The dominant allele Rj4 in soybean interdicts or restricts the nodulation of plants by certain strains of bacteria, most of which are classified as Bradyrhizobium elkanii, while the recessive allele permits normal nodulation with the same strains. The near isogenic lines BARC-2 (Rj4) and BARC-3 (rj4) are calculated to be 99.95% identical in their nuclear DNA, but differ specifically in the allele present at the Rj4 locus. These lines were used to identify spontaneous mutants of the Rj4-restricted Bradyrhizobium elkanii strain USDA 61 Nal(r) that had the ability to effectively nodulate plants of the Rj4 genotype. Of the eight rare nodules found on roots of soybean plants of the Rj4 genotype inoculated with the genetically marked strain USDA 61 Nal(r), four were identified as containing mutants with the ability to overcome the effects of the Rj4 allele.

To determine the relationship between nodulation restriction by the Rj4 allele of soybean, rhizobitoxine-induced chlorosis, and taxonomic grouping of bradyrhizobia, 119 bradyrhizobial isolates were tested in Leonard jar culture for nodulation response and chlorosis induction. In addition to strain USDA 61, the strain originally reported as defining the Rj4 response, eight other isolates (i.e., USDA 62, 83, 94, 238, 252, 259, 260, and 340) were discovered to elicit the nodulation interdiction of the Rj4 allele. Only 16% of all the bradyrhizobial strains tested induced chlorosis, but seven of the nine strains (78%) interdicted by the Rj4 allele were chlorosis-inducing strains. Furthermore, in tests for antibiotic resistance profile, eight of the nine interdicted strains (89%) were classed in DNA homology group II. This evidence suggests that the Rj4 allele has a positive value to the host plant in shielding it from nodulation by certain chlorosis-inducing bradyrhizobia of a DNA homology group with impaired efficiency of nitrogen fixation with soybean.

Several qualitatively inherited genes (Rj1, Rj2, and Rj4) controlling nodulation response have been reported in soybean, Glycine max (L.) Merr. This study was undertaken to determine whether these alleles were located at distinct genetic loci, or whether one or more were located at the same locus. The F3 progeny from crosses of genetic stocks of each allele were evaluated for nodulation response. The responses due to the Rjl allele were evaluated in the field. The responses due to the Rj2 allele and the Rj4 allele were evaluated in sterile vermiculite in growth trays following controlled inoculation with bradyrhizobial strain USDA 7 or USDA 61, respectively. The F3 seed lots were subdivided to permit assay for more than one allele. Genotypes of F2 plants were rationalized from F3 progeny. Allelism tests established that the Rjl, Rfl and Rj4 alleles were distinct in their segregation and must be located at three distinct genetic loci. The Rjl gene has been located 40 ± 2 centimorgans from the F gene. Both Rj2 and Rj4 were found to segregate independently of the F locus, confirming that they are not at the Rjl locus. Therefore, soybean breeders have the option of constructing soybean genotypes containing all or any combination of these alleles if desired.