Influence of flagellin polymorphisms, gene regulation, and responsive memory on the motility of Xanthomonas species that cause bacterial spot disease of solanaceous plants

Increasingly, new evidence has demonstrated variability in the epitope regions of bacterial flagellin, including in regions harboring the microbe-associated molecular patterns flg22 and flgII-28 that are recognized by the pattern recognition receptors FLS2 and FLS3, respectively. Additionally, since bacterial motility is known to contribute to pathogen virulence and chemotaxis, reductions in or loss of motility can significantly reduce bacterial fitness. In this study, we determined that variations in flg22 and flgII-28 epitopes allow some, but not all, Xanthomonas species to evade both FLS2-and FLS3-mediated oxidative burst responses. We observed variation in the motility for many isolates, irrespective of their flagellin sequence. Instead, we determined that past growth conditions may have a significant impact on the motility status of isolates, as we could minimize this variability by inducing motility using chemoattractant assays. Additionally, motility could be significantly suppressed under nutrient-limited conditions, and bacteria could “remember” its prior motility status after storage at ultra-cold temperatures. Finally, we observed larger bacterial populations of strains with flagellin variants predicted not to be recognized by either FLS2 or FLS3, suggesting that these bacteria can evade flagellin recognition in tomato plants. While some flagellin variants may impart altered motility and differential recognition by the host immune system, external growth parameters and gene expression regulation appear to have more significant impacts on the motility phenotypes for these Xanthomonas species.

Genetic and phenotypic characterization of Xanthomonas species pathogenic of wheat in Uruguay

Bacterial diseases affecting wheat production in Uruguay are an issue of growing concern yet remain largely uninvestigated in the region. Surveys of 61 wheat fields carried out from 2017 to 2019 yielded a regional collection of 63 strains identified by 16S rRNA gene analysis as Xanthomonas spp. A real-time PCR protocol using species-specific primers previously reported allowed the identification of 44 strains as X. translucens, the causal agent of bacterial leaf streak (BLS) in wheat and other cereal crops. Multilocus sequence analysis (MLSA) of four housekeeping genes (dnaK, fyuA, gyrB, and rpoD) revealed that these strains were most closely related to X. translucens pv. undulosa, the pathovar that is most commonly associated with BLS of wheat. Multilocus sequence typing (MLST) was applied to examine the genetic diversity among X. translucens strains. Strains were assigned to four different sequence types, three of which have been previously reported globally. Additionally, 17 Xanthomonas strains not belonging to X. translucens were obtained from diseased wheat leaves. Phylogenetic analysis showed that these strains are closely related to Xanthomonas prunicola, and clustered together with previously uncharacterized Xanthomonas strains isolated from wheat in Minnesota, US. In planta pathogenicity assays carried out on a BLS susceptible wheat cultivar showed that X. translucens pv. undulosa strains caused brown necrosis symptoms typical of BLS, while non-translucens Xanthomonas sp. strains elicited an atypical symptom of dry necrosis. These findings suggest that local wheat fields are affected by X. translucens pv. undulosa, and by a new wheat pathogen within the Xanthomonas genus.

Insights into the cell-cell signaling and iron homeostasis in Xanthomonas virulence and lifestyle

The Xanthomonas group of phytopathogens causes economically important diseases, which cause severe yield loss in major crops. Some Xanthomonas species are known to have an epiphytic and in planta lifestyle which are coordinated by several virulence-associated functions, cell-cell signaling (DSF; diffusible signaling factor), and environmental conditions, including iron availability. In this review, we described the role of cell-cell signaling by the DSF molecule and iron in the regulation of virulence-associated functions. Although DSF and iron are involved in the regulation of several virulence-associated functions, members of the Xanthomonas group of plant pathogens exhibit atypical patterns of regulation. Atypical patterns contribute to the adaptation to different lifestyles. Studies on DSF and iron biology indicate that virulence-associated functions can be regulated in completely contrasting fashions by the same signaling system in closely related xanthomonads.

Selection of target genes for pcr diagnostics of Xanthomonas arboricola virulent for cereals and brassicas

Plant pathogenic xanthomonads virulent to wheat, rye, barley, tomato, sunflower, and brassicas were isolated in Russia in 2001–2008. Physiological tests and multilocus sequence typing analysis confirmed their position within the Xanthomonas arboricola species. The obtained draft genome sequence of representative strain 3004 from barley plants, which is also virulent to sunflower, brassicas, and chestnut, demonstrated an absence of the Type 3 Secretion System T3SS and an evidence for the lateral gene transfer of some other virulence genes from distantly related bacteria. It was concluded that T4SS genes can be used as the target for group-specific PCR analysis of the emerging pathogen. It was proposed to use virD4, virB3, virB4, and virB9 genes to design a detection system. After preliminary experiments with classic PCR for the chosen genes, primers and TaqMan(R) probe were designed to specifically amplify a 121 bp fragment of the VirD4 gene. Amplification products were obtained for all target Xanthomonas arboricola strains and were not detected in other Xanthomonas species, or in other pathogenic or epiphytic bacteria occurring on these host plants. The assay readily detected Xanthomonas arboricola infection in diseased plants and from bacterial colonies isolated on semi-selective media, and was more sensitive and specific than traditional plating methods.

Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn).

