The Type III Effector XopM from //Xanthomonas//

Author: Ralf Koebnik
Expert reviewer: Frederik Börnke

Class: XopM
Family: XopM
Prototype: XCV0442 (Xanthomonas euvesicatoria pv. euvesicatoria, ex Xanthomonas campestris pv. vesicatoria; strain 85-10)
RefSeq ID: WP_011346113.1
3D structure: Unknown

xopM _Xee85-10 was identified as a candidate T3E gene based on the presence of a PIP box in the promoter region and its conservation in other plant pathogens (Schulze et al., 2012). XopM was also discovered as a T3E in Xee_85-10 using a machine-learning approach (Teper et al., 2016).

Xanthomonas bacteria containing a translational fusion between XopM and an N-terminally truncated variant of AvrBs3 were shown by imumunodetection with an AvrBs3-specific antibody to secrete the chimeric protein into the culture medium in a T3SS-dependent manner (Schulze et al., 2012).

To test for T3SS-dependent translocation into plant cells, Xee _85-10 expressing the XopM-AvrBs3 chimeric protein was inoculated into leaves of AvrBs3-responsive pepper plants (ECW-30R) and the near-isogenic susceptible pepper line ECW, which lacks the Bs3 resistance gene. Bacteria expressing the XopM_1–520-AvrBs3delta2 chimeric protein induced the hypersensitive response (HR) in ECW-30R, but not in ECW. As expected, no HR induction was observed in plants infected with a strain mutated in the T3SS gene hrcV (Schulze et al., 2012).

Expression of xopM _Xee85-10 was shown to be controlled by both HrpG and HrpX (Schulze et al., 2012).

The Xee_85-10 strain deleted in xopM showed no difference in the induction of disease symptoms on pepper plants compared to the wild-type strain 85-10 (Schulze et al., 2012).

To identify defense reactions, mediated by xopM, leaves of pepper ECW, Nicotiana benthamiana and N. tabacum, the latter two being nonhost plants of Xee _85-10, were inoculated with Agrobacterium strains mediating the in planta expression of the effector gene fused to GFP. In this assay, XopM elicited a cell death reaction in N. benthamiana at 3–5 dpi (Schulze et al., 2012). When ectopically expressed in plants, XopM supports growth of a non-pathogenic bacterial strain and dampens the production of reactive oxygen species, indicating its ability to suppress plant immunity (Brinkmann et al., 2024). The abilty to repress a flg22 induced ROS burst is independent of XopM binding to VAP but requires localization at the host cell plasma membrane (see below, Brinkmann et al., 2024).

XopM-GFP fusions have been shown to localize to the plant plasma membrane (PM) in N. benthamiana leaf epidermal cells. Possibly concentrated in ER-PM contact sites (Brinkmann et al., 2024)

Unknown.

XopM interacts with vesicle-associated membrane protein (VAMP)-associated proteins (VAPs) in an isoform specific manner. XopM displays two FFAT [two phenylalanines (FF) in an acidic tract (AT)] motifs that cooperatively mediate the interaction with VAP. Binding to VAP is not required for XopM's ability to suppress PTI, indicating that it has other virulence targets. In planta pull-down assays using XopM-GFP as a bait, suggest that it interacts with additional membrane components (Brinkmann et al., 2024).

xopM is typically encoded next to the hrp gene cluster and considered a core effector gene in several Xanthomonas species (Jiang et al., 2009; Merda et al., 2017; Pesce et al., 2017).

Yes (Acidovorax, Pseudomonas, and Ralstonia) (Schulze et al., 2012; Pesce et al., 2017)

Brinkmann C, Bortlik J, Raffeiner M, González-Fuente M, Börnke LF, Üstün S, Börnke F (2024). XopM, an FFAT motif-containing type III effector protein from Xanthomonas, suppresses MTI responses at the plant plasma membrane. Mol. Plant Pathol. 25: e70038. DOI: 10.1111/mpp.70038

Jiang W, Jiang BL, Xu RQ, Huang JD, Wei HY, Jiang GF, Cen WJ, Liu J, Ge YY, Li GH, Su LL, Hang XH, Tang DJ, Lu GT, Feng JX, He YQ, Tang JL (2009). Identification of six type III effector genes with the PIP box in Xanthomonas campestris pv. campestris and five of them contribute individually to full pathogenicity. Mol. Plant Microbe Interact. 22: 1401-1411. DOI: 10.1094/MPMI-22-11-1401

Pesce C, Jacobs JM, Berthelot E, Perret M, Vancheva T, Bragard C, Koebnik R (2017). Comparative genomics identifies a novel conserved protein, HpaT, in proteobacterial type III secretion systems that do not possess the putative translocon protein HrpF. Front. Microbiol. 8: 1177. DOI: 10.3389/fmicb.2017.01177

Merda D, Briand M, Bosis E, Rousseau C, Portier P, Barret M, Jacques MA, Fischer-Le Saux M (2017). Ancestral acquisitions, gene flow and multiple evolutionary trajectories of the type three secretion system and effectors in Xanthomonas plant pathogens. Mol. Ecol. 26: 5939-5952. DOI: 10.1111/mec.14343

Schulze S, Kay S, Büttner D, Egler M, Eschen-Lippold L, Hause G, Krüger A, Lee J, Müller O, Scheel D, Szczesny R, Thieme F, Bonas U (2012). Analysis of new type III effectors from Xanthomonas uncovers XopB and XopS as suppressors of plant immunity. New Phytol. 195: 894-911. DOI: 10.1111/j.1469-8137.2012.04210.x

Teper D, Burstein D, Salomon D, Gershovitz M, Pupko T, Sessa G (2016). Identification of novel Xanthomonas euvesicatoria type III effector proteins by a machine-learning approach. Mol. Plant Pathol. 17: 398-411. DOI: 10.1111/mpp.12288