====== The Type III Effector XopC from //Xanthomonas// ======

Author: [[https://www.researchgate.net/profile/Alice_Castaing|Alice Boulanger]]\\
Internal reviewer: [[https://www.researchgate.net/profile/Ralf_Koebnik|Ralf Koebnik]]\\

Class: XopC\\
Families: XopC1 and XopC2\\
Prototype (XopC1): XCV2435 (//Xanthomonas euvesicatoria// pv. //euvesicatoria//, ex //Xanthomonas campestris// pv. //vesicatoria//; strain 85-10)\\
GenBank ID (XopC1): [[https://www.ncbi.nlm.nih.gov/protein/CAJ24112.1|CAJ24112.1]] (834 aa)\\
Prototype (XopC2): XOC_1264 (//Xanthomonas oryzae// pv. //oryzicola//; strain BLS256)\\
GenBank ID (XopC2): [[https://www.ncbi.nlm.nih.gov/protein/AEQ95452.1|AEQ95452.1]] (549 aa - likely too short)\\
GenBank ID (XopC2; strain GX01): [[https://www.ncbi.nlm.nih.gov/protein/QEO98660.1|QEO98660.1]] (596 aa)\\
RefSeq ID (XopC1): [[https://www.ncbi.nlm.nih.gov/protein/WP_011347616.1|WP_011347616.1]] (834 aa)\\
RefSeq ID (XopC2): [[https://www.ncbi.nlm.nih.gov/protein/WP_041183113.1|WP_041183113.1]] (412 aa - likely too short)\\
3D structure: Unknown. XopC2 from //Xanthomonas axonopodis// pv. //punicae// is predicted to be a 661 amino-acids protein with 5 alpha helices and 17 beta strands. It has 21 protein binding and one helical transmembrane region of 18 amino acids (Mondal et al., 2020).

===== Biological function =====

=== How discovered? ===

XopC was discovered in //X. campestris// pv. //vesicatoria// (//Xcv//) in a cDNA-AFLP screen (Noël //et al//., 2001). XopC was also identified in a genetic screen, using a Tn//5//-based transposon construct harboring the coding sequence for the HR-inducing domain of AvrBs2, but devoid of the effectors' T3SS signal, that was randomly inserted into the genome of //Xcv//strain 85-10. The XopC::AvrBs2 fusion protein triggered a //Bs2//-dependent hypersensitive response (HR) in pepper leaves (Roden //et al//., 2004).
=== (Experimental) evidence for being a T3E ===

A chimeric protein consisting of XopC fused to a c-myc epitope (first 466 amino acids plus 5 kDa epitope) 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//., 2003). Another chimeric protein consisting of XopC fused to an N-terminally deleted derivative of the effector protein AvrBs3 (XopC<sub>200</sub>-AvrBs3<sub>∆2-153</sub>) was used to assay the translocation of XopC into plant cells (Noël //et al//., 2003). AvrBs3<sub>∆2-153</sub> was no longer delivered by the T3SS but was still able to induce the HR response in the pepper cultivar ECW-30R when artificially delivered by //Agrobacterium// (Szurek //et al//., 2002). XopC<sub>200</sub>-AvrBs3<sub>∆2-153</sub> was detected in supernatant of a strain with a constitutively active form of //hrpG// in a type III secretion-dependent manner. Translocation of this chimeric protein into plant cells was confirmed by the observation of HR obtained on pepper cultivar ECW-30R.

Type III-dependent secretion was also confirmed using a calmodulin-dependent adenylate cyclase reporter assay, with a Δ//hrpF// mutant strain serving as negative control (Roden //et al//., 2004).

