Soybean preference for Bradyrhizobium japonicum for nodulation: Occupation of serogroup USDA110 in Nodules of soybean plants harboring various Rj-genes grown in a field

For examining the probability of increase in the occupation ratio of inoculated rhizobium in nodules, various Rj-soybean cultivars including the Rj₂Rj₃Rj₄-lines of soybean were grown in a field of the Kyushu University Farm. Bradyrhizobium japonicum USDA110 that carries uptake hydrogenase (Hup⁺) was used as an inoculum. The relative efficiency of nitrogen fixation generally increased by the inoculation. However, there were no significant differences in the effects among the genotypes of the host plants. The occupation ratio of serogroup USDA110 in the nodules on the taproot of the inoculated plants was in the range of 77–100%, suggesting that the B. japonicum strain USDA110 infected taproots immediately after inoculation. The occupation ratios in the nodules on the lateral roots were 53–67, 40–86, 63–83, and 62–77% in inoculated plants of the non-Rj-, Rj₂Rj₃-, Rj₄-, and Rj₂Rj₃Rj₄-genotypes, respectively, and they decreased in all the genotypes with the progression of growth. At the time of the first sampling, the occupation ratios on the lateral roots of these Rj₂Rj₃Rj₄-genotypes showed values intermediate between those of IAC-2 (Rj₂Rj₃) and Hill (Rj₄), which were the parent cultivars of the Rj₂Rj₃Rj₄-lines, B340, B349, and C242. The reduction in the occupation ratio of the serogroup USDA110 for about 1 month after the first sampling was the lowest (0.13–0.16) in the Rj₂Rj₃Rj₄-genotypes, excluding B349, followed by the non-Rj- and Rj₂Rj₃-genotypes and highest (0.52–0.69) in the Rj₄-genotypes, excluding Hill. Therefore, it was considered that the population of compatible rhizobia with host soybean plants increased in the rhizosphere with the progression of the development and growth. The results showed that with the expansion of the root area of host plants, the occupation ratio of type A rhizobia including the serogroup USDA110 was high. Therefore, the Rj₂Rj₃Rj₄-genotypes were superior to other Rj-genotypes in terms of the inoculation effects of nodulation type A rhizobium, B. japonicum USDA110. However, the preference of the Rj₂Rj₃Rj₄-genotype for serogroup USDA110 is not sufficient to rule out the competition with the other serogroups in this study. Therefore, the study should be centered on the isolation of more efficient (Hup⁺) and highly compatible rhizobial strains with the Rj₂Rj₃Rj₄- genotypes.