Competitive modulation ability of Bradyrhizobium japonicum strains incubated in soil

Abstract

Six doubly-marked antibiotic-resistant strains of BradyRhizobium japonicum were introduced separately into soil, incubated, and examined for competitive nodulation ability. Mixtures of these strains in soil suspension and strains cultured in yeast extract mannitol broth were used to inoculate soybean ( Glycine max) seedlings. Nodule occupancy was determined by the antibiotic resistance of bacteria in each nodule. N50 (the inoculum ratio yielding a 50% nodule occupancy rate) was used to measure the competitive nodulation ability of each strain. Three weeks of soil incubation increased the competitive ability of strains 31Nal fRif R and 123 Ery RTet Rand decreased that of strains 29Ery RTet R and 138Nal RRif R. Soil incubation had no significant effect on the competitive nodulation abilities of strains 122Ery'Tet' and 143Nal'RiP. These results show that the soil environment of B. japonicum strains can affect their competitiveness for nodulation.