum. These results demonstrated that although Rh. fredii strains USDA 192 and USDA 205 are either completely ineffective or poorly effective on American soybean cultivars, C. cajan and M. atropurpureum, they have the inherent capacity to establish effective nitrogen‐fixing symbioses with these legumes. Transfer of the symbiotic plasmid of strain AB‐7 (a broad host‐range mutant derivative of USDA 192) to Rh. fredii strain SVQ‐127 (USDA192‐background) produced transconjugants that formed nitrogen‐fixing nodules 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 symbiotic plasmid. Rhizobium fredii strain USDA 192 was more competitive than its derivative AB‐7 to nodulate on the unbred soybean cultivar Malayan. Moreover, the presence of USDA 192 in the inocula severely inhibited nodulation on soybean cultivar Williams by the effective derivative AB‐7. Strain SVQ‐127 was able to nodulate effectively on the Asiatic soybean cultivar Peking although its symbiotic plasmid was no longer visible by agarose gel electrophoresis. DNA‐DNA hybridization experiments showed that the symbiotic plasmid, or part of it, was still present in the bacterial genome.