Production of growth-promoting substances and high colonization ability of rhizobacteria enhance the nitrogen fixation of soybean when coinoculated with Bradyrhizobium japonicum

Understanding the interaction mechanisms between plant growth-promoting rhizobacteria (PGPR), leguminous crops, and rhizobia is necessary to effectively use PGPR in increasing the biological nitrogen fixation of legumes. We determined the coinoculation effects of Bradyrhizobium japonicum A1017 and a gusA-marked strain of Pseudomonas fluorescens 2137, P. fluorescens WCS365, Azomonas agilis 125, and Azospirillum lipoferum 137 on soybean [Glycine max (L.) Merr] cv. Enrei grown under axenic conditions. The gusA-marked rhizobacteria effectively colonized the root tips and surfaces near the roots tips with a colonization rate ranging from 7.50 to 8.62 log colony forming units (cfu) gfw super(-1). P. fluorescens 2137 had the highest colonization activity on soybean roots whether inoculated alone or coinoculated with B. japonicum A1017. Coinoculation of P. fluorescens 2137 and B. japonicum A1017 increased the colonization of B. japonicum A1017 on soybean roots, nodule number, and acetylene reduction activity (ARA) at 10 and 20 days after inoculation. Moreover, the addition of sterile spent medium of P. fluorescens 2137 increased the growth of B. japonicum A1017 in yeast mannitol broth (YMB), indicating that P. fluorescens 2137 may have released substances that increased the rhizobial population. The results of this study suggest that the enhanced nodulation and ARA of soybean due to the high colonization of P. fluorescens on soybean roots could depend on the production of growth-promoting substances that stimulate the growth of B. japonicum. However, coinoculation with P. fluorescens WCS365 decreased the nodule number and ARA, despite its slight stimulation of the growth of B. japonicum on the roots, indicating that coinoculation effects are strain dependent.