A simplified SynCom based on core-helper strain interactions enhances symbiotic nitrogen fixation in soybean

Synthetic microbial communities (SynComs) are a promising tool for making full use of the beneficial functions imparted by whole bacterial consortia. However, the complexity of reconstructed SynComs often limits their application in sustainable agriculture. Furthermore, inter-strain interactions are often neglected during SynCom construction. Here, we propose a strategy for constructing a simplified and functional SynCom (sfSynCom) by using elite helper strains that significantly improve the beneficial functions of the core symbiotic strain, here Bradyrhizobium elkanii BXYD3, to sustain the growth of soybean (Glycine max). We first identified helper strains that significantly promote nodulation and nitrogen fixation in soybean mediated by BXYD3. Two of these helper strains assigned to the Pantoea taxon produce acyl homoserine lactones, which significantly enhanced the colonization and infection of soybean by BXYD3. Finally, we constructed a sfSynCom from these core and helper strains. This sfSynCom based on the core-helper strategy was more effective at promoting nodulation than inoculation with BXYD3 alone and achieved effects comparable to those of a complex elite SynCom previously constructed on the basis of potential beneficial functions between microbes and plants alone. Our results suggest that considering interactions between strains as well as those between strains and the host plant might allow construction of sfSynComs.