| Summary: | Plant colonization by <i>Streptomyces scabiei</i>, the main cause of common scab disease on root and tuber crops, is triggered by cello-oligosaccharides, cellotriose being the most efficient elicitor. The import of cello-oligosaccharides via the ATP-binding cassette (ABC) transporter CebEFG-MsiK induces the production of thaxtomin phytotoxins, the central virulence determinants of this species, as well as many other metabolites that compose the ‘virulome’ of <i>S</i>. <i>scabiei</i>. Homology searches revealed paralogues of the CebEFG proteins, encoded by the <i>cebEFG2</i> cluster, while another ABC-type transporter, PitEFG, is encoded on the pathogenicity island (PAI). We investigated the gene expression of these candidate alternative elicitor importers in <i>S</i>. <i>scabiei</i> 87-22 upon cello-oligosaccharide supply by transcriptomic analysis, which revealed that <i>cebEFG2</i> expression is highly activated by both cellobiose and cellotriose, while <i>pitEFG</i> expression was barely induced. Accordingly, deletion of <i>pitE</i> had no impact on virulence and thaxtomin production under the conditions tested, while the deletion of <i>cebEFG2</i> reduced virulence and thaxtomin production, though not as strong as the mutants of the main cello-oligosaccharide transporter <i>cebEFG1</i>. Our results thus suggest that both <i>ceb</i> clusters participate, at different levels, in importing the virulence elicitors, while PitEFG plays no role in this process under the conditions tested. Interestingly, under more complex culture conditions, the addition of cellobiose restored thaxtomin production when both <i>ceb</i> clusters were disabled, suggesting the existence of an additional mechanism that is involved in sensing or importing the elicitor of the onset of the pathogenic lifestyle of <i>S</i>. <i>scabiei</i>.
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