Résumé
Xylan is a major structural component of plant cell wall and the second most abundant plant polysaccharide in nature. Here, by combining genomic and functional analyses, we provide a comprehensive picture of xylan utilization by X anthomonas campestris pv campestris ( X cc ) and highlight its role in the adaptation of this epiphytic phytopathogen to the phyllosphere. The xylanolytic activity of X cc depends on xylan‐deconstruction enzymes but also on transporters, including two T on B ‐dependent outer membrane transporters ( TBDT s) which belong to operons necessary for efficient growth in the presence of xylo‐oligosaccharides and for optimal survival on plant leaves. Genes of this xylan utilization system are specifically induced by xylo‐oligosaccharides and repressed by a L ac I ‐family regulator named X yl R . Part of the xylanolytic machinery of X cc , including TBDT genes, displays a high degree of conservation with the xylose‐regulon of the oligotrophic aquatic bacterium C aulobacter crescentus . Moreover, it shares common features, including the presence of TBDT s, with the xylan utilization systems of B acteroides ovatus and P revotella bryantii , two gut symbionts. These similarities and our results support an important role for TBDT s and xylan utilization systems for bacterial adaptation in the phyllosphere, oligotrophic environments and animal guts.