| Plant bacterial pathogens use the type III secretion system to transport bacterial effector proteins into the host cell. As related Gram-negative bacterial pathogens of animals can deliver a number of effectors into the host cell, it is possible the same may be true for plant bacterial pathogens. This suggests there remain unidentified effectors used by the pathogen to elicit disease. The research presented in this dissertation was based on two related hypotheses: (i) bacterial plant pathogens deliver a large number of effector proteins into the host cell, many of which are unidentified, and (ii) these effectors target key components of the plant's cellular machinery, which contribute to disease formation. My goals were to identify and characterize a novel bacterial effector from the bacterial plant pathogen Pseudomonas syringe pv. tomato (Pst) DC3000 or Xanthomonas campestris pv. vesicatoria (Xcv). A combination of genetics, biochemical, cell biological, molecular, and bioinformatics techniques were used to achieve these goals.; Studies in Pst DC3000 identified 6 novel effectors. Although five effectors were demonstrated to be delivered into the host cell, in planta growth curves showed no significant change in bacterial growth. Additional secretion and translocation assays demonstrated that none of these effectors is essential for the delivery of other effectors into the host. Therefore, the role of these effector proteins in Pst DC3000 remains unclear.; Previous studies using Xcv identified a novel effector protein, called XopX. XopX is delivered into the plant cell, where it appeared to elicit a strong cell death response. Although Agrobacterium-mediated transient expression of XopX in Nicotiana benthamiana did not elicit the cell death response, it was recapitulated when XopX was transiently expressed and co-inoculated with either XcvDeltaXopX or Xcc. Transgenic 35S:XopX expressing N. benthamiana plants also have the visible plant cell death response to XcvDeltaXopX and Xcc strains. Transgenic 35S:XopX N. benthamiana plants displayed enhanced susceptibility to bacterial growth of Xcc, consistent with a demonstrated increase in bacterial growth on N. benthamiana observed for Xcc with in trans XopX expression. This suggests that XopX targets basic innate immunity in plants, which in turn allows for greater pathogen virulence. |
