A stable 15-member bacterial SynCom promotes Brachypodium growth under drought stress

Rhizosphere microbiomes are known to drive soil nutrient cycling and influence plant fitness during adverse environmental conditions. Field-derived robust Synthetic Communities (SynComs) of microbes that mimic the diversity of rhizosphere microbiomes can greatly advance a deeper understanding of such processes. However, assembling stable, genetically tractable, reproducible, and scalable SynComs remains challenging. Here, we present a systematic approach using a combination of network analysis and cultivation-guided methods to construct a 15-member SynCom from the rhizobiome of Brachypodium distachyon . This SynCom incorporates diverse strains from five bacterial phyla and demonstrates strong stability both in vitro and in planta . Genomic analysis of the individual strains revealed that they encode multiple plant growth-promoting traits, some of which were validated by laboratory phenotypic assays. Additionally, most strains encoded genes both for the synthesis of osmoprotectants (trehalose and betaine) and Na ⁺ /K ⁺ transporters. These traits likely enabled the resilience of Brachypodium to drought stress where plants amended with SynCom recovered better than without. We further observed preferential colonization of SynCom strains around root tips under stress, likely due to active interactions between plant root metabolites and bacteria. Our results represent significant progress towards building and testing stable model SynComs for a better understanding of plant-microbe interactions.