Multiple species of Xanthomonas cause bacterial spot of tomato (BST) and pepper. We sequenced five Xanthomonas euvesicatoria strains isolated from three continents (Africa, Asia, and South America) to provide a set of representative genomes with temporal and geographic diversity. LMG strains 667, 905, 909, and 933 were pathogenic on tomato and pepper, except LMG 918 elicited a hypersensitive reaction (HR) on tomato. Furthermore, LMG 667, 909, and 918 elicited a HR on Early Cal Wonder 30R containing Bs3. We examined pectolytic activity and starch hydrolysis, two tests which are useful in differentiating X. euvesicatoria from X. perforans, both causal agents of BST. LMG strains 905, 909, 918, and 933 were nonpectolytic while only LMG 918 was amylolytic. These results suggest that LMG 918 is atypical of X. euvesicatoria. Sequence analysis of all the publicly available X. euvesicatoria and X. perforans strains comparing seven housekeeping genes identified seven haplotypes with few polymorphisms. Whole genome comparison by average nucleotide identity (ANI) resulted in values of >99% among the LMG strains 667, 905, 909, 918, and 933 and X. euvesicatoria strains and >99.6% among the LMG strains and a subset of X. perforans strains. These results suggest that X. euvesicatoria and X. perforans should be considered a single species. ANI values between strains of X. euvesicatoria, X. perforans, X. allii, X. alfalfa subsp. citrumelonis, X. dieffenbachiae, and a recently described pathogen of rose were >97.8% suggesting these pathogens should be a single species and recognized as X. euvesicatoria. Analysis of the newly sequenced X. euvesicatoria strains revealed interesting findings among the type 3 (T3) effectors, relatively ancient stepwise erosion of some T3 effectors, additional X. euvesicatoria-specific T3 effectors among the causal agents of BST, orthologs of avrBs3 and avrBs4, and T3 effectors shared among xanthomonads pathogenic against various hosts. The results from this study supports the finding that T3 effector repertoire and host range are fundamental for the study of host—microbe interaction but of little relevance to bacterial speciation.

Vascular plant pathogens travel long distances through host veins, leading to life-threatening, systemic infections. In contrast, nonvascular pathogens remain restricted to infection sites, triggering localized symptom development. The contrasting features of vascular and nonvascular diseases suggest distinct etiologies, but the basis for each remains unclear. Here, we show that the hydrolase CbsA acts as a phenotypic switch between vascular and nonvascular plant pathogenesis. cbsA was enriched in genomes of vascular phytopathogenic bacteria in the family Xanthomonadaceae and absent in most nonvascular species. CbsA expression allowed nonvascular Xanthomonas to cause vascular blight, while cbsA mutagenesis resulted in reduction of vascular or enhanced nonvascular symptom development. Phylogenetic hypothesis testing further revealed that cbsA was lost in multiple nonvascular lineages and more recently gained by some vascular subgroups, suggesting that vascular pathogenesis is ancestral. Our results overall demonstrate how the gain and loss of single loci can facilitate the evolution of complex ecological traits.

Xanthomonas translucens is the causal agent of bacterial leaf streak, the most common bacterial disease of wheat and barley. To cause disease, most xanthomonads depend on a highly conserved type III secretion system, which translocates type III effectors into host plant cells. Mutagenesis of the conserved type III secretion gene hrcT confirmed that the X. translucens type III secretion system is required to cause disease on the host plant barley and to trigger a non-host hypersensitive response (HR) in pepper leaves. Type III effectors are delivered to the host cell by a surface appendage, the Hrp pilus, and a translocon protein complex that inserts into the plant cell plasma membrane. Homologs of the Xanthomonas HrpF protein, including PopF from Ralstonia solanacearum and NolX from rhizobia, are thought to act as a translocon protein. Comparative genomics revealed that X. translucens strains harbor a noncanonical hrp gene cluster, which rather shares features with type III secretion systems from Ralstonia solanacearum, Paraburkholderia andropogonis, Collimonas fungivorans, and Uliginosibacterium gangwonense than other Xanthomonas spp. Surprisingly, none of these bacteria, except R. solanacearum, encode a homolog of the HrpF translocon. Here, we aimed at identifying a candidate translocon from X. translucens. Notably, genomes from strains that lacked hrpF/popF/nolX instead encode another gene, called hpaT, adjacent to and co-regulated with the type III secretion system gene cluster. An insertional mutant in the X. translucens hpaT gene, which is the first gene of a two-gene operon, hpaT-hpaH, was non-pathogenic on barley and did not cause the HR or programmed cell death in non-host pepper similar to the hrcT mutant. The hpaT mutant phenotypes were partially complemented by either hpaT or the downstream gene, hpaH, which has been described as a facilitator of translocation in Xanthomonas oryzae. Interestingly, the hpaT mutant was also complemented by the hrpF gene from Xanthomonas euvesicatoria. These findings reveal that both HpaT and HpaH contribute to the injection of type III effectors into plant cells.

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During the last 20 years, the causative agents of bacterial spot of tomato and pepper have been subjected to many studies and reclassifications. According to the current data, the species are four (X. euvesicatoria, X. vesicatoria, X. gardneri, and X. perforans) and cause similar symptoms in plants but possess different phenotypic properties. This work provides the full metabolic characteristics obtained by Biolog system of bacterial spot's xanthomonads based on a large selection of strains from different vegetable-producing regions of Bulgaria with accent on their major differentiating properties which could be used for species differentiation by metabolic profiles. The results are compared to the data available in the literature in order to clarify the strong features of each species and distinguish the variable ones. Simple characteristics like amylase activity and utilization of cis-aconitate cannot serve alone for differentiation.

Sub-species diversity of pepper populations of Xanthomonas euvesicatoria in Bulgaria and Macedonia in 2012 was the object of this study. Species determination of 44 strains was performed by molecular methods using two pairs of species-specific primers and RFLP (restriction fragment length polymorphism) analysis of the 16S-23S ITS region with HpaII. The populations were characterized by genotypic and phenotypic properties. The genotypic diversity of the strains was evaluated by RAPD (random amplified polymorphic DNA) technique. Primer CUGEA-6 differentiated the strains in two groups, one of which included only Bulgarian strains and revealed a mixed profile of the type strain. BiologTM metabolite profiles separated the strains in four groups: two of which were composed only of Bulgarian or Macedonian strains. Correlation between the RAPD and the metabolic profiles was observed. Twelve antibiotics and copper ions in five concentrations (1–5 g kg−1) were tested for biological activity. The inhibition zones of the Bulgarian strains were statistically proven to be considerably larger than the Macedonian ones in the tests with kanamycin, streptomycin, polymyxin B sulphate, tetracycline and vankomycin. The inhibition zones of the Bulgarian strains were statistically proven to be relatively larger than the Macedonian ones in the copper tests. Based on our studies the Macedonian population of X. euvesicatoria manifested a relative homogeneity while a greater diversity was observed in the Bulgarian population.

Xanthomonas translucens pv. translucens causes bacterial leaf streak and bacterial blight diseases of barley. This pathogen limits barley production globally but remains understudied, with limited genomic resources. To better understand the biology of this X. translucens subgroup, we sequenced the complete genome of the X. translucens pv. translucens strain UPB886.

We report the draft genome sequences of two Xanthomonas euvesicatoria strains from the Balkan Peninsula, which were isolated from symptomatic pepper plants. The availability of these genome sequences will facilitate the development of modern genotyping assays for X. euvesicatoria strains and to define targets for resistance breeding.