Author: Leonor Martins
Internal reviewer: Jaime Cubero
Expert reviewer: Kalyan K Mondal

Class: XopF
Family: XopF1, XopF2, XopF3
Prototype: XopF (Xanthomonas euvesicatoria pv. euvesicatoria, ex Xanthomonas campestris pv. vesicatoria; strain 85-10)
RefSeq ID: XopF1_Xe WP_011346095.1 (670 aa), XopF1_Xoo WP_AHK80891 (661 aa), XopF2 AAV74205.1 (667 aa)
Synonym: Hpa4
3D structure: Unknown

XopF1 and XopF2 were 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 XopF1::AvrBs2 and XopF2::AvrBs2 fusion proteins triggered a Bs2-dependent hypersensitive response (HR) in pepper leaves (Roden et al., 2004).

Type III-dependent secretion of XopF1 and XopF2 was confirmed using a calmodulin-dependent adenylate cyclase reporter assay, with a ΔhrpF mutant strain serving as negative control (Roden et al., 2004, Mondal et al., 2016).

Fragments of the xopF1 gene are located within the hrp cluster of many Xanthomonas spp., although a complete ORF is present only in the Xcv and Xanthomonas oryzae pv. oryzae (Xoo) hrp clusters (Roden et al., 2004).

XopF1 belongs to the class A effectors (Büttner et al., 2006). XopF2 is 59% identical and 68% similar to XopF1 when analysed with the pairwise BLAST algorithm. xopF2 appears to be co-transcribed with ORF1. ORF1 analysis revealed characteristics shared by type III chaperones, and is suggested to encode an Xcv chaperone (Roden et al., 2004).

RT-PCR analysis revealed xopF1 is regulated by hrpG and hrpX and that xopF1, hpaD, hpaI belong to the same operon. Upstream there is a PIP box which provides binding site for HrpX (Büttner et al., 2007).

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 xopF, 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 ΔxopF1 or ΔxopF2 mutant in susceptible pepper and tomato leaves (Roden et al., 2004)
• To study the possible virulence function of the putative xopF1 operon encoding HpaD, HpaI, and XopF1 these three genes were deleted from the genome of Xcv 85-10. The resultant mutant strain 85-10ΔEF displayed a wild-type phenotype when infiltrated into susceptible and resistant plants. To investigate a possible functional redundancy due to homologous genes, xopF2 and the flanking ORF XCV2943 were also deleted in strain 85-10ΔEF. Since the resulting multiple mutant strain 85-10ΔEFΔxopF2 also behaved like the wild type in infection tests, xopF1 and xopF2 regions did not seem to play an obvious role in the bacterial interaction with the host plant (Büttner et al., 2007).
• Later, Xoo XopF1 was proven to contribute to virulence in rice, as infection with xopF1 mutant has shown a reduced lesion size comparing to wild type (Mondal et al., 2016).
• Additionally, XopF1 and XopF2 of X. euvesicatoria and Xoo seem to have a role in PTI suppression in planta, namely by inhibiting callose deposition and by suppressing the induction of PTI marker genes, overall contributing to development of symptoms (Mondal et al., 2016; Popov et al., 2016).
• Xoo XopF1 triggered an HR in non-host plants (Li et al., 2016).

XopF2 localizes in the Golgi apparatus, while XopF1 has been found in cytoplasm (Popov et al., 2016) and plasma membrane (Mondal et al., 2016). XopF1 is encoded within hrp region, between hpaB and hrpF, while XopF2 is encoded elsewhere in the bacterial chromosome (Roden et al., 2004; Büttner et al., 2007).

Unknown.

XopF1 secretion and translocation is T3SS-dependent; HpaH, HpaC and T3S chaperone HpaB are required for efficient secretion XopF1 (Büttner et al., 2006, 2007).

Yes (e.g., X. arboricola, X. bromi, X. citri, X. oryzae pv. oryzae, X. euvesicatoria, X. translucens, X. vasicola). Since the G+C content of the xopF1 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).

Unknown.

Büttner D, Lorenz C, Weber E, Bonas U (2006). Targeting of two effector protein classes to the type III secretion system by a HpaC- and HpaB-dependent protein complex from Xanthomonas campestris pv. vesicatoria. Mol. Microbiol. 59: 513-527. DOI: 10.1111/j.1365-2958.2005.04924.x

Büttner D, Noël L, Stuttmann J, Bonas U (2007). Characterization of the nonconserved hpaB-hrpF region in the hrp pathogenicity island from Xanthomonas campestris pv. vesicatoria. Mol. Plant Microbe Interact. 20: 1063-1074. DOI: 10.1094/MPMI-20-9-1063

Li S, Wang Y, Wang S, Fang A, Wang J, Liu L, Zhang K, Mao Y, Sun W (2015). The type III effector AvrBs2 in Xanthomonas oryzae pv. oryzicola suppresses rice immunity and promotes disease development. Mol. Plant Microbe Interact. 28: 869-880. DOI: 10.1094/MPMI-10-14-0314-R

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

Mondal K K, Verma G, Manju, Junaid A, Mani C (2016). Rice pathogen Xanthomonas oryzae pv. oryzae employs inducible hrp-dependent XopF type III effector protein for its growth, pathogenicity and for suppression of PTI response to induce blight disease. Eur. J. Plant Pathol. 144: 311-323. DOI: 10.1007/s10658-015-0768-7

Popov G, Fraiture M, Brunner F, Sessa G (2016). Multiple Xanthomonas euvesicatoria type III effectors inhibit flg22-triggered immunity. Mol. Plant Microbe Interact. 29: 651-660. DOI: 10.1094/MPMI-07-16-0137-R

Roden J, Belt B, Ross J, Tachibana T, Vargas J, Mudgett M (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

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