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

Author: [[https://www.researchgate.net/profile/Claude_Bragard|Claude Bragard]]\\
Internal reviewer: Harrold van den Burg\\
Expert reviewer: **WANTED!**

Class: XopP\\
Family: XopP\\
Prototype: XopP (//Xanthomonas euvesicatoria// pv. //euvesicatoria//, ex //Xanthomonas campestris// pv. //vesicatoria//; strain 85-10)\\
GenBank ID: [[https://www.ncbi.nlm.nih.gov/protein/AAV74208.1|AAV74208.1]] (658 aa)\\
RefSeq ID: [[https://www.ncbi.nlm.nih.gov/protein/WP_104613784.1|WP_104613784.1]] (714 aa)\\
3D structure: Unknown

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

=== How discovered? ===

XopP was 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 //X. campestris// pv. //vesicatoria// (//Xcv//) strain 85-10. The XopP::AvrBs2 fusion protein triggered a //Bs2//-dependent hypersensitive response (HR) in pepper leaves (Roden //et al//., 2004). XopP was also identified in //X. campestris// pv. //campestris// (//Xcc//) strain 8004 as a candidate T3E due to the presence of a plant-inducible promoter (PIP) box in its gene, XC_2994 (Jiang //et al.//, 2009).
=== (Experimental) evidence for being a T3E ===

Type III-dependent secretion was confirmed using a calmodulin-dependent adenylate cyclase reporter assay, with a Δ//hrpF// mutant strain serving as negative control (Roden //et al.//, 2004). Using an AvrBs1 reporter fusion, XopP<sub>Xcc8004</sub> was shown to be translated into plant cells in a //hrpF//- and //hpaB//-dependent manner (Jiang //et al.//, 2009).
=== Regulation ===

The //xopP// <sub>Xcc8004</sub> gene contains a PIP box and was shown to be controlled by //hrpG// and //hrpX// (Jiang et al., 2009).

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 //xopP//, 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 ===

  * Roden //et al.//  did not find significant growth defects of a //Xcv//  Δ//xopP//  mutant in susceptible pepper and tomato leaves (Roden et al., 2004).
  * XopQ<sub>Xcc8004</sub>  is required for full virulence and growth of //X. campestris//  pv. //campestris//  in the host plant Chinese radish (Jiang //et al.//, 2009).
  * XopP<sub>Xoo</sub>  is able to suppress rice pathogen associated molecular pattern (PAMP)-immunity and resistance to //Xanthomonas oryzae//  pv. //oryzae//. Although XopP<sub>Xoo</sub>  is classified within the XopP, it shows only 40% sequence identity with the XopP homologue of //X. campestris//  pv. //campestris//  (Furutani //et al//., 2009). Therefore, it remains unclear if such interaction is similar in different pathosystems where XopP has been found.
  * //Agrobacterium//-mediated transient expression of both XopQ and XopX in rice cells resulted in induction of rice immune responses, which were not observed when either protein was individually expressed. A screen for //Xanthomonas//  effectors which can suppress XopQ-XopX induced rice immune responses, led to the identification of five effectors, namely XopU, XopV, XopP, XopG and AvrBs2, that could individually suppress these immune responses. These results suggest a complex interplay of //Xanthomonas//  T3SS effectors in suppression of both pathogen-triggered immunity and effector-triggered immunity to promote virulence on rice (Deb //et al.//, 2020).
  * XopP inhibits the function of the host-plant exocyst complex by direct targeting of Exo70B, a subunit of the exocyst complex, which plays a significant role in plant immunity. XopP interferes with exocyst-dependent exocytosis, and can do this without activating a plant NLR (NOD-like receptor) that guards Exo70B in Arabidopsis. In this way, //Xanthomonas//  efficiently inhibits the host's pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) by blocking exocytosis of pathogenesis-related protein-1A (PR1a), callose deposition and localization of the FLS2 immune receptor to the plasma membrane, thus promoting successful infection (Michalopoulou //et al.//, 2022).
  * Using biophysical, biochemical, and molecular assays in combination with structural and functional predictions, utilizing AplhaFold and DALI online, it was shown that XopP<sub>Xcc</sub>  functions as a novel serine/threonine kinase upon its host target AtExo70B1 but also protects the latter from the innate AtCPK5 phosphorylation, in order to bypass the host’s immune responses (Kotsaridis //et al.//, 2023).

