The crucial roles played by arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) in enhancing plant nutrient uptake and soil quality are widely recognized across various plant species. This study explored the effects and potential of Bacillus velezensis S141 as a plant growth-promoting rhizobacterium on promoting a symbiotic relationship of AMF, Rhizophagus irregularis with Lotus japonicus. B. velezensis S141 inoculation positively influenced fungal growth and development. B. velezensis S141 promoted fungal abundance, such as AM root colonization and spore number. It also boosted plant nutrient uptake, enhancing the nitrogen and phosphorus concentration by 1.65 and 1.51 times, respectively, under tripartite interaction conditions. However, the indole-3-acetic acid (IAA) producing capability of B. velezensis S141, based on the inoculation experiment test of S141 mutants defective in IAA synthesis, was not the key mechanism for promoting this symbiotic interaction. Interestingly, the S141 strain, originating from rhizospheric soil fields of soybeans, was found to penetrate plant root cells and establish itself as an endophyte. The presence of B. velezensis S141 not only triggered the expression of marker genes associated with early stages of AMF colonization and nutrient uptake in the host plant, but it also led to an upregulation of AMF genes responsible for cell cycle regulation. These results suggest that B. velezensis S141 holds promise as a helper bacterium in promoting plant-AMF symbiosis.
