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

Abstract

Taxonomic status: Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadaceae; Genus Xanthomonas; Species Xanthomonas citri ssp. citri (Xcc). Host range: Compatible hosts vary in their susceptibility to citrus canker (CC), with grapefruit, lime and lemon being the most susceptible, sweet orange being moderately susceptible, and kumquat and calamondin being amongst the least susceptible. Microbiological properties: Xcc is a rod-shaped (1.5-2.0 × 0.5-0.75 µm), Gram-negative, aerobic bacterium with a single polar flagellum. The bacterium forms yellow colonies on culture media as a result of the production of xanthomonadin. Distribution: Present in South America, the British Virgin Islands, Africa, the Middle East, India, Asia and the South Pacific islands. Localized incidence in the USA, Argentina, Brazil, Bolivia, Uruguay, Senegal, Mali, Burkina Faso, Tanzania, Iran, Saudi Arabia, Yemen and Bangladesh. Widespread throughout Paraguay, Comoros, China, Japan, Malaysia and Vietnam. Eradicated from South Africa, Australia and New Zealand. Absent from Europe.

Keywords: Asiatic citrus canker; Xanthomonas axonopodis pv. citri.

Figure 1

The transcription activator‐like effector (TALE), PthA4, contains 17.5 repeats in which the repeat‐variable diresidues (RVDs) bind to the effector‐binding element (EBE) upstream of the susceptibility gene, CsLob1, and turn on CsLob1. The arrangement of the RVDs dictates the target of TALE, which, in this case, is EBE_PthA4. aa, amino acid; NLS, nuclear localization signal.

Figure 2

Characterization of the transit peptide located at the N‐terminal moiety of the AvrGF1 effector. (A) AvrGF1‐GFP, AvrGf1_ΔN87‐GFP and green fluorescent protein (GFP) empty vector were transiently expressed in Nicotiana benthamiana leaves through Agrobacterium‐mediated infiltration. Images were taken at 3 days post‐inoculation (dpi) using confocal microscopy. (B) Agrobacterium strains harbouring AvrGF1 and AvrGf1ΔN87 were needlelessly infiltrated into sweet orange leaves with different optical densities (ODs) as presented. Hypersensitive response (HR) induction was observed and photographed at 3 dpi. (C) Xanthomonas citri strains harbouring AvrGF1, HpaA‐AvrGf1 and HpaA‐AvrGf1_ΔN87 were inoculated into grapefruit leaves to test the HR. Images (a) and (b) were photographed at 2 dpi, and (c) was photographed at 5 dpi. (a) 306/p53‐AvrGf1; (b) 306/p53‐HpaA‐AvrGf1; (c) 306/p53‐HpaA‐AvrGf1_ΔN87. (D) Different lengths of amino acid residues from AvrGF1 N‐terminus were fused with GFP. Chimeric proteins were transiently expressed in N. benthamiana leaves through Agrobacterium‐mediated infiltration. Images were photographed at 3 dpi using confocal microscopy.