Author: 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: AAV74208.1 (658 aa)
RefSeq ID: WP_104613784.1 (714 aa)
3D structure: Unknown

XopP was identified in a genetic screen, using a Tn5-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).

Type III-dependent secretion was confirmed using a calmodulin-dependent adenylate cyclase reporter assay, with a ΔhrpF mutant strain serving as negative control (Roden et al., 2004). Using an AvrBs1 reporter fusion, XopP_Xcc8004 was shown to be translated into plant cells in a hrpF- and hpaB-dependent manner (Jiang et al., 2009).

The xopP _Xcc8004 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 ΔxrvC mutant compared with those in the wild-type strain PXO99^A (Liu et al., 2016).

• Roden et al. did not find significant growth defects of a Xcv ΔxopP mutant in susceptible pepper and tomato leaves (Roden et al., 2004).
• XopQ_Xcc8004 is required for full virulence and growth of X. campestris pv. campestris in the host plant Chinese radish (Jiang et al., 2009).
• XopP_Xoo is able to suppress rice pathogen associated molecular pattern (PAMP)-immunity and resistance to Xanthomonas oryzae pv. oryzae. Although XopP_Xoo 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_Xcc 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).

XopP_Xoo co-localizes with OsPUB44 in the cytoplasm (Ishikawa et al., 2014).

XopP_Xcc 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).

XopP_Xoo interacts with the U-box domain of a rice ubiquitin E3 ligase, OsPUB44 and inhibits its activity (Ishikawa et al., 2014). XopP_Xcc 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).

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).

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

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: 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: 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: 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: 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: 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: 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: 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: 10.1073/pnas.0407383101

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