Xanthomonas axonopodis pv. eucalyptorum pv. nov. Causing Bacterial Leaf Blight on Eucalypt in Brazil

Xanthomonas citri pv. eucalyptorum strain 4866-2_S43 (formerly X. axonopodis pv. eucalyptorum): the causal agent of bacterial leaf blight on eucalypts recovered in Argentina

We report the draft genome assembly of strain 4866-2_S43 isolated from a eucalyptus lesion in Argentina and what until recently was caused by Xanthomonas citri pv. eucalyptorum (Xce). The genome size is 5 188 607 bp, with a G+C content of 64.66%. Comparative analysis reveals that the closest relative of strain 4866-2_S43 is Xce LPF 602, isolated in Brazil. Comparison of the whole genome sequences revealed an average nucleotide identity (ANI) of 99.96% between the two strains. ANIs were determined between the whole genome sequence of strain 4866-2_S43 and the genomes of all currently validated Xanthomonas spp. These results revealed that strain 4866-2_S43 shared >95% similarity with X. citri pv. citri and X. citri pv. phaseoli, and <95% with X. euvesicatoria pv. alfalfae, X. euvesicatoria pv. perforans, and X. euvesicatoria pathovars euvesicatoria and eucalyptii.

A Multilocus Sequence Typing Scheme for Rapid Identification of Xanthomonas citri Based on Whole-Genome Sequencing Data

Xanthomonas citri is a plant-pathogenic bacterium associated with a diverse range of host plant species. It has undergone substantial reclassification and currently consists of 14 different subspecies or pathovars that are responsible for a wide range of plant diseases. Whole-genome sequencing (WGS) provides a cutting-edge advantage over other diagnostic techniques in epidemiological and evolutionary studies of X. citri because it has a higher discriminatory power and is replicable across laboratories. WGS also allows for the improvement of multilocus sequence typing (MLST) schemes. In this study, we used genome sequences of Xanthomonas isolates from the NCBI RefSeq database to develop a seven-gene MLST scheme that yielded 19 sequence types (STs) that correlated with phylogenetic clades of X. citri subspecies or pathovars. Using this MLST scheme, we examined 2,911 Xanthomonas species assemblies from NCBI GenBank and identified 15 novel STs from 37 isolates that were misclassified in NCBI. In total, we identified 545 X. citri assemblies from GenBank with 95% average nucleotide identity to the X. citri type strain, and all were classified as one of the 34 STs. All MLST classifications correlated with a phylogenetic position inferred from alignments using 92 conserved genes. We observed several instances where strains from different pathovars formed closely related monophyletic clades and shared the same ST, indicating that further investigation of the validity of these pathovars is required. Our MLST scheme described here is a robust tool for rapid classification of X. citri pathovars using WGS and a powerful method for further comprehensive taxonomic revision of X. citri pathovars.

Trends in Molecular Diagnosis and Diversity Studies for Phytosanitary Regulated Xanthomonas

Bacteria in the genus Xanthomonas infect a wide range of crops and wild plants, with most species responsible for plant diseases that have a global economic and environmental impact on the seed, plant, and food trade. Infections by Xanthomonas spp. cause a wide variety of non-specific symptoms, making their identification difficult. The coexistence of phylogenetically close strains, but drastically different in their phenotype, poses an added challenge to diagnosis. Data on future climate change scenarios predict an increase in the severity of epidemics and a geographical expansion of pathogens, increasing pressure on plant health services. In this context, the effectiveness of integrated disease management strategies strongly depends on the availability of rapid, sensitive, and specific diagnostic methods. The accumulation of genomic information in recent years has facilitated the identification of new DNA markers, a cornerstone for the development of more sensitive and specific methods. Nevertheless, the challenges that the taxonomic complexity of this genus represents in terms of diagnosis together with the fact that within the same bacterial species, groups of strains may interact with distinct host species demonstrate that there is still a long way to go. In this review, we describe and discuss the current molecular-based methods for the diagnosis and detection of regulated Xanthomonas, taxonomic and diversity studies in Xanthomonas and genomic approaches for molecular diagnosis.

Imidazolium salts as alternative compounds to control diseases caused by plant pathogenic bacteria

AIMS

To evaluate the inhibitory effect of five structurally different imidazolium salts on the in vitro growth of plant pathogenic bacteria that belong to divergent taxonomic genera as well as their ability to reduce the severity of common bacterial blight of common bean caused by Xanthomonas axonopodis pv. phaseoli and bacterial speck of tomato caused by Pseudomonas syringae pv. tomato.

METHODS AND RESULTS

Growth inhibition of Xanthomonas, Pseudomonas, Erwinia, Pectobacterium and Dickeya strains by imidazolium salts was assessed in vitro by radial diffusion on agar medium and by ressazurin reduction in liquid medium. The reduction of common bacterial blight and bacterial speck symptoms and the area under de disease progress curves were determined by spraying two selected imidazolium salts on healthy plants 48 h prior to inoculation with virulent strains of the bacterial pathogens. All imidazolium salts inhibited the growth of all plant pathogenic bacteria when tested by radial diffusion on agar medium. The strength of inhibition differed among imidazolium salts when tested on the same bacterial strain and among bacterial strains when tested with the same imidazolium salt. In liquid medium, most imidazolium salts presented the same minimum inhibitory concentration (MIC) and minimum bactericidal concentration values (200 µmol l^-1 ), the most notable exception of which was the MIC (at least 1000 µmol l^-1 ) for the dicationic MImC₁₀ MImBr₂ . The imidazolium salts C₁₆ MImBr and C₁₆ MImCl caused significant reductions in the severity of common bacterial blight symptoms when compared with nontreated plants.

CONCLUSION

Imidazolium salts inhibit the in vitro growth of plant pathogenic bacteria and reduce plant disease symptoms to levels comparable to an authorized commercial antibiotic product.

SIGNIFICANCE AND IMPACT OF THE STUDY

New compounds exhibiting broad-spectrum antibacterial activity with potential use in agriculture were identified.

First Draft Genome Sequence of Xanthomonas axonopodis pv. eucalyptorum, Causal Agent of Bacterial Leaf Blight on Eucalypt

Here, we report the annotated draft genome sequence of Xanthomonas axonopodis pv. eucalyptorum pathotype strain LPF602 (synonym Xanthomonas axonopodis BSC45a), isolated from eucalypt leaves showing bacterial blight symptoms in Brazil. The availability of these genomic data will help improve the understanding of the evolution and molecular mechanisms involved in the pathogenesis of this microorganism.

Here, we report the annotated draft genome sequence of Xanthomonas axonopodis pv. eucalyptorum pathotype strain LPF602 (synonym Xanthomonas axonopodis BSC45a), isolated from eucalypt leaves showing bacterial blight symptoms in Brazil. The availability of these genomic data will help improve the understanding of the evolution and molecular mechanisms involved in the pathogenesis of this microorganism.