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

Author: [[https://www.researchgate.net/profile/Joana_Costa12|Joana Costa]] & Trainees from the 2<sup>nd</sup>  EuroXanth Training School (Maria Laura Destefanis, [[https://www.researchgate.net/profile/Katarina_Gasic|Katarina Gašić]], [[https://www.researchgate.net/profile/G_Licciardello|Grazia Licciardello]], Tamara Popović)\\
Internal reviewer: [[https://www.researchgate.net/profile/Isabel-Rodrigues-12|Isabel Rodrigues]]

Class: XopI\\
Families: XopI1, XopI2\\
Prototype: XCV0806 (//Xanthomonas euvesicatoria// pv. //euvesicatoria//, ex //Xanthomonas campestris// pv. //vesicatoria//; strain 85-10)\\
GenBank ID: [[https://www.ncbi.nlm.nih.gov/protein/CAJ22437.1|CAJ22437.1]] (450 aa)\\
RefSeq ID: XopI1 [[https://www.ncbi.nlm.nih.gov/protein/WP_011346406.1|WP_011346406.1]] (450 aa), XopI2 [[https://www.ncbi.nlm.nih.gov/protein/WP_010367684.1|WP_010367684.1]] (419 aa)\\
3D structure: Unknown

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

=== How discovered? ===

Effector proteins (T3Es) can suppress the plant innate immunity and alter the plant metabolism to the pathogen’s advantage. The T3E XopI was identified in //Xcv// strain 85-10 due to a F-box motif based on the presence of a PIP (pathogen-inducible promoter) box in its promoter region. XopI secretion and translocation was shown during the interaction of //Xcv// with resistant pepper plants (Schulze //et al//., 2012). Moreover, interaction studies in yeast showed that XopI specifically interacts with one out of 21 //Arabidopsis thaliana// Skp1-like proteins (ASK), suggesting that upon infection, XopI integrates into particular Skp1-Cullin-F-box protein (SCF) which target proteins for ubiquitination (Salomon //et al//., 2011). A yeast-two-hybrid screen with XopI as bait identified five proteins, that presumably are involved in the regulation of stomatal movement. Silencing of two of these potential interactors confirmed that they mediate stomatal closure after PAMP treatment in //Nicotiana benthamiana//. In tomato plants, virulence of //xopI// knockout strains is dramatically reduced. The stomatal aperture is as well reduced, suggesting that XopI is essential for //Xcv// entry into the host plant apoplast. Stomata assays with stable xopI transgenic //N. benthamiana// lines showed, that XopI suppresses tomatal closure induced by different treatments, suggesting that XopI maybe affects different pathways of stomatal immunity (Nagel & Bonas, 2018).
=== (Experimental) evidence for being a T3E ===

The transcripts of //xopI// were amplified from //Xcv// derivative 85* strain, which expresses the constitutively active HrpG* mutant resulting in constitutive expression of the T3S system, suggesting co‐expression with T3S genes (Schulze //et al//., 2012). To investigate whether //xopI// was indeed T3SS dependently secreted and translocated into the plant cell, a translational fusion with the reporter protein AvrBs3Δ2, a derivative of the TAL effector AvrBs3 which lacks a T3S and translocation signal, was performed. Fusion of a functional T3S signal to AvrBs3Δ2 enables its translocation and thus the induction of the HR in pepper cultivar ECW-30R plants that harbor the corresponding resistance gene //Bs3//. When the bacteria were incubated in T3S medium, XopI<sub>1–140</sub>-AvrBs3Δ2 was detected in the culture supernatant of strain 85*, but not of 85*Δ//hrcV//, by an AvrBs3-specific antibody. These results demonstrate that the XopI effector contained functional T3S signals in the N-terminal regions (Schulze //et al//., 2012) To test for T3SS-dependent translocation, //Xcv// strains 85* and 85*Δ//hrcV// expressing avrBs3Δ2 or //xopI// fusion were inoculated into leaves of AvrBs3-responsive pepper plants (ECW-30R) and the near-isogenic susceptible pepper line ECW, which lacks the //Bs3// resistance gene. Derivatives of strain 85* expressing XopI<sub>1–140</sub>-AvrBs3Δ2 induced the HR in ECW-30R, but not in ECW. No HR induction was observed in plants infected with derivatives of strain 85*Δ//hrcV//. Taken together, these findings confirm the T3SS secretion and translocation of XopI, and thus their nature as T3Es.

