Pepper and tomato, as two common, popular, and important vegetables grown worldwide, provide human beings with high quality fruit of flavor and aroma, and a high concentration of vitamins and antioxidants. Pepper and tomato production is frequently affected by various pathogens, including nematodes, fungi, and bacteria. Among those phytopathogens, Xanthomonas euvesicatoria (Xe) causes a severe bacterial spot (BS) disease on pepper and tomato. The BS disease could cause a loss of approximately 10% of the total crop yield in the world. Breeding tomato and pepper cultivars with improved BS disease resistance is one of the most important breeding goals. A better understanding of the virulence mechanism of Xe could help breeders design new strategies for resistance breeding. In this dissertation, we characterized the virulence and avirulence functions of four Xe Type Three Secretion Effectors (T3Es): Xe-XopQ, Xe-XopX, Xe-XopN, and Xe-avrRxo1. Xe-XopQ is a Xe T3E that functions as a determinant of host specificity. Here, we further explored the virulent and avirulent functions of Xe-XopQ. We identified another T3E Xe-XopX that could interact with XopQ and subsequently elicit the hypersensitive response in N. benthamiana in the Agrobacterium-mediated transient assay and Xe-mediated disease assay. The interaction is confirmed by bimolecular fluorescence complementation, co-immunoprecipitation and split luciferase assay. Intriguingly, we also revealed that XopX also interacts with multiple Xe T3Es including AvrBS2, XopN, XopB, and XopD in the co-IP assay. The virulent and avirulent functions of XopQ and AvrBS2 are compromised in the absence of Xe-XopX. Since XopX is conserved in diverse Xanthomonas spp., we speculate that Xe-XopX may have a general role required for the pathogenesis of Xe. Xe-XopN has been reported to be a T3E with virulence function via targeting host defense-related proteins, including atypical receptor-like kinase named TARK1 and a 14-3-3 protein to suppress the PAMPs (pathogen-associated molecular patterns) triggered immunity upon Xe colonization of tomato. In this study, we revealed additional virulence mechanisms of Xe-XopN, where Xe-XopN, is required for triggering the water-soaking symptom on Nicotiana benthamiana and pepper plants infected with Xe. In addition, we identified that XopN interacts with a transcription factor, NbVOZ, and represses the expression of NPR1, a key component of the basal defense. Therefore, XopN has a role in maintaining a water-affluent environment for better replication of Xe, and it can also interact with NbVOZ1/2 to regulate plant immunity. AvrRxo1, a T3E of Xanthomonas oryzae pv. oryzicola (Xoc), was previously identified to function as a NAD kinase. Here, we characterized a Xe T3E, Xe avrRxo1, that is a functional homologue of AvrRxo1, which is required for the full virulence of Xe to colonize the pepper and N. benthamiana plants. Overexpression of AvrRxo1 in bacterial or plant cells is toxic. Our group previously demonstrated AvrRxo1-ORF2 functions as an antitoxin that binds to AvrRxo1 to suppress its toxicity. In this study, we identified Xe4429 as the homologue of AvrRxo1-ORF2, which could interact with Xe-avrRxo1 to suppress its toxicity. We also revealed that Xe4429 could bind to the promoter of Xe-avrRxo1 and suppress its transcription. Therefore, we found Xe4429 encodes protein functions as an antitoxin and a transcription repressor in Xe bacterial cells. / Doctor of Philosophy / Peppers and tomatoes are two of the most important vegetables grown worldwide, providing humans with high quality of flavor and aroma, vitamins, and antioxidants. The pepper and tomato production is frequently threatened by various pathogens, including nematodes, fungi, and bacteria. Among those phytopathogens, Xanthomonas euvesicatoria (Xe) causes a severe bacterial spot (BS) disease on peppers and tomatoes. The BS disease can be easily identified due to the appearance of the dark, irregular, water-soaked areas on the leaf, which can cause approximately 10% loss of the total yield of peppers and tomatoes. Breeding tomato and pepper cultivars with improved BS disease resistance is one of the most critical breeding goals. A better understanding of the virulence mechanism of Xe could help breeders to design new strategies for resistance breeding. In my seminar, I will discuss the virulence and avirulence functions of Xe type three secretion (T3S) effectors: Xe XopN, Xe XopQ, and Xe XopX. In my study, I identified Xe XopN is a key factor that regulates the development of the water-soaking symptom on pepper plants infected with Xe. In addition, we revealed Xe XopN interacts with a transcription factor NbVOZ to regulate the expression of NbNPR1 and PR1 genes expression, which may also contribute to the development of water-soaking phenotype. In addition, I identified that Xe XopN could interact with a transcription factor, NbVOZ, and represses the expression of NbNPR1, a key component of the basal defense, and the pathogenesis-related gene PR1. Therefore, Xe XopN has a role in regulating a water-affluent environment to promote bacterial proliferation in the infected plant tissue. Xe XopQ is a Xe T3S effector that functions as a determinant of host specificity. In my study, I identified another T3S effector Xe XopX that could interact with Xe XopQ to trigger the defense response in Nicotiana benthamiana. I also confirmed Xe XopQ physically interacts with Xe XopX inside of plant cells by using bimolecular fluorescence complementation, co-immunoprecipitation and split luciferase assay. Intriguingly, Xe XopX could also interact with multiple Xe T3Es including AvrBS2 in a co-IP assay. The virulence and avirulent functions of Xe XopQ and AvrBS2 are compromised in the absence of Xe XopX.