====== XopQ ======

Author: Valérie Olivier & Tamara Popović\\
Internal reviewer: [[https://www.researchgate.net/profile/Ralf_Koebnik|Ralf Koebnik]]\\
Expert reviewer: FIXME

Class: XopQ\\
Family: XopQ\\
Prototype: XCV4438 (//Xanthomonas euvesicatoria// pv. //euvesicatoria//, ex //Xanthomonas campestris// pv. //vesicatoria//; strain 85-10)\\
RefSeq ID: [[https://www.ncbi.nlm.nih.gov/protein/WP_011349176.1|WP_011349176.1]] (464 aa)\\
3D structure: [[https://www.rcsb.org/structure/4kl0|4KL0]] (Yu //et al.//, 2013); [[https://www.rcsb.org/structure/4P5F|4P5F]] (Yu //et al.//, 2014)

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

=== How discovered? ===

XopQ 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 XopQ::AvrBs2 fusion protein triggered a //Bs2//-dependent hypersensitive response (HR) in pepper leaves (Roden //et al//., 2004). XopQ 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_3177 (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, XopQ<sub>Xcc8004</sub> was shown to be translated into plant cells in a //hrpF//- and //hpaB//-dependent manner Jiang et al., 2009).
=== Regulation ===

The //xopQ// <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//) were significantly reduced in the //Xanthomonas oryzae// pv. //oryzae// Δ//xrvC// mutant compared with those in the wild-type strain PXO99<sup>A</sup>  , but this did not apply to //xopQ// (Liu //et al.//, 2016).
=== Phenotypes ===

  * Roden //et al//. did not find significant growth defects of a //Xcv//  Δ//xopQ//  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).
  * In //X. oryzae//  pv. //oryzae//  (//Xoo//), XopQ was described by Sinha //et al//. to suppress DAMP-induced PTI in rice. Indeed, //Xoo//  secretes hydrolytic enzymes such as LipA (Lipase/Esterase) that damage rice cell walls and induce innate immune responses. XopQ was found to suppress LipA-induced innate immune responses in rice (Sinha //et al//., 2013).
  * XopQ<sub>Xcv</sub>  suppresses cell death controlled by components of the MAP kinase cascade MAPKKKα/MEK2/SIPK and induced by certain //R/////avr//  gene pairs, such as //Pto/////avrPto//  and Gpa2/RBP–1, but not that induced by other gene pairs (//Bs3/////avrBs3//, //RPS2/////avrRpt2//, //C9/////avr9//, //Rx2/////Cp//) (Teper //et al//., 2014).
  * Consistent with a role in ETI, TFT4 mRNA abundance increased during the incompatible interaction of tomato and pepper with //Xcv//  (Teper //et al//., 2014).
  * Mutations of two potential active site residues, D116 and Y279, resulted in //Xoo//  mutants with reduced virulence on rice and reduced hypersensitive response (HR) on //Nicotiana benthamiana//, a nonhost. However, Arabidopsis lines expressing either xopQ or //xopQ// <sub>Y279A</sub>  were equally proficient at suppression of LipA-induced callose deposition (Gupta //et al.//, 2015).
  * Compatibility studies with //X. euvesicatoria//  pv. //perforans//  revealed that a double deletion of //avrBsT//  and //xopQ//  allows a host range expansion for //Nicotiana benthamiana//  (Schwartz //et al.//, 2015).
  * The avirulence activity of XopQ derivatives did not correlate with macroscopically visible plant reactions upon transient expression in //N. benthamiana//. It was therefore speculated that //N. benthamiana//  might encode two resistance proteins for the recognition of XopQ (Adlung, 2016).
  * Transient co-expression of XopQ::GFP and XopS::GFP in //N. benthamiana//  triggered cell death reactions, which were not observed when each effector was expressed alone. Bimolecular fluorescence complementation using split-YFP derivatives revealed that XopQ and XopS co-localize in the nucleus. These results suggested that both effectors may form a protein-protein complex i//n planta //  (Adlung, 2016).
  * XopQ suppressed cell death reactions in //N. benthamiana//  that were triggered by three //Xcv//  type III effectors (XopB, XopJ, XopL), whereas cell death reactions triggered by AvrBsT were not suppressed by XopQ (Adlung, 2016).
  * XopQ-mediated cell death suppression in //N. benthamiana//  during transient expression assays was later shown to result from an attenuation of //Agrobacterium//  ‐mediated protein expression rather than reflecting a genuine XopQ virulence activity (Adlung & Bonas, 2017).
  * A Δ//xopN//–Δ//xopQ //double knock-out mutant in //X. phaseoli//  pv. //manihotis//  (//Xpm//) was less aggressive in the cassava host plant than its single mutation counterparts. In addition, //in planta //  bacterial growth was reduced at 5 dpi in the double mutant with respect to the wild-type strain CIO151 and individual knock-out strains. The phenotype of the double mutant could be complemented when transforming a plasmid containing //xopQ//. These results confirmed that //xopN //and// xopQ //are functionally redundant in //Xpm//  (Medina //et al.//, 2017).
  * A reverse genetics screen identified Recognition of XopQ 1 (Roq1), a nucleotide-binding leucine-rich repeat (NLR) protein with a Toll-like interleukin-1 receptor (TIR) domain, which mediates XopQ recognition in //N. benthamiana//. Roq1 orthologs appear to be present only in the //Nicotiana//  genus. Expression of Roq1 was found to be sufficient for XopQ recognition in both the closely-related //Nicotiana sylvestris//  and the distantly-related beet plant (//Beta vulgaris//) (Schultink //et al.//, 2017).
  * Roq1 is able to recognize XopQ alleles from various //Xanthomonas//  species, as well as HopQ1 from //Pseudomonas//, demonstrating widespread potential application in protecting crop plants from these pathogens (Schultink //et al.//, 2017).
  * The coiled-coil NLR protein N requirement gene 1 (NRG) interacts with EDS1 and acts downstream of Roq1 and EDS1 to mediate XopQ/HopQ1-triggered ETI. In addition, Roq1, EDS1, and NRG1 mediate XopQ-triggered transcriptional changes in //N. benthamiana//  and regulate resistance to //Xanthomonas//  and //Pseudomonas//  species that carry the effectors XopQ or HopQ1. This study suggests that NRG1 may be a conserved key component in TNL-mediated signaling pathways (Qi //et al.//, 2018).
  * Roq1 is also involved in the recognition of RipB, the homolog of XopQ in //Ralstonia solanacearum//: The RipB‐induced resistance against //R. solanacearum//  was abolished in Roq1‐silenced plants (Nakano & Mukaihara, 2019).
  * Effectors that interact with 14–3–3 proteins may provide plant-pathogenic bacteria with the ability to modulate PTI as well as ETI. Suppression of immune responses induced by a //xopN//–//xopQ//–//xopX//–//xopZ//  quadruple mutant by the XopQ effector may be both suppression of ETI as well as suppression of DTI (damage-triggered immunity) caused by the release of DAMPs by the quadruple mutant strain (Deb //et al//., 2019).
  * Roq1 was found to confer immunity to //Xanthomonas//  (containing XopQ), //P. syringae//  (containing the XopQ homolog HopQ1), and //Ralstonia//  (containing the XopQ homolog RipB) when expressed in tomato (Thomas //et al.//, 2020).
  * Strong resistance to //Xanthomonas euvesicatoria//  pv. //perforans//  was observed with transgenic tomato plants expressing Roq1 from //N. benthamiana//  in three seasons of field trials with both natural and artificial inoculation. The Roq1 gene can therefore be used to provide safe, economical, and effective control of these pathogens in tomato and other crop species and reduce or eliminate the need for traditional chemical controls (Thomas //et al.//, 2020).
  * Agrobacterium-mediated transient expression of both XopQ and XopX in rice cells resulted in induction of rice immune responses. These immune responses were not observed when either protein was individually expressed in rice cells. XopQ-XopX induced rice immune responses were not observed with a XopX mutant that is defective in 14-3-3 binding (Deb //et al.//, 2020).
  * 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).