XopI belongs to the translocation class of T3SS-secreted proteins, based on HpaB dependence (Büttner //et al.//, 2006).
=== Regulation ===

XopI is presumably controlled by both HrpG and HrpX. The HrpX-dependent induction of //xopR// has been described previously (Koebnik //et al.//, 2006). HrpG‐ and HrpX‐dependent co‐regulation with the T3S system.
=== Phenotypes ===

Bacterial strains carrying deletions of XopI showed no difference in the induction of disease symptoms and the HR compared with wild-type strain 85-10 (Schulze //et al//., 2012). In tomato plants, virulence of xopI knockout strains is dramatically reduced. The stomatal aperture is as well reduced, suggesting that XopI is essential for Xcv entry into the host plant apoplast
=== Localization ===

XopI is translocated by the 85*Δ//hpaB// strain, thus belonging to the class B (Büttner //et al.//, 2006).
=== Enzymatic function ===

The T3Es XopE1, XopF2, XopH, **XopI**, XopM, XopQ, XopV, AvrBs1 and AvrXv4 partially or fully inhibited cell death triggered by at least one of the cell death inducers (Teper //et al.//, 2015).
=== Interaction partners ===

XopR and XopS belong to //Xcv// translocation class A, comprising T3Es whose translocation into plant cells depends on HpaB, whereas XopB, XopG, **XopI**, XopK, XopM and XopV were assigned to class B, because they are still translocated in the absence of HpaB (Büttner //et al.//, 2006). Both new class A effectors lack homology to known proteins or motifs, so that their molecular function remains elusive. By contrast, the class B effectors comprise the putative enzyme XopG, a member of the HopH family of putative zinc metalloproteases. Other effectors possess interesting features, for example XopI contains an F‐box motif typical for eukaryotic proteins playing a role in the ubiquitin‐26S proteasome system (UPS). The UPS controls protein stability in eukaryotes and appears to be a favorable target for many T3Es, for example members of the GALA family, which strongly contribute to the virulence of //R. solanacearum// and the E3 ubiquitin ligase AvrPtoB from //P. syringae.//

===== Conservation =====

=== In xanthomonads ===

Yes.

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

No.

===== References =====

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.13652958.2005.04924.x]]

Nagel O, Bonas U (2018). The //Xanthomonas// effector protein XopI suppresses the stomatal immunity of tomato. Poster. 6<sup>th</sup> //Xanthomonas// Genomics Conference & 2<sup>nd</sup> Annual EuroXanth Conference. PDF: [[https://euroxanth.eu/wp-content/uploads/2018/07/EuroXanth_Second-Annual-Conference-Abstract-Book.pdf|https://euroxanth.eu/wp-content/uploads/2018/07/EuroXanth_Second-Annual-Conference-Abstract-Book.pdf]]

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]]

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: [[https://doi.org/10.1111/j.1469-8137.2012.04210.x|10.1111/j.1469-8137.2012.04210.x]]

Teper D, Sunitha S, Martin GB, Sessa G (2015). Five //Xanthomonas// type III effectors suppress cell death induced by components of immunity-associated MAP kinase cascades. Plant Signal. Behav. 10: e1064573. DOI: [[https://doi.org/10.1080/15592324.2015.1064573|10.1080/15592324.2015.1064573]]

Üstün S, Börnke F (2014). Interactions of //Xanthomonas// type-III effector proteins with the plant ubiquitin and ubiquitin-like pathways Front. Plant Sci. 5: 736. DOI: [[https://doi.org/10.3389/fpls.2014.00736|10.3389/fpls.2014.00736]]

===== Further reading =====

Thieme F (2006). Genombasierte Identifizierung neuer potentieller Virulenzfaktoren von //Xanthomonas// //campestris// pv. //vesicatoria//. Doctoral Thesis, Martin-Luther-Universität Halle-Wittenberg, Germany. PDF: [[http://sundoc.bibliothek.uni-halle.de/diss-online/06/06H103/prom.pdf|http://sundoc.bibliothek.uni-halle.de/diss-online/06/06H103/prom.pdf]]

===== Acknowledgements =====

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