The timing of bacterial mesophyll infection shapes the leaf chemical landscape.

Chemistry in eukaryotic intercellular spaces is shaped by both hosts and symbiotic microorganisms such as bacteria. Pathogenic microorganisms like barley-associated Xanthomonas translucens (Xt) swiftly overtake the inner leaf tissue becoming the dominant microbial community member during disease development. The dynamic metabolic changes due to Xt pathogenesis in the mesophyll spaces remain unknown. Genomic group I of Xt consists of two barley-infecting lineages: pathovar translucens (Xtt) and pathovar undulosa (Xtu). Xtu and Xtt, although genomically distinct, cause similar water-soaked lesions. To define the metabolic signals associated with inner leaf colonization, we used untargeted metabolomics to characterize Xtu and Xtt metabolism signatures associated with mesophyll growth. We found that mesophyll apoplast fluid from infected tissue yielded a distinct metabolic profile and shift from catabolic to anabolic processes over time compared to water-infiltrated control. The pathways with the most differentially expressed metabolites by time were glycolysis, tricarboxylic acid cycle, sucrose metabolism, pentose interconversion, amino acids, galactose, and purine metabolism. Hierarchical clustering and principal component analysis showed that metabolic changes were more affected by the time point rather than the individual colonization of the inner leaves by Xtt compared to Xtu. Overall, in this study, we identified metabolic pathways that explain carbon and nitrogen usage during host-bacterial interactions over time for mesophyll tissue colonization. This foundational research provides initial insights into shared metabolic strategies of inner leaf colonization niche occupation by related but phylogenetically distinct phyllosphere bacteria. IMPORTANCE: The phyllosphere is a habitat for microorganisms including pathogenic bacteria. Metabolic shifts in the inner leaf spaces for most plant-microbe interactions are unknown, especially for Xanthomonas species in understudied plants like barley (Hordeum vulgare). Xanthomonas translucens pv. translucens (Xtt) and Xanthomonas translucens pv. undulosa (Xtu) are phylogenomically distinct, but both colonize barley leaves for pathogenesis. In this study, we used untargeted metabolomics to shed light on Xtu and Xtt metabolic signatures. Our findings revealed a dynamic metabolic landscape that changes over time, rather than exhibiting a pattern associated with individual pathovars. These results provide initial insights into the metabolic mechanisms of X. translucens inner leaf pathogenesis.

Bacterial Leaf Streak Diseases of Plants: Symptom Convergence in Monocot Plants by Distant Pathogenic Xanthomonas Species.

Bacterial leaf streak (BLS) is a disease of monocot plants caused by Xanthomonas translucens on small grains, X. vasicola on maize and sorghum, and X. oryzae on rice. These three pathogens cause remarkably similar symptomology in their host plants. Despite causing similar symptoms, BLS pathogens are dispersed throughout the larger Xanthomonas phylogeny. Each aforementioned species includes strain groups that do not cause BLS and instead cause vascular disease. In this commentary, we hypothesize that strains of X. translucens, X. vasicola, and X. oryzae convergently evolved to cause BLS due to shared evolutionary pressures. We examined the diversity of secreted effectors, which may be important virulence factors for BLS pathogens and their evolution. We discuss evidence that differences in gene regulation and abilities to manipulate plant hormones may also separate BLS pathogens from other Xanthomonas species or pathovars. BLS is becoming an increasing issue across the three pathosystems. Overall, we hope that a better understanding of conserved mechanisms used by BLS pathogens will enable researchers to translate findings across production systems and guide approaches to control this (re)emerging threat.

Comparative Genomics of Xanthomonas translucens pv. undulosa Strains Isolated from Weedy Grasses and Cultivated Wild Rice.

Bacterial leaf streak (BLS) of wheat (Triticum aestivum), caused by Xanthomonas translucens pv. undulosa, is a disease of major concern in the Northern Great Plains. The host range for X. translucens pv. undulosa is relatively broad, including several small grains and perennial grasses. In Minnesota, X. translucens pv. undulosa was isolated from weedy grasses in and around wheat fields that exhibited BLS symptoms and from cultivated wild rice (Zizania palustris) with symptomatic leaf tissue. Currently, no genomic resources are available for X. translucens pv. undulosa strains isolated from non-wheat hosts. In this study, we sequenced and assembled the complete genomes of five strains isolated from weedy grass hosts, foxtail barley (Hordeum jubatum), green foxtail (Setaria viridis), and wild oat (Avena fatua), and from cultivated wild rice and wheat. These five genomes were compared with the publicly available genomes of seven X. translucens pv. undulosa strains originating from wheat and one genome of an X. translucens pv. secalis strain originating from rye (Secale cereale). Global alignments of the genomes revealed little variation in genomic structures. Average nucleotide identity-based phylogeny and life identification numbers revealed that the strains share ≥99.25% identity. We noted differences in the presence of Type III secreted effectors, including transcription activator-like effectors. Despite differences between strains, we did not identify unique features distinguishing strains isolated from wheat and non-wheat hosts. This study contributes to the availability of genomic data for X. translucens pv. undulosa from non-wheat hosts, thus increasing our understanding of the diversity within the pathogen population.

Comparative Genomics of Barley-Infecting Xanthomonas translucens Shows Overall Genetic Similarity but Globally Distributed Virulence Factor Diversity.

Xanthomonas translucens pv. translucens (Xtt) is a global barley patho-gen and a concern for resistance breeding and regulation. Long-read whole genome sequences allow in-depth understanding of pathogen diversity. We have completed long-read PacBio sequencing of two Minnesotan Xtt strains and an in-depth analysis of available Xtt genomes. We found that average nucleotide identity (ANI)-based approaches organize Xtt strains different from the previous standard multilocus sequencing analysis approach. According to ANI, Xtt forms a separate clade from X. translucens pv. undulosa and consists of three main groups which are represented on multiple continents. Some virulence factors, such as 17 Type III-secreted effectors, are highly conserved and offer potential targets for the elicitation of broad resistance. However, there is a high degree of variation in virulence factors, meaning that germplasm should be screened for resistance with a diverse panel of Xtt.