Author: Christian Vernière & Trainees from the 2nd EuroXanth Training School (Songül Erken, Damla Ertimurtaş, Jelena Menković, Andjelka Prokić)
Internal reviewer: Tamás Kovács
Expert reviewer: Nian Wang
Class: XopAG
Family: XopAG1, XopAG2
Prototype: AvrGf1 (Xanthomonas citri pv. citri; Xac‐Aw strain 12879), AvrGf2 (Xanthomonas fuscans pv. aurantifolii; Xac‐Aw strain Xfa-C51302)
GenBank ID (AvrGf1): ABB84189.1 (532 aa)
GenBank ID (AvrGf2): AIP90071.1 (508 aa)
RefSeq ID (XopAG1): WP_272820829.1 (511 aa)
RefSeq ID (XopAG2): WP_007970248.1 (508 aa)
Synonym: AvrGf1, AvrGf2
3D structure: Unknown
Xanthomonas citri pv. citri (Xcc-A) causing citrus bacterial canker can infect most of the commercial citrus species and are worldwide distributed. Strains that were pathogenic on Key lime (Citrus aurantifolia), but that did not cause canker symptoms on grapefruit, were reported in Florida and designated as Xcc-Aw . Three clones were selected from a genomic library of the 12879 strain of Xcc-Aw that caused rapid necrosis in grapefruit leaves, but not in tomato leaves when they were expressed in X. perforans (Rybak et al., 2009). A 1599-bp open reading frame (ORF) was found within the nucleotide sequence of DNA from a 2.3-kb subclone from pL799 that caused HR in grapefruit leaves. The complete sequence of the ORF, designated as avrGf1 (Rybak et al., 2009). Genes avrGf1 and avrGf2 were found to share low sequence similarity at the nucleotide level, except for a small region in the last 200 nucleotides of the genes, which showed a high level of identity (68%) (Gochez et al., 2017). The alignment of translated proteins AvrGf1 (533 amino acids) and AvrGf2 (509 amino acids) determined that AvrGf2 had a low degree of sequence identity (45% amino acid identity) with the previously identified AvrGf1. The highest sequence similarities were observed between AvrGf1 and AvrGf2 in the C-terminal portions of the effector proteins (74.5% identity at the amino acid level over 51 amino acids) (Gochez et al., 2017).
An active TTSS is necessary for HR produced by AvrGf1 in grapefruit leaves, as it was proven by transconjugation experiments (Rybak et al., 2009).
No data available. The effector gene xopAG was however shown to be induced in XVM2 medium compared to NB medium in X. citri subsp. citri AW 12879 strain, a variant strain restricted to Mexicanl lime (Jalan et al., 2013).
All xopAG-containing strains of X. citri pv. citri induced the hypersensitive response (HR) on grapefruit (Citrus paradisi) and sweet orange (C. sinensis) but express canker symptoms on Key lime (Escalon et al., 2013). After infiltration of grapefruit leaves with inoculum adjusted to 5×108 cfu/mL, internal bacterial populations of Xcc-A (strain A 40) and Xcc-Aw (strain 12879) were similar through the second day, but populations of Xcc-A were significantly greater than those of Xcc-Aw after six days. The symptoms caused by the Xac-Aw ΔavrGf1 strain that was mutated on avrGf1 were more similar to those produced by the wild-type Xac-A strain than to those produced by the wild-type Xac-Aw strain (Rybak et al., 2009). So the whole pathogenicity was not restored.
AvrGf1 (Figueiredo et al., 2011) and AvrGf2 (Gochez et al., 2017) possess a N-terminal chloroplast localization signal. The signal is not shared by all members of the XopAG effector family (Gochez et al., 2017). Transient expression of the protein with the first 116 amino acids deleted in grapefruit leaves resulted in the elimination of the HR and a lack of accumulation of the protein in the chloroplast.
AvrGf2 elicited rapid cell death in grapfruit leaves (Gonchez et al., 2015), detailed enzymatic function has not been determined yet.
The XopAG AvrGf2 effector contains a Cyp-binding site that is essential for the elicitation of HR in citrus (Gochez et al., 2017). Yeast two-hybrid experiments showed strong interaction of AvrGf2 with grapefruit cyclophilin (GfCyp), whereas mutation of the GPLL motif in the cyclophilin-binding domain abolished the interaction.
Yes (e.g., X. campestris, X. vasicola) (Gochez et al., 2017).
Yes (e.g., P. syringae. pv. phaseolicola (HopG1), P. syringae pv. tomato (HopG1), Ralstonia solanacearum, Acidovorax citrulli) (Gochez et al., 2017).
Escalon A, Javegny S, Vernière C, Noël LD, Vital K, Poussier S, Hajri A, Boureau T, Pruvost O, Arlat M, Gagnevin L (2013). Variations in type III effector repertoires, pathological phenotypes and host range of Xanthomonas citri pv. citri pathotypes. Mol. Plant Pathol. 14: 483-496. DOI: 10.1111/mpp.12019
Figueiredo JF, Romer P, Lahaye T, Graham JH, White FF, Jones JB (2011). Agrobacterium-mediated transient expression in citrus leaves: a rapid tool for gene expression and functional gene assay. Plant Cell Rep. 30: 1339-1345. DOI: 10.1007/s00299-011-1045-7 Gochez AM, Minsavage GV, Potnis N, Canteros BI, Stall RE, Jones JB (2015). A functional XopAG homologue in Xanthomonas fuscans pv. aurantifolii strain C limits host range. Plant Pathol, 64: 1207-1214. DOI: 10.1111/ppa.12361
Gochez AM, Shantharaj D, Potnis N, Zhou X, Minsavage GV, White FF, Wang N, Hurlbert JC, Jones JB (2017). Molecular characterization of XopAG effector AvrGf2 from Xanthomonas fuscans ssp. aurantifolii in grapefruit. Mol. Plant Pathol. 18: 405-419. DOI: 10.1111/mpp.12408
Jalan N, Kumar D, Andrade MO, Yu F, Jones JB, Graham JH, White FF, Setubal JC, Wang N (2013). Comparative genomic and transcriptome analyses of pathotypes of Xanthomonas citri subsp. citri provide insights into mechanisms of bacterial virulence and host range. BMC Genomics 14: 551. DOI: 10.1186/1471-2164-14-551
Rybak M, Minsavage GV, Stall RE, Jones JB (2009). Identification of Xanthomonas citri ssp. citri host specificity genes in a heterologous expression host. Mol. Plant Pathol. 10: 249-262. DOI: 10.1111/j.1364-3703.2008.00528.x
This fact sheet is based upon work from COST Action CA16107 EuroXanth, supported by COST (European Cooperation in Science and Technology).