Bacterial tolerance to host-exuded specialized metabolites structures the maize root microbiome
Plants exude specialized metabolites from their roots and these compounds are known to structure the root microbiome. However, the underlying mechanisms are poorly understood. We established a representative collection of maize root bacteria and tested their tolerance against benzoxazinoids, the dominant specialized and bioactive metabolites in the root exudates of maize plants. In vitro experiments revealed that benzoxazinoids inhibited bacterial growth in a strain- and compound-dependent manner. Tolerance against these selective antimicrobial compounds depended on bacterial cell wall structure. Further, we found that native root bacteria isolated from maize tolerated the benzoxazinoids better compared to non-host Arabidopsis bacteria. This finding suggests the adaptation of the root bacteria to the specialized metabolites of their host plant. Bacterial tolerance to 6-methoxy-benzoxazolin-2-one (MBOA), the most abundant and selective antimicrobial metabolite in the maize rhizosphere, correlated significantly with the abundance of these bacteria on benzoxazinoid-exuding maize roots. Thus, strain-dependent tolerance to benzoxazinoids largely explained the abundance pattern of bacteria on maize roots. Abundant bacteria generally tolerated MBOA, while low abundant root microbiome members were sensitive to this compound. Our findings reveal that tolerance to plant specialized metabolites is an important competence determinant for root colonization. We propose that bacterial tolerance to plant-secreted antimicrobial compounds is an underlying mechanism determining the structure of host-specific microbial communities. Significance StatementDiverse microbial communities colonize plant roots. They feed on carbon rich root exudates which contain a diverse mix of chemicals including primary and specialized metabolites. Here we show that specialized metabolites act as selective antibiotics to shape the root bacterial communities. By growing single isolates of maize root bacteria in the presence of benzoxazinoids in vitro, we find that the strains differ greatly in their tolerance to benzoxazinoids. Their different levels of tolerance largely explained their abundance on benzoxazinoid-exuding roots. Our work shows how plant specialized metabolites act to shape the maize root microbial community and thus deepened our mechanistic understanding of how plants shape their microbiome.