Occurrence of diverse plant growth promoting bacteria in soybean [Glycine max (L.) Merrill] root nodules and their prospective role in enhancing crop yield

Root nodules are the small nitrogen fixing factories housing endophytic bacteria. In this study, seventeen endophytic bacteria were isolated from the root nodules of soybean, which were subjected to the in vitro testing of plant growth-promoting characteristics. Bacterial isolates were able to produce indole acetate ranging from 0.7 to 18.3 μg ml⁻¹. Five isolates were able to solubilize inorganic phosphate, in which NSB15 exhibited maximum solubilization potential. All bacterial isolates showed growth on nitrogen-free mineral media, except NSB7 and NSB11. Bacterial isolates were able to form biofilm, therein NSB10 displayed a maximum efficiency of 2.6 at OD_{570 nm}. Seven isolates were positive for cellulase activity and five for pectinase activity. Based on the in vitro characterization, five efficient endophytic bacteria were phylogenetically identified by 16 S rRNA sequence analysis. NSB5 showed 99.52% identity with different species of Pseudomonas, and NSB9 showed 99.68% sequence identity with Bradyrhizobium. NSB15 and NSB16 showed maximum sequence identity with Comamonas terrigena, and NSB17 showed 100% sequence identity with Pseudomonas indica. This study represents the first international report of C. terrigena and P. indica in soybean host. In a pot experiment, inoculation of NSB9 and NSB17 significantly increased plant dry weight by 29% and 35%, respectively, compared to control. In a field trial, inoculation of NSB9, NSB15 and NSB17 significantly improved grain yield by 30%, 31% and 45%, respectively, compared to control. Conclusively, Bradyrhizobium sp. NSB9, C. terrigena NSB15 and P. indica NSB17 could be used as biofertilizers to improve the yield of soybean in a sustainable manner.