| Bacterial pathogens often manipulate the normal signaling and metabolic pathways of eukaryotic cells to successfully colonize plant or animal hosts. One strategy many bacteria employ is delivery of bacterial proteins into host cells during infection using a large molecular machine called a type III secretion system (TTSS). Mutations in genes that encode the TTSS system of the phytopathogen Xanthomonas campestris pathovar (pv.) vesicatoria (Xcv) make these bacteria non-pathogenic on susceptible pepper and tomato host plants. The substrates of the TTSS, called type III effector proteins, are therefore predicted to be important in the infection process. Several Xcv type III effectors had been identified at the initiation of this work. Other plant pathogens like Pseudomonas syringae may contain as many as forty of these proteins, suggesting that there were additional type III effectors to be found in Xcv.; In this dissertation, several different aspects of Xcv type III effectors were explored. First, a biochemical assay helped to demonstrate that type III effectors are targeted into plant cells during infection. Second, a functional genetic screen was performed in Xcv strain 85-10 that identified eight type III effectors, called Xanthomonas outer proteins (Xops), which are sent into pepper cells during infection. Mutations in one effector gene, xopN, caused a reduction in growth of Xcv 85-10 on susceptible pepper and tomato host plants. This phenotype is striking, since type III effectors have redundant targets or are found in homolog families, and mutations in other xop genes did not affect Xcv growth on susceptible plants.; Because XopN was predicted to contain a protein-protein interaction domain, I performed a yeast two-hybrid screen to find plant interactors of XopN. XopN interacted with several proteins, including a leucine-rich repeat class receptor-like kinase and a fourteen-three-three protein. The roles of these proteins in the XopN-dependent growth defect are currently being explored. Further analysis of the identity and functions of XopN and other Xcv type III effectors in pathogenesis will help extend the knowledge about how bacterial phytopathogens cause disease. |
