A synthetic microbial community for soybean biofertilization designed via chlorophyll-based iterative selection

Abstract

Improving the effectiveness effectiveness effectiveness of microbial inoculants for soybean is essential to enhance biological nitrogen fixation and reduce fertilizer dependence; however, inoculated Bradyrhizobium strains frequently display inconsistent field performance. Inoculation is usually carried out with single-strain formulations, overlooking the possible influence influence of the native soil microbiota on nodulation success. This limitation may be addressed by formulating inoculants with consortia that include selected members of the soil microbiota. To this end, a synthetic microbial community (SynCom) was developed through a host-mediated microbiome engineering approach guided by leaf chlorophyll content as a rapid, non-destructive plant trait. The experiment was initiated by inoculating soybean plants with a consortium of 9 Bradyrhizobium spp. and 14 non-rhizobial soil isolates. Across eight consecutive selection rounds under gnotobiotic conditions, rhizosphere communities associated with superior plant performance were pooled and propagated. Recurrent selection induced significant significant significant shifts in community composition, consistently favoring Bradyrhizobium diazoefficiens diazoefficiensdiazoefficiens as the dominant nodulating member and enriching taxa from Pseudomonadales, Burkholderiales, and Sphingomonadales. Sequencing-based profiling profiling and network analysis suggested the emergence of a cohesive and functionally enriched community, with increased potential for nitrogen transformations and organic matter turnover. When evaluated in non-sterile soil, the SynCom derived from the sixth selection round increased nodule number and biomass relative to an uninoculated control and a commercial inoculant strain. These results suggest that plant-guided selection can steer rhizosphere community assembly toward beneficial configurationsconfigurations configurations and support the development of improved soybean bioinoculants.