Searching for Myanmar indigenous Bradyrhizobium type C strains that best identify Rj₄ genotypes in soybean

Bradyrhizobium species are symbiotic partners of soybean plants. However, some Bradyrhizobium bacteria do not form functional nodules on the roots of Rj₄ genotype soybean cultivars. Our objective was to identify the strains of Bradyrhizobium (i.e., type C strains) that are least competent to form nodules on the roots of this plant genotype. We checked (i) previously isolated type C strains of Myanmar Bradyrhizobium elkanii (MMY6-1, MMY6-2, and MMY6-5), (ii) previously isolated type C strains of Myanmarese Bradyrhizobium spp. (MMY3-5 and MMY3-7), and (iii) strain Is-34 of B. japonicum, for nodule formation when associated with Rj₄ and other Rj genotype soybeans. Strains in groups (i), (ii), and (iii) are known to be incompatible with Rj₄ soybean genotypes. MMY3-5 and MMY3-7 produced functional nodules when associated with Rj₄ and other Rj genotype soybean cultivars, except Hill (Rj₄) cultivar. The ratios of ineffective nodule numbers/total nodule numbers (I/T ratios) for MMY6-1, MMY6-2, MMY6-5, and Is-34 in association with Rj₄ soybean cultivars were > 0.5, demonstrating incompatibility between these bacterial strains and the Rj₄ genotype. Interestingly, the I/T ratios of MMY6-1 and MMY6-2 were higher than that of Is-34 in almost all Rj₄ soybean cultivars. Thus, the nodule-forming abilities of the B. elkanii strains MMY6-1 and MMY6-2 were strongly suppressed in Rj₄ soybean cultivars; these strains may therefore be useful to identify the Rj₄ genotype in soybean cultivars.