Inheritance of soybean nodulation response with a fast-growing strain of Rhizobium

Until recently, the soybean [Glycine max (L.) Merr.], was thought to form a successful nitrogen fixing symbiosis only with slow-growing strains of Rhizoblum japonicum. However, in 1982 the soybean cultivar Peking was reported to nodulate and fix nitrogen with several newly isolated fast-growing rhizobial strains. Several U.S.-bred cultivars did not form a successful symbiosis with these fast-growing strains. In this study, the mode of inheritance of the nodulation response was determined by hybridizing the parent cultivars Peking and Kent and then evaluating the parents and the subsequent F1, F2 and F3 progeny for nodulation response with the fast-growing rhizobial strain USDA 205. Seedlings of Peking displayed normal nodulation. Kent and the F1 hybrids exhibited Ineffective nodulation, the presence of swellings or small rudimentary nodules rather than true nodules on the roots. The F2 population fit a 3:1 segregation (ineffective/nodulated) (P = 0.95–0.80). The F3 lines fit a 1:2:1 segregation (P = 0.95–0.80). Therefore, a genetic locus characterized by Mendellan segregation with a major phenotypic effect on nodulation was found to govern the nodulation response of Kent and Peking soybeans with the fast-growing rhizobial strain USDA 205. Peking carries the recessive allele conditioning effective nodulation and Kent carries the dominant allele conditioning ineffective nodulation.