Plant growth-promoting rhizobacteria (PGPR) native to African yam bean (Sphenostylis stenocarpa) enhance the early growth of three economically important legume species

African yam bean, AYB (Sphenostylis stenocarpa) root nodules are colonized by various strains of plant growth-promoting rhizobacteria (PGPR) which, when isolated and applied to other legume species seeds, may assist early seedling growth. Four PGPR isolated from AYB root nodules, Bradyrhizobia isolates 1-1, 4-4 (OQ586405), 6-2-1, and Paenibacillus sp. 4-5-1 (OQ586406) were tested in a pot study for their vegetative growth promotion of the temperate legume, alfalfa (Medicago sativa) and the tropical legumes, AYB and Bambara groundnut (Vigna subterranea L. Verdc) and compared to control plants which were not inoculated. Biomass, shoot and root growth, along with nodule number was assessed six weeks after treatment (6 WAT). Legumes without PGPR served as the control. Bradyrhizobium spp. 6-2-1, 4-4, 1-1, and Paenibacillus sp. 4-5-1, all significantly (P < 0.05) supported the biomass dry weight (g) of alfalfa, 0.06 g, 0.07 g, 0.05 g, and 0.04 g respectively, when compared to the control with 0.01 g at 6 WAT. All PGPR enhanced the early growth of the legumes studied in the nitrogen-free vermiculite medium when compared to the control with yellowish leaves, indicating a lack of nitrogen at 6 WAT. These PGPR could be useful as bioinoculants for the sustainable production of temperate and tropical legumes in nitrogen-depleted soil conditions. Further studies are necessary for more information about the nitrogen fixation efficacy of these strains and the mechanisms underlying their plant growth-promotion.