Author: Fernando Tavares
Reviewer: Amandine Cunty

Class: XopR
Family: XopR
Prototype: XOO4134 (Xanthomonas oryzae pv. oryzae; strain T7174R)
GenBank ID: BAE70889.1 (437 aa)
RefSeq ID: XopR WP_011260681.1 (437 aa)
3D structure: Unknown

xopR was firstly identified as a putative T3E ORF (XOO4134) shown to be under regulation of HrpX preceded by both a PIP box and a -10 box-like motif (Furutani et al., 2006). Later, translocation of XOO4134::Cya fusion proteins into plant cells were shown to occur via a T3SS (Furutani et al., 2009; White et al., 2009). XopR_Xoo was confirmed to have a functional type III secretion signal using a reporter fusion with AvrBs1 (Zhao et al., 2013).

Evidence for T3SS-dependent secretion and translocation of XopR into plant cells was mainly based on calmodulin-dependent adenylate cyclase (Cya) reporter assays of fusion proteins (Furutani et al., 2009).

Functional studies using hrp-inducing and non-hrp-inducing media and reverse-transcriptase PCR in wild type and Xoo ∆hrpX mutants showed that the expression of xopR is hrpX dependent (Verma et al., 2019). These results are indirectly supported by previous findings showing that X. oryzae pv. oryzae (Xoo) deficient mutants for xrvB, a gene coding for a repressor of hrp gene expression, leads to an increase of XopR into plant cells (Kametani-Ikawa et al., 2011).

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) were significantly reduced in the Xanthomonas oryzae pv. oryzae ΔxrvC mutant compared with those in the wild-type strain PXO99^A, but this did not apply to xopR (Liu et al., 2016).

In the last few years a comprehensive body of experimental evidence has been gathered supporting a multiple action of XopR in hampering host plant defenses, namely by fostering bacterial growth in planta, and suppressing pathogen-associated molecular patterns (PAMP) triggered host plant immunity (PTI) (Akimoto-Tomiyama et al., 2012; Wang et al., 2016; Medina et al., 2018; Verma et al., 2018; Verma et al., 2019).

A xopR deletion mutant in the Chinese Xoo strain 13751 showed a significant reduction in virulence in hybrid rice cv. Teyou63 compared to the wild type (Zhao et al., 2013). However, the growth of the mutant in host plant rice was not affected. These results indicated that xopR was required for full virulence of Xoo strain 13751 by inducing rice disease tolerance (Zhao et al., 2013).

Later studies suggested that XopR suppress PAMP-triggered stomatal closure in transgenic Arabidopsis expressing XopR (Wang et al., 2016). More recently, when compared with a Xoo wild type strain, xopR deficient mutants (Xoo ∆xopR) infiltrated in rice leaves led to an increase of callose deposits, and a significant higher production of reactive oxygen species (ROS), namely of hydrogen peroxide (H₂O₂) and superoxide anion (O_2^-), known as the main components of the plant oxidative burst (reference ). Furthermore, reverse transcriptase expression analyses of eight rice genes linked to plant disease resistance (BRI1, GST1, PR2, PR5, RAC1, SERK1, WRKY29 and WRKY71) were shown to be up-regulated in rice leaves inoculated with Xoo ∆xopR (Verma et al., 2018; Verma et al., 2019). To further support these findings, complementation of Xoo ∆xopR with xopR was able to restore the disease phenotype of the wild type Xoo strain (Verma et al., 2018; Verma et al., 2019).

Confocal microscopy studies of XopR::EYFP (enhanced yellow fluorescent protein) fusion protein transiently expressed in Nicotiana benthaminiana, suggested that XopR is localized to the plasma membrane of plant epidermal cells (Akimoto-Tomiyama et al., 2012; Verma et al., 2019). These results are further corroborate by findings assigning XopR localization to the plasma membrane of rice protoplasts, contrary to other effectors analysed, namely XopL XopV, XopC, and XopW, which were localized to the cytoplasm (Wang et al., 2016).

