Epidemiology, diversity, and management of bacterial spot of tomato caused by Xanthomonas perforans

Xanthomonas as a Model System for Studying Pathogen Emergence and Evolution

In this review, we highlight studies in which whole-genome sequencing, comparative genomics, and population genomics have provided unprecedented insights into past and ongoing pathogen evolution. These include new understandings of the adaptive evolution of secretion systems and their effectors. We focus on Xanthomonas pathosystems that have seen intensive study and improved our understanding of pathogen emergence and evolution, particularly in the context of host specialization: citrus canker, bacterial blight of rice, and bacterial spot of tomato and pepper. Across pathosystems, pathogens appear to follow a pattern of bursts of evolution and diversification that impact host adaptation. There remains a need for studies on the mechanisms of host range evolution and genetic exchange among closely related but differentially host-specialized species and to start moving beyond the study of specific strain and host cultivar pairwise interactions to thinking about these pathosystems in a community context.

Influence of Co-occurring Weakly Pathogenic Bacterial Species on Bacterial Spot Disease Dynamics on Tomato

Mixed infections caused by multiple pathogenic and weakly pathogenic strains inhabiting the same host plants are common in nature and may modify pathogen dynamics. However, traditional plant pathogen studies have mostly focused on the binary interaction between a single host and a single pathogen. In this study, we have looked beyond this binary interaction and evaluated the impact of coinfection on disease dynamics on tomato using the bacterial spot pathogen Xanthomonas perforans (Xp), the co-occurring weakly pathogenic strain of X. arboricola (Xa), and the co-occurring potential weak pathogenic strain of Pseudomonas capsici (Pc). Time-series coinfection experiments monitoring disease severity and within-host population dynamics revealed higher disease severity in coinfection by three species compared with infection by Xp alone. However, coinfection by dual species, Xp and Pc, or Xa resulted in lower disease severity compared with Xp alone. Thus, coinfection outcomes depend on interacting species. Weak pathogens could exploit Xp to colonize the host plant as indicated by their higher populations in coinfection. However, Xp population dynamics were dependent on the coinfecting partner. While resource competition might be a possible explanation for lower Xp population in dual coinfection, interaction of Pc with the host was found to influence Xp population. Interestingly, Xp population was higher in the presence of three-species interaction compared with Xp and Xa coinfection, suggesting potential modulation of cooperative interactions among Xp and Xa in three-species coinfection rather than competitive interactions. Humidity played a significant role in population dynamics of the three species. Overall, this study highlighted the importance of coinfection dynamics in studying plant disease outbreaks.

High air humidity dampens salicylic acid pathway and NPR1 function to promote plant disease

The occurrence of plant disease is determined by interactions among host, pathogen, and environment. Air humidity shapes various aspects of plant physiology and high humidity has long been known to promote numerous phyllosphere diseases. However, the molecular basis of how high humidity interferes with plant immunity to favor disease has remained elusive. Here we show that high humidity is associated with an "immuno-compromised" status in Arabidopsis plants. Furthermore, accumulation and signaling of salicylic acid (SA), an important defense hormone, are significantly inhibited under high humidity. NPR1, an SA receptor and central transcriptional co-activator of SA-responsive genes, is less ubiquitinated and displays a lower promoter binding affinity under high humidity. The cellular ubiquitination machinery, particularly the Cullin 3-based E3 ubiquitin ligase mediating NPR1 protein ubiquitination, is downregulated under high humidity. Importantly, under low humidity the Cullin 3a/b mutant plants phenocopy the low SA gene expression and disease susceptibility that is normally observed under high humidity. Our study uncovers a mechanism by which high humidity dampens a major plant defense pathway and provides new insights into the long-observed air humidity influence on diseases.

Bacterial Spot of Tomato and Pepper in Africa: Diversity, Emergence of T5 Race, and Management

Bacterial spot disease was first reported from South Africa by Ethel M. Doidge in 1920. In the ensuing century after the initial discovery, the pathogen has gained global attention in plant pathology research, providing insights into host-pathogen interactions, pathogen evolution, and effector discovery, such as the first discovery of transcription activation-like effectors, among many others. Four distinct genetic groups, including Xanthomonas euvesicatoria (proposed name: X. euvesicatoria pv. euvesicatoria), Xanthomonas perforans (proposed name: X. euvesicatoria pv. perforans), Xanthomonas gardneri (proposed name: Xanthomonas hortorum pv. gardneri), and Xanthomonas vesicatoria, are known to cause bacterial spot disease. Recently, a new race of a bacterial spot pathogen, race T5, which is a product of recombination between at least two Xanthomonas species, was reported in Nigeria. In this review, our focus is on the progress made on the African continent, vis-à-vis progress made in the global bacterial spot research community to provide a body of information useful for researchers in understanding the diversity, evolutionary changes, and management of the disease in Africa.

Metabolomics Insights into Chemical Convergence in Xanthomonas perforans and Metabolic Changes Following Treatment with the Small Molecule Carvacrol

Microbes are natural chemical factories and their metabolome comprise diverse arrays of chemicals. The genus Xanthomonas comprises some of the most important plant pathogens causing devastating yield losses globally and previous studies suggested that species in the genus are untapped chemical minefields. In this study, we applied an untargeted metabolomics approach to study the metabolome of a globally spread important xanthomonad, X. perforans. The pathogen is difficult to manage, but recent studies suggest that the small molecule carvacrol was efficient in disease control. Bacterial strains were treated with carvacrol, and samples were taken at time intervals (1 and 6 h). An untreated control was also included. There were five replicates for each sample and samples were prepared for metabolomics profiling using the standard procedure. Metabolomics profiling was carried out using a thermo Q-Exactive orbitrap mass spectrometer with Dionex ultra high-performance liquid chromatography (UHPLC) and an autosampler. Annotation of significant metabolites using the Metabolomics Standards Initiative level 2 identified an array of novel metabolites that were previously not reported in Xanthomonas perforans. These metabolites include methoxybrassinin and cyclobrassinone, which are known metabolites of brassicas; sarmentosin, a metabolite of the Passiflora-heliconiine butterfly system; and monatin, a naturally occurring sweetener found in Sclerochiton ilicifolius. To our knowledge, this is the first report of these metabolites in a microbial system. Other significant metabolites previously identified in non-Xanthomonas systems but reported in this study include maculosin; piperidine; β-carboline alkaloids, such as harman and derivatives; and several important medically relevant metabolites, such as valsartan, metharbital, pirbuterol, and ozagrel. This finding is consistent with convergent evolution found in reported biological systems. Analyses of the effect of carvacrol in time-series and associated pathways suggest that carvacrol has a global effect on the metabolome of X. perforans, showing marked changes in metabolites that are critical in energy biosynthesis and degradation pathways, amino acid pathways, nucleic acid pathways, as well as the newly identified metabolites whose pathways are unknown. This study provides the first insight into the X. perforans metabolome and additionally lays a metabolomics-guided foundation for characterization of novel metabolites and pathways in xanthomonad systems.