Translocation of the XopC::AvrBs3 chimeric protein was independent of the export control protein, HpaC (Büttner //et al//., 2006).
=== Regulation ===

The //xopC// gene was shown to be expressed in a //hrpG//- and //hrpX//-dependent manner. No PIP box was identified in the promoter region (Noël //et al//., 2001; Noël //et al//., 2003).

qRT-PCR revealed that transcript levels of 15 out of 18 tested non-TAL effector genes (as well as the regulatory genes //hrpG// and //hrpX//), including //xopC//, were significantly reduced in the //Xanthomonas oryzae// pv. //oryzae// Δ//xrvC// mutant compared with those in the wild-type strain PXO99<sup>A</sup>  (Liu //et al.//, 2016).
=== Phenotypes ===

  * A deletion of //xopC//  did not affect pathogenicity or bacterial growth in plants (Noël //et al//., 2003).
  * Roden et al. did not find significant growth defects of a //Xcv//  Δ//xopC//  mutant in susceptible pepper and tomato leaves (Roden //et al.//, 2004)
  * Later, 86 //Solanaceae//  lines mainly of the genus //Nicotiana//  were screened for phenotypical reactions after //Agrobacterium tumefaciens//-mediated transient expression of 21 different //Xcv//  effectors. Transient expression of XopC exclusively induced plant reactions in lines of the genus //Solanum//  (Adlung //et al//., 2006). //Xcv//  85-10 strain deleted for //xopC//  induced weaker reactions than the wild type in //S. americanum//, which could be complemented by ectopic expression of //xopC//. Deletion of //xopC//  did not affect visible reactions in //N. benthamiana//  and //N. tabacum//  to infection with //Xcv//. Thus, XopC contributes to //Xcv//-induced phenotypes in certain non-host plants (Adlung //et al//., 2006).
  * The absence of //xopC//  in the genome of //Xcv//  led to an accelerated AvrBs1-induced HR in resistant pepper plants, if the plants were additionally stressed by exogenous application of salicylic acid (SA). This phenotype was complemented by //xopC//, but not by a //xopC//  derivative carrying a mutation in the predicted HAD-like hydrolase sequence (Herzfeld, 2013).
  * Virus-induced gene silencing (VIGS) of OAS-TL in planta abolished the acceleration of AvrBs1-mediated HR formation induced by the absence of //xopC//  in //Xcv//  in resistant pepper plants dependent on SA. These data suggest, that the induction of the AvrBs1-dependent HR in resistant pepper plants is SA-stress dependently delayed by XopC, which is reliant on a HAD-like hydrolase domain in XopC. This delay is mediated by the XopC plant interaction partner OAS-TL. Furthermore, expression analysis showed an increased accumulation of β-1,3-Glucanase transcript in //Xcv//-infected, resistant pepper plants by the presence of //xopC//. These findings indicated that XopC influences different mechnisms of the plant metabolism (Herzfeld, 2013).
  * XopC2 of //X. citri//  pv. //punicae//  was found to contribute to the bacterial blight development on pomegranate fruit plants. Xap //ΔxopC2//  was demonstrated to cause reduced the blight lesions when inflitrated on pomegranate leaves, induce defense responses like callose deposition, ROS production and upregulate immune-responsive genes in its natural host plants (Mondal //et al.//, 2020).
  * Ectopic expression of XopC2 was found to promote jasmonate signaling and stomatal opening in transgenic rice plants, which were more susceptible to //X. oryzae//  pv. //oryzicola//  infection (Wang //et al.//, 2021).
  * The small regulatory noncoding RNA (sRNA) Xonc3711 was found to repress production of the DNA-binding protein Xoc_3982 by binding to the xoc_3982 mRNA, and both ChIP-seq and electrophoretic mobility shift assays showed that Xoc_3982 repressed the transcription of the effector XopC2, which contributes to virulence in //Xoc// <sub>BLS256</sub>  (Wu //et al.//, 2021).
  * XopC2 from //Xanthomonas phaseoli//  pv. //manihotis//  was found to repress host immune responses in cassava, thus promoting bacterial pathogen infection (Wei //et al.//, 2024).

