Streptomyces scabies is an economically important plant pathogen well-known for damaging root and tuber crops by causing scab lesions. Thaxtomin A is the main causative agent responsible for the pathogenicity of S. scabies and cello-oligosaccharides are environmental triggers that induce the production of this phytotoxin. How cello-oligosaccharides are sensed or transported in order to induce the virulent behavior of S. scabies? Here we report that the cellobiose and cellotriose binding protein CebE, and MsiK, the ATPase providing energy for carbohydrates transport, are the protagonists of the cello-oligosaccharide mediated induction of thaxtomin production in S. scabies. Our work provides the first example where the transport and not the sensing of major constituents of the plant host is the central mechanism associated with virulence of the pathogen. Our results allow to draw a complete pathway from signal transport to phytotoxin production where each step of the cascade is controlled by CebR, the cellulose utilization regulator. We propose the high affinity of CebE to cellotriose as possible adaptation of S. scabies to colonize expanding plant tissue. Our work further highlights how genes associated with primary metabolism in nonpathogenic Streptomyces species have been recruited as basic elements of virulence in plant pathogenic species.

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2Common scab disease on root and tuber plants is caused by Streptomyces scabies and related 2 3 species which use the cellulose synthase inhibitor thaxtomin A as main phytotoxin. 4Thaxtomin production is primarily triggered by the import of cello-oligosaccharides. Once 2 5 inside the cell, the fate of the cello-oligosaccharides is dichotomized into i) fueling glycolysis 2 6with glucose for the saprophytic lifestyle through the action of β -glucosidase(s) (BG), and ii) reduced production of thaxtomin when the mutant was cultivated on media containing cello- mediated induction of thaxtomin production and suggests that the role of BglC in the route to 4 0 the pathogenic lifestyle of S. scabies is more complex than currently presented.

2Common scab disease on root and tuber plants is caused by Streptomyces scabies and related 2 3 species which use the cellulose synthase inhibitor thaxtomin A as main phytotoxin. 4Thaxtomin production is primarily triggered by the import of cello-oligosaccharides. Once 2 5 inside the cell, the fate of the cello-oligosaccharides is dichotomized into i) fueling glycolysis 2 6with glucose for the saprophytic lifestyle through the action of β -glucosidase(s) (BG), and ii) reduced production of thaxtomin when the mutant was cultivated on media containing cello- mediated induction of thaxtomin production and suggests that the role of BglC in the route to 4 0 the pathogenic lifestyle of S. scabies is more complex than currently presented.

Streptomyces scabies is responsible for common scab disease on root and tuber vegetables. Production of its main phytotoxin thaxtomin A is triggered upon transport of cellulose byproducts cellotriose and cellobiose, which disable the repression of the thaxtomin biosynthesis activator gene txtR by the cellulose utilization regulator CebR. To assess the intracellular response under conditions where S. scabies develops a virulent behavior, we performed a comparative proteomic analysis of wild-type S. scabies 87-22 and its cebR null mutant (hyper-virulent phenotype) grown in the absence or presence of cellobiose. Our study revealed significant changes in abundance of proteins belonging to metabolic pathways known or predicted to be involved in pathogenicity of S. scabies. Among these, we identified proteins of the cello-oligosaccharide-mediated induction of thaxtomin production, the starch utilization system required for utilization of the carbohydrate stored in S. scabies's hosts, and siderophore synthesis utilization systems, which are key features of pathogens to acquire iron once they colonized the host. Thus, proteomic analysis supported by targeted mass spectrometry-based metabolite quantitative analysis revealed the central role of CebR as a regulator of virulence of S. scabies.