Comparative genomics of a cannabis pathogen reveals insight into the evolution of pathogenicity in Xanthomonas

Abstract

Pathogenic bacteria in the genus Xanthomonas cause diseases on over 350 plant species, including cannabis (Cannabis sativa L.). Because of regulatory limitations, the biology of the Xanthomonas-cannabis pathosystem remains largely unexplored. To gain insight into the evolution of Xanthomonas strains pathogenic to cannabis, we sequenced the genomes of two geographically distinct Xanthomonas strains, NCPPB 3753 and NCPPB 2877, which were previously isolated from symptomatic plant tissue in Japan and Romania. Comparative multilocus sequence analysis of housekeeping genes revealed that they belong to Group 2, which comprises most of the described species of Xanthomonas. Interestingly, both strains lack the Hrp Type III secretion system and do not contain any of the known Type III effectors. Yet their genomes notably encode two key Hrp pathogenicity regulators HrpG and HrpX, and hrpG and hrpX are in the same genetic organization as in the other Group 2 xanthomonads. Promoter prediction of HrpX-regulated genes suggests the induction of an aminopeptidase, a lipase and two polygalacturonases upon plant colonization, similar to other plant-pathogenic xanthomonads. Genome analysis of the distantly related Xanthomonas maliensis strain 97M, which was isolated from a rice leaf in Mali, similarly demonstrated the presence of HrpG, HrpX, and a HrpX-regulated polygalacturonase, and the absence of the Hrp Type III secretion system and known Type III effectors. Given the observation that some Xanthomonas strains across distinct taxa do not contain hrpG and hrpX, we speculate a stepwise evolution of pathogenicity, which involves (i) acquisition of key regulatory genes and cell wall-degrading enzymes, followed by (ii) acquisition of the Hrp Type III secretion system, which is ultimately accompanied by (iii) successive acquisition of Type III effectors.

Using Ecology, Physiology, and Genomics to Understand Host Specificity inXanthomonas

“…isolated in Rwanda have reduced T3E repertoires (5,17,59). Xanthomonas sacchari and strains of X. arboricola and Xanthomonas cannabis do not present known T3Es (67,70,126). Interestingly, in several Xanthomonas species, a correlation was observed between pathovars and the composition of T3E repertoires (57)(58)(59), further suggesting that T3E repertoires play a major role in determining the host specificity of strains.…”

“…Variations in Flg22 residues required for FLS2-mediated recognition are observed in xanthomonads, allowing evasion of FLS2-mediated immunity (128). Noneliciting variants of flg22 are present not only in Xcc (56,128) but also in xanthomonads pathogenic to, for example, cannabis (70).…”

“…An emerging picture of the acquisition of pathogenicity in strains of Xanthomonas involves as a first step the acquisition of master regulators, such as HrpG and HrpX, and key CWDEs (70). The acquisition of a T3SS and core T3Es as a second step could lead to the emergence of generalist pathogenic strains (70,97).…”

“…Nonpathogenic strains of Xanthomonas devoid of T3SSs and T3Es (or featuring a very restricted number of T3E genes) were isolated from several ecosystems (42,70,131). An emerging picture of the acquisition of pathogenicity in strains of Xanthomonas involves as a first step the acquisition of master regulators, such as HrpG and HrpX, and key CWDEs (70).…”

Genómica comparativa de cepas de Xanthomonas arboricola asociada a Prunus spp.: caracterización de los procesos de infección de la mancha bacteriana de frutales de hueso y almendro

“…mahaleb, open the discussion about the origin and evolution of pathogenicity in this species as it has been proposed previously for the genus Xanthomonas (Jacobs et al, 2015). Based in the current data, it is not possible to determine if these non-pathogenic strains were predecessors of the pathogenic groups or the result of the loss of their pathogenicity.…”