=== Localization ===

XopC localises to the plant cell cytoplasm (Mondal //et al.//, 2020) and the nucleus (Herzfeld, 2013).

=== Enzymatic function ===

XopC contains a predicted phosphoribosyl transferase domain and a putative haloacid dehalogenase (HAD)-like hydrolase domain in its C-terminal end. Phenotype of point mutation in catalytic domain have shown that HAD-like hydrolase activity is required for the XopC deleterious effect in yeast (Salomon //et al//., 2011). XopC2 represents a family of atypical kinases that specifically phosphorylate OSK1, a universal adaptor protein of the Skp1-Cullin-F-box ubiquitin ligase complexes (Wang //et al.//, 2021).

=== Interaction partners ===

Yeast-2-hybrid studies revealed a XopC interactor, which also interacted with XopC //in planta//. The interactor localises to the plant cell cytoplasm and carries typical features of plant cytosolic //O//-acetylserine (thiol)lyases (OAS-TL). It shows OAS-TL activity //in vivo//  and //in vitro//. The latter one is enhanced by adding XopC (Herzfeld, 2013).

XopC2 from //Xanthomonas phaseoli//  pv. //manihotis//  physically associated with MeHSP90.9 from cassava to inhibit its interaction with MeCPK1 and the corresponding protein phosphorylation by MeCPK1, so as to repress host immune responses and promote bacterial pathogen infection (Wei //et al.//, 2024).

===== Conservation =====

=== In xanthomonads ===

Close, full-length homologs (>90% sequence identity) of XopC1 have only been found in several strains of clade-2 xanthomonads, such as //X. citri//, //X. euvesicatoria//, //X. fragariae//, //X. gardneri//, //X. hortorum//, and //X. phaseoli// (BLASTP and TBLASTN performed in June 2020).

The distantly related XopC2 has homologs in //X. citri//, //X. axonopodis//, //X. euvesicatoria//, //X. oryzae//, //X. phaseoli//, and //X. translucens// (BLASTP and TBLASTN performed in June 2020)

=== In other plant pathogens/symbionts ===

XopC1: //Ralstonia solanacearum// (RipC2), //Trinickia caryophylli// (//Paraburkholderia caryophylli//), //Xylophilus ampelinus// (BLASTP and TBLASTN performed in June 2020).

XopC2: //Acidovorax// ssp., //Pseudomonas cissicola// [a pathovar of //Xanthomonas citri//], //Ralstonia solanacearum// (RipC1) (BLASTP and TBLASTN performed in June 2020).

===== References =====

Adlung N, Prochaska H, Thieme S, Banik A, Blüher D, John P, Nagel O, Schulze S, Gantner J, Delker C, Stuttmann J, Bonas U (2006). Non-host resistance induced by the //Xanthomonas// effector XopQ is widespread within the genus //Nicotiana// and functionally depends on EDS1. Front. Plant Sci. 30: 1796. DOI: [[https://doi.org/10.3389/fpls.2016.01796|10.3389/fpls.2016.01796]]

Büttner D, Lorenz C, Weber E, Bonas U (2006). Targeting of two effector protein classes to the type III secretion system by a HpaC- and HpaB-dependent protein complex from //Xanthomonas campestris// pv. //vesicatoria//. Mol Microbiol. 59: 513-527. DOI: [[https://doi.org/10.1111/j.1365-2958.2005.04924.x|10.1111/j.1365-2958.2005.04924.x]]

Herzfeld EM (2013). Identifizierung und Charakterisierung von dem pflanzlichen Interaktionspartner OAS-TL des Typ-III-Effektors XopC. Doctoral Thesis, Martin-Luther-Universität Halle-Wittenberg, Germany. PDF: [[https://opendata.uni-halle.de/handle/1981185920/7783|opendata.uni-halle.de/handle/1981185920/7783]]

