Author: Naama Wagner
Internal reviewer: Ralf Koebnik

Class: XopBH
Family: XopB
Prototype: XCV0581 (Xanthomonas euroxanthea, strain 85-10)
GenBank ID: CAJ22212.1 (613 aa)
RefSeq ID: WP_039417318.1 (515 aa)
3D structure: Unknown

XopB was discovered in a cDNA-AFLP screen (Noël et al., 2001).

A chimeric protein consisting of a C-terminally truncated XopB where the last 52 residues (5 kDa) were replaced by the triple c-myc epitope (5 kDa) was secreted into culture supernatants of a strain with a constitutively active form of hrpG in a type III secretion-dependent manner (Noël et al., 2001). XopB belongs to translocation class B (Schulze et al., 2012). Mutation studies of a putative translocation motif (TrM) showed that the proline/arginine-rich motif is required for efficient type III-dependent secretion and translocation of XopB and determines the dependence of XopB transport on the general T3S chaperone HpaB (Prochaska et al., 2018).

The xopB gene was shown to be expressed in a hrpG- and hrpX-dependent manner (Noël et al., 2001). Presence of a PIP and ‐10 box (TTCGB‐N₁₅ ‐TTCGB‐N_30–32 ‐YANNNT) (Schulze et al., 2012).

A deletion of xopB did not affect pathogenicity or bacterial growth in plants (Noël et al., 2001). Later it was found that XopB contributes to disease symptoms and bacterial growth (Schulze et al., 2012; Priller et al., 2016). Infection of susceptible pepper plants with a strain lacking xopB resulted in increased formation of salicylic acid (SA) and expression of pathogenesis-related (PR) genes (Priller et al., 2016). When expressed in yeast, XopB attenuated cell proliferation (Salomon et al., 2011). XopB caused a fast and confluent cell death when transiently expressed in the non-host Nicotiana benthamiana leaves, whereas its expression in host tomato leaves did not result in a visible phenotype, even seven days after agroinfiltration (Salomon et al., 2011). XopB suppresses pathogen‐associated molecular pattern (PAMP)‐triggered plant defense gene expression and inhibits cell death reactions induced by different T3Es, thus suppressing defense responses related to both PAMP‐triggered immunity (PTI) and effector‐triggered immunity (ETI) (Schulze et al., 2012). For instance, XopB inhibited the flg22-triggered burst of reactive oxygen species (ROS) (Priller et al., 2016). Interestingly, a XopB point mutant derivative was defective in the suppression of ETI‐related responses, but still interfered with vesicle trafficking and was only slightly affected with regard to the suppression of defense gene induction, suggesting that XopB‐mediated suppression of PTI and ETI is dependent on different mechanisms that can be functionally separated (Schulze et al., 2012). A deletion of xopB caused a prominent increase in cell wall-bound invertase activity, which might be linked to defense responses because an increase in the apoplastic hexose-to-sucrose ratio has been suggested to strengthen plant defense (Sonnewald et al., 2012). Expression of xopB in Arabidopsis thaliana promoted the growth of the virulent Pseudomonas syringae pv. tomato DC3000 strain, which was paralleled by a decreased salicylic acid (SA)-pool and a lower induction of SA-dependent pathogenicity-related (PR) gene expression (Priller et al., 2016).

XopB localizes to the Golgi apparatus and cytoplasm of the plant cell and interferes with eukaryotic vesicle trafficking (Schulze et al., 2012). Interestingly, a short ORF is found between the PIP box/-10 promoter region and the predicted translation start codon of xopB in Xcv_85-10, which encodes a 25-aa peptide (MGLCSSKPRVQAQLNIMRPRHRAD) with a strong palmitoylation signal (Koebnik, unpublished). Whether this peptide once belonged to XopB or to another candidate effector, if at all, remains unknown.

Unknown.

Unknown.

Yes (e.g., X. fragariae, X. cynarae pv. gardneri (syn. X. gardneri), X. oryzae, X. vasicola) (Harrison et al., 2014).

Yes (e.g., Pseudomonas spp., Ralstonia solanacearum, Acidovorax spp., Pantoea agglomerans) (Schulze et al., 2012).

Koebnik R, Krüger A, Thieme F, Urban A, Bonas U (2006). Specific binding of the Xanthomonas campestris pv. vesicatoria AraC-type transcriptional activator HrpX to plant-inducible promoter boxes. J. Bacteriol. 188: 7652-7660. DOI: 10.1128/JB.00795-06

Wagner N, Baumer E, Lyubman I, Shimony Y, Bracha N, Martins L, Potnis N, Chang JH, Teper D, Koebnik R, Pupko T (2025). Effectidor II: A pan-genomic AI-based algorithm for the prediction of type III secretion system effectors. Bioinformatics, in press. DOI: 10.1093/bioinformatics/btaf272

This fact sheet is based upon work from COST Action CA16107 EuroXanth, supported by COST (European Cooperation in Science and Technology).