EPISTASIS OF RJ1 NONNODULATION OF SOYBEAN TO NODULATION BY SINORHIZOBIUM-FREDII

Utilization of highly efficient N-fixing, improved strains of Bradyrhi zobium japonicum in soybean [Glycine max (L.) Merr.] inoculants has be en hampered because these strains cannot compete in the rhizosphere wi th indigenous strains. Development of host genotypes that selectively nodulate with introduced strains could improve the field response of s oybean to inoculation. Our objective was to determine whether Sinorhiz obium fredii USDA 205 could overcome the nodulation block in soybean p lants homozygous for the rj1 gene. The rj1 gene produces a nonnondulat ing phenotype with most strains of B. japonicum. 'Peking' is homozygou s for the recessive allele conferring the ability to nodulate with S. fredii. Peking was crossed with an isoline of 'Harosoy' beating rj1. W e identified F2 plants homozygous for the recessive allele for S. fred ii nodulation by selecting for the ability to form nodules with S..fre dii. F3 progeny of these F2 selections segregated for the ability to n odulate with B. japonicum 110, indicating segregation for rj1. The res ults indicate that the two traits are controlled by different loci tha t are not tightly linked. In a separate test, F2 plants homozygous for rj1 were selected and allowed to self. Ten seeds from each of four F2 rj1 plants were inoculated with S. fredii, but none formed nodules, s howing that rj1 nonnodulation is epistatic to nodulation by S. fredii. We surmise that rj1 cannot be used in conjunction with an S. fredii i noculum to overcome the problem of bacterial competition for nodule oc cupancy.