Liu Y, Long J, Shen D, Song C (2016). //Xanthomonas oryzae// pv. //oryzae// requires H-NS-family protein XrvC to regulate virulence during rice infection. FEMS Microbiol. Lett. 363: fnw067. DOI: [[https://doi.org/10.1093/femsle/fnw067|10.1093/femsle/fnw067]]

Mondal KK, Soni M, Verma G, Kulshreshtha A, Mrutyunjaya S, Kumar R ( 2020). //Xanthomonas axonopodis// pv. //punicae// depends on multiple non-TAL (Xop) T3SS effectors for its coveted growth inside the pomegranate plant through repressing the immune responses during bacterial blight development. Microbiol Res. 240: 126560 DOI: [[https://doi.org/10.1016/j.micres.2020.126560|10.1016/j.micres.2020.126560]]

Noël L, Thieme F, Gäbler J, Büttner D, Bonas U (2003). XopC and XopJ, two novel type III effector proteins from //Xanthomonas campestris// pv. vesicatoria. J. Bacteriol. 185: 7092-7102. DOI: [[https://doi.org/10.1128/jb.185.24.7092-7102.2003|10.1128/jb.185.24.7092-7102.2003]]

Noël L, Thieme F, Nennstiel D, Bonas U (2001). cDNA-AFLP analysis unravels a genome-wide //hrpG//-regulon in the plant pathogen //Xanthomonas campestris// pv. //vesicatoria//. Mol. Microbiol. 41: 1271-1281. DOI: [[https://doi.org/10.1046/j.1365-2958.2001.02567.x|10.1046/j.1365-2958.2001.02567.x]]

Roden JA, Belt B, Ross JB, Tachibana T, Vargas J, Mudgett MB (2004). A genetic screen to isolate type III effectors translocated into pepper cells during //Xanthomonas// infection. Proc. Natl. Acad. Sci. USA 101: 16624-16629. DOI: [[https://doi.org/10.1073/pnas.0407383101|10.1073/pnas.0407383101]]

Salomon D, Dar D, Sreeramulu S, Sessa G (2011). Expression of //Xanthomonas campestris// pv. //vesicatoria// type III effectors in yeast affects cell growth and viability. Mol. Plant-Microbe Interact. 24: 305-314. DOI: [[https://doi.org/10.1094/MPMI-09-10-0196|10.1094/MPMI-09-10-0196]]

Szurek B, Rossier O, Hause G, Bonas U (2002). Type III-dependent translocation of the //Xanthomonas// AvrBs3 protein into the plant cell. Mol. Microbiol. 46: 13-23. DOI: [[https://doi.org/10.1046/j.1365-2958.2002.03139.x|10.1046/j.1365-2958.2002.03139.x]]

Wang S, Li S, Wang J, Li Q, Xin XF, Zhou S, Wang Y, Li D, Xu J, Luo ZQ, He SY, Sun W (2021). A bacterial kinase phosphorylates OSK1 to suppress stomatal immunity in rice. Nat. Commun.12: 5479. DOI: [[https://doi.org/10.1038/s41467-021-25748-4|10.1038/s41467-021-25748-4]]

Wei Y, Zhu B, Zhang Y, Ma G, Wu J, Tang L, Shi H (2024). CPK1-HSP90 phosphorylation and effector XopC2-HSP90 interaction underpin the antagonism during cassava defense-pathogen infection. New Phytol. 242: 2734-2745. DOI: [[https://doi.org/10.1111/nph.19739|10.1111/nph.19739]]

Wu Y, Wang S, Nie W, Wang P, Fu L, Ahmad I, Zhu B, Chen G (2021). A key antisense sRNA modulates the oxidative stress response and virulence in //Xanthomonas oryzae// pv. //oryzicola//. PLoS Pathog. 17: e1009762. DOI: [[https://doi.org/10.1371/journal.ppat.1009762|10.1371/journal.ppat.1009762]]

===== Acknowledgements =====

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