Endophytic bacteria improve nodule function and plant nitrogen in soybean on co-inoculation with Bradyrhizobium japonicum MN110

The potential of bacterial endophytes to improve symbiotic efficiency through synergistic interactions with rhizobia can help to improve nodulation and nitrogen fixation in legume plants. In the present study, we compared the effect of endophytic plant growth-promoting bacteria on nodulation and effective rhizobial symbiosis in soybean. Nodule endophyte Bacillus megaterium LNL6 isolated from root nodules of Lesperdeza sp. and plant endophyte Methylobacterium oryzae CBMB20 isolated from rice leaves were selected as endophytic co-inoculants. Treatment of Bradyrhizobium japonicum MN110 along with B. megaterium LNL6 and M. oryzae CBMB20 exhibited an increase in nodule number in pots at 35 days after sowing compared to single inoculation of MN110. Additionally, both the co-inoculation treatments showed significant increase in nodule activity which was measured in terms of nodule leghemoglobin content, nodulated root ARA and total plant nitrogen content compared to solitary inoculation of B. japonicum MN110. Though ACCD activity of the co-inoculated strains can be attributed to increase in nodule number, the observed increase in root nitrogenase activity and leghemoglobin content in the nodules is understood as due to plant growth promotion traits of the specific endophytic strains. High levels of IAA produced by B. megaterium LNL6 can be considered to have aided development of mature nodules which thereby improved the nodular nitrogen fixation. Thus, endophytic lifestyle combined with plant growth-promoting traits, such as IAA production, ACC deaminase, cellulase and nitrogenase activity by B. megaterium LNL6 and M. oryzae CBMB20, contributes to the improvement of overall symbiotic nitrogen fixation by the plant.