The CebE/MsiK Transporter is a Doorway to the Cello-oligosaccharide-mediated Induction of Streptomyces scabies Pathogenicity

Abstract

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.

Contribution of the β‐glucosidase BglC to the onset of the pathogenic lifestyle of Streptomyces scabies

“…scabies . We generated a bglC null mutant (Δ bglC ) by replacing orf scab57721 with the apramycin resistance cassette, as performed previously for cebR , cebE and msiK (Francis et al ., ; Jourdan et al ., ). Semi‐quantitative analysis by high‐performance liquid chromatography (HPLC) revealed that the Δ bglC /Δ 57721 mutant produced only 37% and 9% of the thaxtomin levels produced by the wild‐type strain when cultivated in liquid minimal medium (MM) with (Glc) 2 or (Glc) 3 as sole carbon source, respectively (Fig.…”

“…As we demonstrated that bglC/BglC is induced by cellooligosaccharides and displays BG activity against (Glc) 2 and (Glc) 3 , we finally assessed whether the catabolic activity of BglC influenced the production levels of thaxtomin A and, as a consequence, the virulence of S. scabies. We generated a bglC null mutant (DbglC) by replacing orf scab57721 with the apramycin resistance cassette, as performed previously for cebR, cebE and msiK (Francis et al, 2015;Jourdan et al, 2016). Semi-quantitative analysis by high-performance liquid chromatography (HPLC) revealed that the DbglC/D57721 mutant produced only 37% and 9% of the thaxtomin levels produced by the wild-type strain when cultivated in liquid minimal medium (MM) with (Glc) 2 or (Glc) 3 as sole carbon source, respectively ( Fig.…”

“…6His-BglC was not able to release glucose from disaccharides unrelated to cellulose, except for lactose, but with much lower efficiency relative to (Glc) 2 or any of the other cellooligosaccharides (data not shown). If BglC displayed activity in vitro against (Glc) 4 , it was unlikely to occur inside the cytoplasm, as the extracellular ABC transporter component CebE of S. scabies only displayed a high binding affinity to (Glc) 2 and (Glc) 3 (Jourdan et al, 2016).…”

“…). However, as stated earlier, in the plant pathogen S. scabies , (Glc) 2 and (Glc) 3 are not only perceived as nutrients used during the course of saprophytic behaviour, but are, above all, signalling molecules eliciting its pathogenic lifestyle (Johnson et al ., ; Jourdan et al ., ; Wach et al ., ). Enzymes with a BG activity could thus potentially play an important role in controlling the onset of the virulence of S. scabies by limiting the intracellular accumulation of signals triggering thaxtomin A biosynthesis (Fig.…”

“…Streptomyces scabies is the causative agent of common scab on tuber and root plants via the production of the phytotoxin thaxtomin A amongst other virulence factors (Bignell et al, 2010;Lerat et al, 2009;Loria et al, 2008). The onset of thaxtomin A is triggered on transport of the cello-oligosaccharides cellobiose [(Glc) 2 ] and cellotriose [(Glc) 3 ], which involves the ATP-binding cassette (ABC) transporter system CebEFG-MsiK (Jourdan et al, 2016). Once inside the cell, mainly (Glc) 2 , but also (Glc) 3 , can interact with the cellulose utilization repressor CebR, preventing it from binding to its operator sequences associated with the thaxtomin biosynthetic gene cluster, and therefore allowing the production of the phytotoxin (Francis et al, 2015).…”