Inheritance of host-controlled restriction of nodulation by Bradyrhizobium japonicum strain USDA 110

The use of soybean [Glycine max (L.) Merr.] genotypes which specifically restrict nodulation by competitive indigenous Bradyrhizobium populations has been proposed as an approach to favor increased nodulation by more effective, inoculant-quality strains. Soybean plant introduction (PI) PI 417566 restricts nodulation by Bradyrhizobium japonicum (Kirchner, Jordan) strains in serogroup 110. In this study, the interaction of PI 417566 with strains in serogroup 110 is being used as a model system to determine whether host- controlled nodulation restriction can be used to alter the competitiveness of indigenous bradyrhizobia. The objectives of this study were to determine the type of inheritance and the number of genes in PI 417566 which condition restriction of nodulation by strain USDA 110 and whether this gene(s) could function to prevent nodulation of serogroup lift strains under field conditions. Inheritance of host-controlled nodulation restriction was determined by crossing PI 417566 with the nonrestricting cultivars Agassiz and Evans. The nodulation phenotype of 126 F₂ plants from Agassiz x PI 417566 and 44 F₂ plants from Evans x PI 417566 was assessed in growth chamber studies. In the former cross, 98 nodulating and 28 non-nodulating plants, and in the latter, 32 nodulating and 12 non-nodulating plants were observed. In both cases, a Chi-squared test indicated a good fit to a 3:1 segregation ratio, indicating that a single recessive gene conditions restricted nodulation in PI 417566. A 2-yr field study utilizing F(2:4) and F(2:5) progeny from Agassiz x PI 417566 and F(2:4) progeny from Evans x PI 417566 demonstrated that the lines had the ability to restrict nodulation by serogroup 110 strains under field conditions and that restriction occurred even in the presence of large numbers (5 X 10⁷ cells/g soil) of added USDA 110.