=== Localization ===

Cytoplasma and nucleus (Deb //et al//., 2019).

=== Enzymatic function ===

XopQ is structurally homologous to an inosine-uridine nucleoside N-ribohydrolase from a protazoan parasite, as shown be [[https://en.wikipedia.org/wiki/Phyre|3D-PSSM]] analysis. Such proteins are implicated in the ability of many organisms to salvage nucleotides from their environment. Like other homologs, XopQ contains conserved aspartate residues found in the active site of the enzyme. It was speculated that XopQ may function as a scavenging hydrolase //in planta//  or may interfere with plant cell processes by binding and sequestering nucleosides important for plant signaling and/or metabolism (Roden //et al.//, 2004).

Despite such similarities, later structural and functional studies revealed that XopQ<sub>Xoo</sub>  does not exhibit the expected activity of a nucleoside hydrolase (Yu //et al//., 2013). Purified XopQ<sub>Xoo</sub>  did not show NH activity on standard nucleoside substrates but exhibited ribose hydrolase activity on the nucleoside substrate analogue 4-nitrophenyl β-D-ribofuranoside (Gupta //et al.//, 2015). The D116A and Y279A mutations cause a reduction in biochemical activity (Gupta //et al.//, 2015).

In 2014, Yu //et al//. reported the crystal structure of XopQ<sub>Xoo</sub>  in complex with adenosine diphosphate ribose (ADPR), which is involved in regulating cytoplasmic Ca<sup>2+</sup>   concentrations in eukaryotic cells, which is one of the key events in the immune response elicited by pathogen invasion of a host plant (Yu //et al//., 2014). ADPR is bound to the active site of XopQ<sub>Xoo</sub>  with its ribosyl end tethered to the Ca<sup>2+</sup>   coordination shell. The binding of ADPR is further stabilized by interactions mediated by hydrophobic residues that undergo ligand-induced conformational changes. XopQ<sub>Xoo</sub>  is capable of binding a novel chemical bearing a ribosyl moiety (Yu //et al//., 2014).

