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.
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.
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