Soil commensal rhizobia promote Rhizobium etli nodulation efficiency through CinR-mediated quorum sensing

The rhizosphere microbiome is composed of diverse microorganisms directly interacting with plants and each other. We sought to achieve a better understanding of how rhizobia interact with other soil bacteria during the initial symbiosis period. In this study, we investigated how soil commensals, particularly other rhizobia, affect Rhizobium etli–Phaseolus vulgaris interactions. We found that R. etli formed significantly more nodules on beans grown in unsterilized soil than those in sterilized soil. Furthermore, a strain identified as Rhizobium fabae, isolated from unsterilized soil, was found to affect R. etli nodulation. Interestingly, we found that the key quorum sensing regulator CinR is important for R. etli nodulation efficiency when it is co-inoculated with R. fabae. Moreover, we found that quorum sensing signals produced by R. fabae promoted CinR-mediated gene expression in R. etli. These data suggest that the effects of R. fabae on R. etli symbiosis may act through multispecies bacterial cell–cell communication.