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.

The new rank and combination Berberis extensiflora (Ahrendt) Bh.Adhikari & Harber is proposed based on morphological and molecular studies of the Himalayan species of Berberis (Berberidaceae). Morphologically, Berberis extensiflora differs from B. concinna Hook.f. by its much longer (3–4 cm) inflorescence with 2 or 3 flowers in umbels (vs < 2 cm long solitary flowers) and 10- to 13-seeded berries (vs 5- to 9-seeded berries). A detailed taxonomic description, notes on habitat and ecology, and colour photographs are provided.