=== Interaction partners ===

Using protein-protein interaction studies in yeast and in planta, XopQ<sub>Xcv</sub>  was shown to physically interacts with the 14–3–3 protein TFT4 from tomato (//Solanum lycopersicum//) (Teper //et al.//, 2014). A mutation in the putative 14–3–3 binding site of XopQ (S65A) impaired interaction of the effector with TFT4 from pepper and tomato (//Capsicum annuum//) and its virulence function //in planta//  (Teper //et al.//, 2014). Yeast 2-hybrid assays revealed that XopQ<sub>Xcv</sub>  interacts with multiple, but perhaps not all 14–3–3 protein isoforms (Teper //et al.//, 2014; Dubrov //et al.//, 2018).

Bimolecular fluorescence complementation assays upon transient expression in //N. benthamiana//  using split-YFP derivatives revealed that XopQ may interact with itself and also with XopS, maybe forming a large protein complex i//n planta //  (Adlung, 2016).

Roq1, a nucleotide-binding leucine-rich repeat (NLR) protein with a Toll-like interleukin-1 receptor (TIR) domain, was found to co-immunoprecipitate with XopQ, suggesting a physical association between the two proteins (Schultink //et al.//, 2017).

XopQ<sub>Xoo</sub>  was shown to interact in yeast and i//n planta//  with two rice 14–3–3 proteins, Gf14f and Gf14g (Deb //et al//., 2019). A serine to alanine mutation (S65A) of a 14–3–3 interaction motif in XopQ abolished the ability of XopQ to interact with the two 14–3–3 proteins and to suppress innate immunity.

Yeast two-hybrid, bimolecular fluorescence complementation (BiFC) and co-IP assays indicated that XopQ and XopX interact with each other (Deb et al., 2020).

===== Conservation =====

=== In xanthomonads ===

XopQ is a widely conserved across //Xanthomonas//  ssp., such as //X. campestris//,// X. citri//, //X. euvesicatoria//, //X. oryzae//  (Roden //et al//., 2004; Furutani et al., 2009; Hajri et al., 2009; Thomas //et al.//, 2020). Since the G+C content of the //xopQ//  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 ===

XopQ shares homology with the //Ralstonia solanacearum//  effector RipB and the //Pseudomonas syringae//  pv. //tomato//  effector HolPtoQ/HopQ (Roden //et al//., 2004; Büttner & Bonas, 2010). Unlike most recognized effectors, alleles of XopQ/HopQ1 are highly conserved and present in most plant-pathogenic strains of //Xanthomonas//  and //P. syringae//, and the homolog of XopQ/HopQ1, named RipB, is present in most Ralstonia strains (Thomas //et al.//, 2020).

===== References =====

Adlung N (2016). Charakterisierung der Avirulenzaktivität von XopQ und Identifizierung möglicher Interaktoren von XopL aus //Xanthomonas campestris// pv. //vesicatoria//. Doctoral Thesis. Martin-Luther-Universität Halle-Wittenberg, Germany. PDF: [[https://d-nb.info/1116951061/34|d-nb.info/1116951061/34]]

Adlung N, Bonas U (2017). Dissecting virulence function from recognition: cell death suppression in //Nicotiana benthamiana// by XopQ/HopQ1-family effectors relies on EDS1-dependent immunity. Plant J. 91: 430-442. DOI: [[https://doi.org/10.1111/tpj.13578|10.1111/tpj.13578]]

Adlung N, Prochaska H, Thieme S, Banik A, Blüher D, John P, Nagel O, Schulze S, Gantner J, Delker C, Stuttmann J, Bonas U (2016). Non-host resistance induced by the //Xanthomonas// effector XopQ is widespread within the genus //Nicotiana// and functionally depends on EDS1. Front. Plant Sci. 7: 1796. DOI: [[https://doi.org/10.3389/fpls.2016.01796|10.3389/fpls.2016.01796]]

Büttner D, Bonas U (2010). Regulation and secretion of //Xanthomonas// virulence factors. FEMS Microbiol. Rev. 34: 107-133. DOI: [[https://doi.org/10.1111/j.1574-6976.2009.00192.x|10.1111/j.1574-6976.2009.00192.x]]

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Deb S, Gupta MK, Patel HK, Sonti RV (2019). //Xanthomonas oryzae// pv. //oryzae// XopQ protein suppresses rice immune responses through interaction with two 14-3-3 proteins but its phospho-null mutant induces rice immune responses and interacts with another 14-3-3 protein. Mol. Plant Pathol. 20: 976-989. DOI: [[https://doi.org/10.1111/mpp.12807|10.1111/mpp.12807]]

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