Role of Alternative Elicitor Transporters in the Onset of Plant Host Colonization by Streptomyces scabiei 87-22

Plant colonization by Streptomyces scabiei, 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 S. scabiei. Homology searches revealed paralogues of the CebEFG proteins, encoded by the cebEFG2 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 S. scabiei 87-22 upon cello-oligosaccharide supply by transcriptomic analysis, which revealed that cebEFG2 expression is highly activated by both cellobiose and cellotriose, while pitEFG expression was barely induced. Accordingly, deletion of pitE had no impact on virulence and thaxtomin production under the conditions tested, while the deletion of cebEFG2 reduced virulence and thaxtomin production, though not as strong as the mutants of the main cello-oligosaccharide transporter cebEFG1. Our results thus suggest that both ceb 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 ceb 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 S. scabiei.

Mannose- and Mannobiose-Specific Responses of the Insect-Associated Cellulolytic Bacterium Streptomyces sp. Strain SirexAA-E

The cellulolytic insect symbiont bacterium Streptomyces sp. strain SirexAA-E secretes a suite of carbohydrate-active enzymes (CAZymes), which are involved in the degradation of various polysaccharides in the plant cell wall, in response to the available carbon sources. Here, we examined a poorly understood response of this bacterium to mannan, one of the major plant cell wall components. SirexAA-E grew well on mannose, carboxymethyl cellulose (CMC), and locust bean gum (LBG) as sole carbon sources in the culture medium. The secreted proteins from each culture supernatant were tested for their polysaccharide-degrading ability, and the composition of secreted CAZymes in each sample was determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results indicated that mannose, LBG, and CMC induced the secretion of mannan and cellulose-degrading enzymes. Interestingly, two α-1,2-mannosidases were abundantly secreted during growth on mannose and LBG. Using genomic analysis, we found a unique 12-bp palindromic sequence motif at 4 locations in the SirexAA-E genome, two of which were found upstream of the above-mentioned α-1,2-mannosidase genes, along with a newly identified mannose and mannobiose-responsive transcriptional regulator, SsManR. Furthermore, the previously reported cellobiose-responsive repressor, SsCebR, was determined to also use mannobiose as an effector ligand. To test whether mannobiose induces the sets of genes under the control of the two regulators, SirexAA-E was grown on mannobiose, and the secretome composition was analyzed. As hypothesized, the composition of the mannobiose secretome combined sets of CAZymes found in both LBG and CMC secretomes, and thus they are likely under the regulation of both SsManR and SsCebR. IMPORTANCE Streptomyces sp. SirexAA-E, a microbial symbiont of biomass-harvesting insects, secretes a suite of polysaccharide-degrading enzymes dependent on the available carbon sources. However, the response of this bacterium to mannan has not been documented. In this study, we investigated the response of this bacterium to mannose, mannobiose, and galactomannan (LBG). By combining biochemical, proteomic, and genomic approaches, we discovered a novel mannose and mannobiose responsive transcriptional regulator, SsManR, which selectively regulates three α-1,2-mannosidase-coding genes. We also demonstrated that the previously described cellobiose responsive regulator, SsCebR, could use mannobiose as an effector ligand. Overall, our findings suggest that the Streptomyces sp. SirexAA-E responds to mannose and mannooligosaccharides through two different transcriptional repressors that regulate the secretion of the plant cell wall-degrading enzymes to extract carbon sources in the host environment.