=== Localization ===

XopP<sub>Xoo</sub>  co-localizes with OsPUB44 in the cytoplasm (Ishikawa //et al//., 2014).

=== Enzymatic function ===

XopP<sub>Xcc</sub>  functions as a novel serine/threonine kinase upon its host target AtExo70B1, which gets phosphorylated at Ser107, Ser111, Ser248, Thr309, and Thr364 (Kotsaridis //et al.//, 2023).

=== Interaction partners ===

XopP<sub>Xoo</sub>  interacts with the U-box domain of a rice ubiquitin E3 ligase, OsPUB44 and inhibits its activity (Ishikawa //et al//., 2014). XopP<sub>Xcc</sub>  interacts with EXO70B1, EXO70B2 and EXO70F1 in a yeast two-hybrid assay (Michalopoulou //et al.//, 2022). Interaction was confirmed in planta by split YFP and coIP assays (Michalopoulou //et al.//, 2022).

===== Conservation =====

=== In xanthomonads ===

Yes (//e.g.//, //X. campestris//, //X. citri//, //X. euvesicatoria//, //X. oryzae//, //X. translucens//). Since the G+C content of the //xopP//  gene is similar to that of the //Xcv//  //hrp//  gene cluster, it may be a member of a “core” group of //Xanthomonas//  spp. effectors (Roden et al., 2004).

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

Yes (//e.g.//, //Ralstonia solanacearum//) (Roden //et al//., 2004).

===== References =====

Deb S, Ghosh P, Patel HK, Sonti RV (2020). Interaction of the //Xanthomonas//  effectors XopQ and XopX results in induction of rice immune responses. Plant J. 104: 332-350. DOI: [[https://doi.org/10.1111/tpj.14924|10.1111/tpj.14924]]

Furutani A, Takaoka M, Sanada H, Noguchi Y, Oku T, Tsuno K, Ochiai H, Tsuge S (2009). Identification of novel type III secretion effectors in //Xanthomonas oryzae//  pv. //oryzae//. Mol. Plant Microbe Interact. 22: 96-106. DOI: [[https://doi.org/10.1094/MPMI-22-1-0096|10.1094/MPMI-22-1-0096]]

Ishikawa K, Yamaguchi K, Sakamoto K, Yoshimura S, Inoue K, Tsuge S, Kojima C, Kawasaki T (2014). Bacterial effector modulation of host E3 ligase activity suppresses PAMP-triggered immunity in rice. Nat. Commun. 5: 5430. DOI: [[https://doi.org/10.1038/ncomms6430|10.1038/ncomms6430]]

Jiang W, Jiang B, Xu R, Huang J, Wei H, 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: [[https://doi.org/10.1094/MPMI-22-11-1401|10.1094/MPMI-22-11-1401]]

Kotsaridis K, Michalopoulou VA, Tsakiri D, Kotsifaki D, Kefala A, Kountourakis N, Celie PHN, Kokkinidis M, Sarris PF (2023). The functional and structural characterization of //Xanthomonas campestris//  pv. //campestris//  core effector XopP revealed a new kinase activity. Plant J., in press. DOI: [[https://doi.org/10.1111/tpj.16362|10.1111/tpj.16362]]

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]]

Michalopoulou VA, Mermigka G, Kotsaridis K, Mentzelopoulou A, Celie PHN, Moschou PN, Jones JDG, Sarris PF (2022). The host exocyst complex is targeted by a conserved bacterial type-III effector that promotes virulence. Plant Cell 34: 3400-3424 DOI: [[https://doi.org/10.1093/plcell/koac162|10.1093/plcell/koac162]]

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]]

===== Acknowledgements =====

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