A functional XopAG homologue in Xanthomonas fuscans pv. aurantifolii strain C limits host range

Abstract

Citrus canker is caused by two Xanthomonas species, Xanthomonas citri, which has become the primary pathogen where citrus canker occurs (type A citrus canker, Xc‐A), and X. fuscans pv. aurantifolii (Xfa), which consists of strains B and C. The B strain is less pathogenic than the A strain, but produces symptoms in all citrus species. The C‐type cankers only infect Key lime (Citrus aurantifolia) and produce a hypersensitive reaction (HR) in grapefruit (Citrus paradisi) leaves. An avirulence gene, avrGf2, was identified in a C strain that was responsible for the HR in grapefruit. AvrGf2 is a member of XopAG effector family and shares 45% identity at amino acid level with another member of the same family, AvrGf1 from strain Xc‐Aw, which was previously shown to elicit an HR in grapefruit. AvrGf2 shares sequence identity with other XopAG effectors present in Xanthomonas vasculorum, Xanthomonas campestris pv. musacearum and Pseudomonas syringae pv. tomato. Mutagenesis of avrGf2 in C strain resulted in a compatible reaction in grapefruit. There was no observable effect on virulence when Xc‐A transconjugants containing either avirulence gene were inoculated on Key lime. Expression of avrGf1 or avrGf2 in Xc‐A resulted in a similar phenotype following infiltration into grapefruit leaves, although the avrGf2 transconjugant elicited a faster HR and lower populations than the transconjugant containing avrGf1. Also, it was shown that all Xfa‐B strains tested contain a transposon in avrGf2 that helps to explain the differences in host range between B and C strains.

Using Ecology, Physiology, and Genomics to Understand Host Specificity in Xanthomonas

“…Generalist: organism with an extended host range; i.e., it interacts with multiple hosts with little selectivity However, when expressed by Xcci-A strains in grapefruit, xopAG or its homolog avrGf2 induces an HR reaction (50). Conversely, T3Es may act as host-range broadening factors by suppressing ETI induced by another T3E.…”

Recent advances in the understanding of Xanthomonas citri ssp. citri pathogenesis and citrus canker disease management

“…Rybak et al, 2009). The XopAG homologue, avrGf2, which also elicits an HR in grapefruit, has been isolated from Xcc-C (Gochez et al, 2015). A Cyp-binding site (GPLL) present in avrGf2 (and in every member of the XopAG effector family) was essential for the expression of an HR (Gochez et al, 2016).…”

Molecular characterization of XopAG effector AvrGf2 from Xanthomonas fuscans ssp. aurantifolii in grapefruit

“…We hypothesized that the sequence similarity of the C‐terminal portions of the two effectors may reflect the importance of the conserved motifs in the function of the proteins. AvrGf2 was shown to elicit a faster HR than AvrGf1 in citrus (Gochez et al ., ). Given that only the carboxy‐terminal amino acids of the two effectors show significant similarity, the first experimental approach was to determine whether the carboxy‐termini of the effectors could be swapped and result in a difference in the timing of HR.…”

“…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. Another avirulence gene, avrGf2 , was identified in Xfa ‐C strains (Gochez et al ., ), and resulted in the elicitation of an HR in all citrus species, except Key lime. Compared with avrGf1 , a faster HR was elicited by avrGf2 in citrus.…”