Unknown.

Co-immunoprecipitation assays indicate that XopR associates with various receptor-like cytoplasmic kinases (RLCKs), including BIK1 known to be involved in pathogen-associated molecular patterns (PAMP) to triggered stomatal closure (Wang et al., 2016). In vitro kinase assays indicate that XopR is phosphorylated by BIK1 likely affecting BIK1 targets, and possibly impairing PAMP-triggered stomatal immunity (Wang et al., 2016).

Yes (e.g. X. arboricola, X. axonopodis, X. campestris, X. citri, X. gardneri, X. oryzae, X. phaseoli, X. populi, X. vasicola, X. bromi, X. cucurbitae inferred from a BlastP search for a query coverage higher than 90% and a percent identity over 35%).

Unknown.

Akimoto-Tomiyama C, Furutani A, Tsuge S, Washington EJ, Nishizawa Y, Minami E, Ochiai H (2012). XopR, a type III effector secreted by Xanthomonas oryzae pv. oryzae, suppresses microbe-associated molecular pattern-triggered immunity in Arabidopsis thaliana. Mol. Plant Microbe Interact. 25: 505-514. DOI: 10.1094/mpmi-06-11-0167

Furutani A, Nakayama T, Ochiai H, Kaku H, Kubo Y, Tsuge S (2006). Identification of novel HrpXo regulons preceded by two cis-acting elements, a plant-inducible promoter box and a −10 box-like sequence, from the genome database of Xanthomonas oryzae pv. oryzae. FEMS Microbiol. Lett. 259: 133-141. DOI: 10.1111/j.1574-6968.2006.00265.x

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

Kametani-Ikawa Y, Tsuge S, Furutani A, Ochiai H (2011). An H-NS-like protein involved in the negative regulation of hrp genes in Xanthomonas oryzae pv. oryzae. FEMS Microbiol. Lett. 319: 58-64. DOI: 10.1111/j.1574-6968.2011.02266.x

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

Medina CA, Reyes PA, Trujillo CA, Gonzalez JL, Bejarano DA, Montenegro NA, Jacobs JM, Joe A, Restrepo S, Alfano JR, Bernal A (2018). The role of type III effectors from Xanthomonas axonopodis pv. manihotis in virulence and suppression of plant immunity. Mol. Plant Pathol. 19: 593-606. DOI: 10.1111/mpp.12545

Verma G, Sharma M, Mondal KK (2018). XopR TTSS-effector regulates in planta growth, virulence of Indian strain of Xanthomonas oryzae pv. oryzae via suppressing reactive oxygen species production and cell wall-associated rice immune responses during blight induction. Funct. Plant Biol. 45: 561-574. DOI: 10.1071/FP17147

Verma G, Mondal KK, Kulshreshtha A, Sharma M (2019). XopR T3SS-effector of Xanthomonas oryzae pv. oryzae suppresses cell death-mediated plant defense response during bacterial blight development in rice. 3 Biotech. 9: 272. DOI: 10.1007/s13205-019-1802-9

Wang S, Sun J, Fan F, Tan Z, Zou Y, Lu D (2016). A Xanthomonas oryzae pv. oryzae effector, XopR, associates with receptor-like cytoplasmic kinases and suppresses PAMP-triggered stomatal closure. Sci. China Life Sci. 59: 897-905. DOI: 10.1007/s11427-016-5106-6

White FF, Potnis N, Jones JB, Koebnik R (2009). The type III effectors of Xanthomonas. Mol. Plant Pathol. 10: 749-766. DOI: 10.1111/j.1364-3703.2009.00590.x

Zhao S, Mo WL, Wu F, Tang W, Tang JL, Szurek B, Verdier V, Koebnik R, Feng JX (2013). Identification of non-TAL effectors in Xanthomonas oryzae pv. oryzae Chinese strain 13751 and analysis of their role in the bacterial virulence. World J. Microbiol. Biotechnol. 29: 733-744. DOI: 10.1007/s11274-012-1229-5

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