Abstract
Citrus bacterial canker (CBC) is an important disease of citrus cultivars worldwide that causes blister-like lesions on host plants and leads to more severe symptoms such as plant defoliation and premature fruit drop. The causative agent, Xanthomonas citri pv. citri, exists as three pathotypes—A, A*, and Aw—which differ in their host range and elicited host response. To date, comparative analyses have been hampered by the lack of closed genomes for the A* pathotype. In this study, we sequenced and assembled six CBC isolates of pathotype A* using second- and third-generation sequencing technologies to produce complete, closed assemblies. Analysis of these genomes and reference A, A*, and Aw sequences revealed genetic groups within the A* pathotype. Investigation of accessory genomes revealed virulence factors, including type IV secretion systems and heavy metal resistance genes, differentiating the genetic groups. Genomic comparisons of closed genome assemblies also provided plasmid distribution information for the three genetic groups of A*. The genomes presented here complement existing closed genomes of A and Aw pathotypes that are publicly available and open opportunities to investigate the evolution of X. citri pv. citri and the virulence factors that contribute to this serious pathogen.
Highlights
Citrus bacterial canker (CBC), a disease caused by the pathogen Xanthomonas citri pv. citri, is a serious phytopathogen of a wide variety of citrus cultivars
Gordon et al observed the presence of nonsynonymous single-nucleotide polymorphisms (SNPs) in xopL specific to the A, Aw, and A* pathotypes; only nonsynonymous SNPs were observed in A* in this study
XopL has previously been shown to be required for full virulence of Xanthomonas campestris pv. campestris on Chinese radish plants, part of the Brassicaceae family [66], and as it was only observed in A* pathotypes in this study, the role of XopL in host range differences should not be discounted
Summary
Citrus bacterial canker (CBC), a disease caused by the pathogen Xanthomonas citri pv. citri, is a serious phytopathogen of a wide variety of citrus cultivars. Canker causes plant defoliation and can lead to premature fruit drop. These effects cause considerable economic loss internationally, with extra implications to regions without CBC and their market access should exotic incursions of CBC arise. Examples of this include the recent incursions into Australia in 2004 and 2018, with the 2004 eradication having been estimated by the Australian Bureau of Agricultural and Resource Economics to have provided a net benefit of AUD 70 million to Queensland alone had eradication not occurred [2]. In 2016-2017, Australia’s citrus industry was valued at AUD 724.4 million, with a large proportion (AUD 462 million) being for exports, and in 2019, Australian citrus export exceed AUD 541 million [3,4]
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