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Epistasis of rj1 Nonnodulation of Soybean to Nodulation by Sinorhizobium fredii

Public Health Res. Inst., 455 First Ave., New York, NY 10016
Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 27695-7629

^* Corresponding author.

Utilization of highly efficient N-fixing, improved strains of Bradyrhizobium japonicum in soybean [Glycine max (L.) Merr.] inoculants has been hampered because these strains cannot compete in the rhizospbere with indigenous strains. Development of host genotypes that selectively nodulate with introduced strains could improve the field response of soybean to inoculation. Our objective was to determine whether Sinorhizobium fredii USDA 205 could overcome the nodulation block in soybean plants homozygous for the rj1 gene. The rj1 gene produces a nonnondulating phenotype with most strains of B.japonicum. ‘Peking’ is homozygous for the recessive allele conferring the ability to nodulate with S. fredii. Peking was crossed with an isoline of ‘Harosoy’ bearing rj1. We identified F₂ plants homozygous for the recessive allele for S. fredii nodulation by selecting for the ability to form nodules with S. fredii. F₃ progeny of these F₂ selections segregated for the ability to nodulate with B. japonicum 110, indicating segregation for rj1. The results indicate that the two traits are controlled by different loci that 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, showing that rj1 nonnodulation is epistatic to nodulatlon by S. fredii. We surmise that rj1 cannot be used in conjunction with an S. fredii inoculum to overcome the problem of bacterial competition for nodule occupancy.

Received for publication April 27, 1992.