NEAR-ISOGENIC LINES OF SOYBEANS AS TOOLS TO IDENTIFY NODULATION SPECIFIC MUTANTS OF BRADYRHIZOBIUM-ELKANII

The dominant allele Rj4 in soybean interdicts or restricts the nodulat ion of plants by certain strains of bacteria, most of which are classi fied as Bradyrhizobium elkanii, while the recessive allele permits nor mal 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 n uclear DNA, but differ specifically in the allele present at the Rj4 l ocus. 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 eig ht rare nodules found on roots of soybean plants of the Rj4 genotype i noculated with the genetically marked strain USDA 61 Nal(r), four were identified as containing mutants with the ability to overcome the eff ects of the Rj4 allele.