Xanthomonas vasicola pv. vasculorum (Xvv) is a bacterial pathogen that causes both bacterial leaf streak of maize and sugarcane gumming disease. After decades limited to South Africa, bacterial leaf streak of maize spread rapidly through maize-growing areas of Argentina, Brazil and the USA since 2014. The origin, method and biological underpinnings of this sudden spread are not well understood but are the subject of active research. Effective control methods remain elusive, but sanitation and crop debris management may limit the disease. Yield impact data are not yet available, but lesions may become severe enough to limit plant productivity in some varieties. The pathogen is not currently considered a quarantine threat by the USDA, EPPO or IPPC. Taxonomic Tree Top of page Domain: Bacteria Phylum: Proteobacteria Class: Gammaproteobacteria Order: Xanthomonadales Family: Xanthomonadaceae Genus: Xanthomonas Species: Xanthomonas vasicola pv. vasculorum Notes on Taxonomy and Nomenclature Top of page The taxonomic nomenclature of the pathogen has undergone several changes and is still being resolved at the time of this report. The Xanthomonas clade causing.

Machine learning approaches to predict the plant-associated phenotype of Xanthomonas strains

The genus Xanthomonas has long been considered to consist predominantly of plant pathogens, but over the last decade there has been an increasing number of reports on non-pathogenic and endophytic members. As Xanthomonas species are prevalent pathogens on a wide variety of important crops around the world, there is a need to distinguish between these plant-associated phenotypes. To date a large number of Xanthomonas genomes have been sequenced, which enables the application of machine learning (ML) approaches on the genome content to predict this phenotype. Until now such approaches to the pathogenomics of Xanthomonas strains have been hampered by the fragmentation of information regarding strain pathogenicity over many studies. Unification of this information into a single resource was therefore considered to be an essential step. Mining of 39 papers considering both plant-associated phenotypes, allowed for a phenotypic classification of 578 Xanthomonas strains. For 65 plant-pathogenic and 53 non-pathogenic strains the corresponding genomes were available and de novo annotated for the presence of Pfam protein domains used as features to train and compare three ML classification algorithms; CART, Lasso and Random Forest. Recursive feature extraction provided further insights into the virulence enabling factors, but also yielded domains linked to traits not present in pathogenic strains.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

Genome sequencing and functional characterization of Xanthomonas cucurbitae, the causal agent of bacterial spot disease of cucurbits

Bacterial leaf spot disease caused by X. cucurbitae has severely affected the pumpkin industries in the Midwestern region of United States, with the bacteria mainly infecting pumpkin leaves and fruits, and leading to significant yield losses. In this study, we utilized genomics and genetics approaches to elucidate Xanthomonas cucurbitae molecular mechanisms of pathogenesis during interaction with its host. We generated the first reference-quality whole-genome sequence of the X. cucurbitae type isolate and compared to other Xanthomonas species, X. cucurbitae has a smaller genome size with fewer virulence-related genes. RNA-seq analysis of X. cucurbitae under plant-mimicking media conditions showed altered transcriptional responses, with upregulation of virulence genes and downregulation of cellular homeostasis genes. Additionally, characterization of key virulence genes using gene deletion methods revealed that both type II enzymes and type III effectors are necessary for X. cucurbitae to cause infection in the pumpkin host.

Arabidopsis bZIP11 is a susceptibility factor during Pseudomonas syringae infection

The induction of plant nutrient secretion systems is critical for successful pathogen infection. Some bacterial pathogens, e.g. Xanthomonas species, use TAL (transcription activator-like) effectors to induce transcription of SWEET sucrose efflux transporters. Pseudomonas syringae pathovar (pv.) tomato strain DC3000 lacks TAL effectors, yet is able to induce multiple SWEETs in Arabidopsis thaliana by unknown mechanisms. Since bacteria require other nutrients besides sugars for efficient reproduction, we hypothesized that Pseudomonas may depend on host transcription factors involved in secretory programs to increase access to essential nutrients. Bioinformatic analyses identified the Arabidopsis basic-leucine zipper transcription factor bZIP11 as a potential regulator of nutrient transporters, including SWEETs and UmamiT amino acid transporters. Inducible downregulation of bZIP11 expression in Arabidopsis resulted in reduced growth of P. syringae pv. tomato strain DC3000, whereas inducible overexpression of bZIP11 resulted in increased bacterial growth, supporting the hypothesis that bZIP11 regulated transcription programs are essential for maximal pathogen titer in leaves. Our data are consistent with a model in which a pathogen alters host transcription factor expression upstream of secretory transcription networks to promote nutrient efflux from host cells.

A Defect in the Twin-Arginine Translocation Pathway Decreases the Tolerance of Xanthomonas campestris pv. campestris to Phenazines

Phenazine-1-carboxylic acid (PCA), a member of phenazines secreted by microorganisms, inhibits the growth of many bacteria and fungi. Xanthomonas campestris pv. campestris is the causal agent of black rot, the most important disease of cruciferous crops worldwide, and is more tolerant to PCA than other Xanthomonas species. Previous studies reported that reactive oxygen species (ROS) scavenging ability is involved in regulating the PCA tolerance of Xanthomonas species. Additionally, the cytochrome c maturation (CCM) system has been found to play a more important role in tolerance to phenazines than the ROS scavenging system. In this study, a highly PCA-sensitive insertion mutant of X. campestris pv. campestris, X-5, was identified and studied. The insertion site of X-5 was found to be in tatB gene (XC_4183), which encodes a subunit of the twin-arginine translocation (TAT) complex. Disruption of the three genes of TAT pathway resulted in decreased biological fitness and reduced tolerance to phenazines in comparison with the wild-type strain 8004. These results imply that the tolerance mechanism of the TAT pathway to phenazines is related to the CCM system, but not due to the ROS scavenging system. Furthermore, respiration-related characteristic tests and peptide analysis suggested that disruption of the TAT complex causes a defect in the cytochrome bc1 complex, which may be involved in the tolerance to phenazines. In summary, this study sheds new light on the critical role of the TAT pathway in influencing the fitness and phenazines tolerance of Xanthomonas species.