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Bacterial virulence factors

Plant resistance genes

Molecular Diagnosis and Diversity for Regulated Xanthomonas

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

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Author: Joana Costa & Trainees from the 2nd EuroXanth Training School (Maria Laura Destefanis, Katarina Gašić, Grazia Licciardello, Tamara Popović)
Internal reviewer: Isabel Rodrigues
Expert reviewer: FIXME

Class: XopI
Family: XopI
Prototype: (Xanthomonas euvesicatoria pv. euvesicatoria, ex Xanthomonas campestris pv. vesicatoria; strain 85-10)
RefSeq ID:
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 a constitutively active HrpG point 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, XopI1–140-AvrBs3Δ2 was detected in the culture supernatant of strain 85*, but not of 85*ΔhrcV, 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*ΔhrcV 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 XopI1–140-AvrBs3Δ2 induced the HR in ECW-30R, but not in ECW. No HR induction was observed in plants infected with derivatives of strain 85*ΔhrcV. Taken together, these findings confirm the T3SS secretion and translocation of XopI, and thus their nature as T3Es.

Translocation class; classification based on HpaB dependence (Büttner et al., 2006).


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.


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


XopI is translocated by the 85*ΔhpaB strain, that belong to the class B.

Enzymatic function

These phenotypes can be ascribed either to the virulence activity of the effectors in plant cells, or to their recognition by the plant surveillance system. As shown in **Fig. 1A** and **Table 1**, the XopE1, XopF2, XopH, XopI, XopM, XopQ, XopV, AvrBs1 and AvrXv4 effectors partially or fully inhibited cell death triggered by at least one of the cell death inducers.

Interaction partners

XopR and XopS belong to Xcv translocation class A, comprising T3Es whose translocation into plant cells is completely dependent 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.


In xanthomonads


In other plant pathogens/symbionts



Büttner D, Lorenz C, Weber E, Bonas U (2006). Targeting of two effectorprotein classes to the type III secretion system by a HpaC- andHpaB-dependent protein complex from Xanthomonas campestris pv. Vesicatoria. Mol Microbiol.59: 513–527. DOI: 10.1111/j.13652958.2005.04924.x

Nagel O, Bonas U (2018). The Xanthomonas effector protein XopI suppresses the stomatal immunity of tomato. Poster. 6th Xanthomonas Genomics Conference & 2nd Annual EuroXanth Conference. 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: 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: 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: 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: 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:

bacteria/t3e/xopi.txt · Last modified: 2020/07/09 10:16 by rkoebnik