Bacterial spot of tomato and pepper: insights into host-pathogen interactions

The Solanaceae family, containing approximately 100 genera and 2,500 species, is the third most valuable crop family in the world, and the most valuable vegetable crop. Some of the most economically important Solanaceae are tomato (Solanum lycopersicum), pepper (Capsicum spp.), eggplant (S. melongena) and tobacco (Nicotiana tabacum). Bacterial infection and subsequent disease symptoms cause enormous yield losses to crops in this family. Xanthomonas spp., the causal agent of bacterial spot disease, can result in millions of dollars in losses of solanaceous crop production worldwide. It is almost impossible to control the disease once it is present in the plant. Therefore, it is necessary to have a better understanding of host-pathogen interactions at the molecular level to detail how some of the Xanthomonas and host molecules interact. 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 in early stages of infection and chemotaxis, reductions in or loss of motility can significantly reduce bacterial fitness. In Chapter 2, I observed variation in the motility for many isolates, irrespective of their flagellin sequence. Instead, I 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. While some flagellin variants may impart altered motility, external growth parameters and gene expression regulation appear to have more significant impacts on the motility phenotypes for these Xanthomonas species. In Chapter 3, I determined that variations in flg22 and flgII-28 epitopes led to differential recognition by the immune systems in tomato, chili pepper, and bell pepper plants. One variant of flgII-28 had a polymorphism that allowed it to evade recognition by tomato and chili pepper plants; however, the FLS3 variant present in bell pepper plants recognizes this flgII-28 variant, as shown by differential immune responses in the production of reactive oxygen species and in the inhibition of seedling growth. In tomato and chili pepper plants, I observed larger bacterial populations of strains with flagellin variants predicted not to be recognized by either FLS2 and FLS3, suggesting that these bacteria can evade flagellin recognition. However, there were no difference in bacterial populations in bell pepper plants, indicating that chili pepper plants possess an FLS3 receptor that can recognize all flagellin variants tested. Overall, this flgII-28 polymorphism allows some, but not all, Xanthomonas species to evade both FLS2-and FLS3-mediated oxidative burst responses, and impacts the virulence of the bacteria after infection into different plant hosts. In Chapter 4, I wanted to further investigate the FLS3 polymorphisms that may be causing differential immune recognition of flgII-28 in tomato, chili, and bell pepper plants. I first obtained the FLS3 sequence information from chili and bell pepper plants that showed polymorphic amino acids at 12 residues located in different parts of the FLS3 proteins. I then performed in silico predictions to identify a region that may potentially function as the binding pocket of the receptor and further narrowed down my findings to two putative amino acid polymorphisms. Finally, I found charge and surface accessibility predictions in the flgII-28 epitope that is not recognized by tomato and chili pepper FLS3 proteins that may influence its binding to FLS3. In conclusion, I anticipate that these results will improve our understanding of host-pathogen interactions between Xanthomonas species and tomato and pepper plants, specifically at the initial events in pattern-triggered immunity. I hope that these findings could be used in the development of genetically engineered crops with enhanced resistance to bacterial diseases.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste