Development of multiple-occupancy nodules in correlation of the density-dependent infection by soybean nodulating rhizobia

抄録

Two soybean (Glycine max (L.) Merr.) cultivars-Sachiyutaka and Kyushu 151-were inoculated with rhizobial mixtures composed of either known USDA strains (U-mix) or representative isolates obtained from field-grown Sachiyutaka (S-mix). This study examined the effects of the host cultivars and inoculum composition on the share of Sinorhizobium (Ensifer) fredii isolates and multiple-occupancy formation. The composition of isolates in the S-mix-infected Sachiyutaka plants, in principle, depended on their relative abundance in the inoculum. Forty-seven to 48% of re-isolated rhizobia were identified as S. fredii. Nearly half of the tested nodules were inhabited by two or more distinct genotypes of rhizobia (multiple occupancy). Cohabitants mostly comprised the Bradyrhizobium japonicum and S. fredii genotypes. When the U-mix was inoculated, B. japonicum USDA 6 preferentially infected both hosts. The frequency of multiple-occupancy nodules was less than that in the case of S-mix inoculation. The two soybean cultivars showed similar results, suggesting that the effect of the plant was minor comparing to that of the inoculum under given conditions. The results suggested that the relative abundance of rhizobial groups determined principally the composition of cohabiters in single nodules under given conditions. In addition, the pairs of B. japonicum and S. fredii developed widely.