Co-inoculation of Bradyrhizobium stimulates the symbiosis efficiency of Rhizobium with common bean

Abstract

Common bean (Phaseolus vulgaris L.) nodulates with a wide range of rhizobia. Amongst these is Bradyrhizobium, which is inefficient but able to induce profuse nodulation on this crop. Based on this observation, we tested whether co-inoculating bradyrhizobia with a more standard common bean symbiont, Rhizobium tropici, could stimulate growth and nodulation of common bean, thus contributing to a more effective symbiosis. Rhizobium tropici was co-inoculated with two Bradyrhizobium strains applied at three different doses (104, 106, and 108 CFU seed−1) under sterile conditions, and at a single dose (108 CFU seed−1) in non-sterile soil. Plant biomass, nodulation, and N accumulation in plant tissues were evaluated. Co-inoculated plants produced more nodules, and accumulated more shoot dry biomass and nitrogen than plants inoculated with R. tropici alone under gnotobiotic conditions. Significant responses were observed at the highest inoculum dose and a significant correlation between dose and shoot dry weight was observed. Co-inoculation increased biomass and N accumulation in non-sterile soil, although with a smaller magnitude. Altogether, our findings suggest that the co-inoculation with bradyrhizobia contributed to an improved symbiotic interaction between R. tropici and common beans.