Searching for Myanmar indigenous Bradyrhizobium type C strains that best identify Rj₄ genotypes in soybean
Htwe, Aung Zaw | Taira, Kenjirou | Yamakawa, Takeo
Bradyrhizobium species are symbiotic partners of soybean plants. However, some Bradyrhizobium bacteria do not form functional nodules on the roots of Rj ₄ 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 Rj ₄ and other Rj genotype soybeans. Strains in groups (i), (ii), and (iii) are known to be incompatible with Rj ₄ soybean genotypes. MMY3-5 and MMY3-7 produced functional nodules when associated with Rj ₄ and other Rj genotype soybean cultivars, except Hill (Rj ₄) 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 Rj ₄ soybean cultivars were > 0.5, demonstrating incompatibility between these bacterial strains and the Rj ₄ genotype. Interestingly, the I/T ratios of MMY6-1 and MMY6-2 were higher than that of Is-34 in almost all Rj ₄ soybean cultivars. Thus, the nodule-forming abilities of the B. elkanii strains MMY6-1 and MMY6-2 were strongly suppressed in Rj ₄ soybean cultivars; these strains may therefore be useful to identify the Rj ₄ genotype in soybean cultivars.
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