| Phytophthora infestans, a plant pathogenic oomycete, causes late blight on potato and tomato resulting in devastating economic losses each year. During infection, P. infestans secretes a diverse set of effector molecules that aim to reprogram the host into a susceptible state. Recognition of effector molecules however, leads to induction of the hypersensitive response (HR) a form of programmed cell death and resistance. Interestingly, both resistant host and nonhost plants display localized HR upon P. infestans ingress, indicating mechanistic similarities or overlap between resistance types. In this dissertation, I investigated the molecular determinants of nonhost resistance and cell death in plants. In order to characterize nonhost resistance, we explored Arabidopsis thaliana-P. infestans associations as a model system. Cytological investigations as well as gene expression analysis revealed that activation of HR and induction of defense responses occurs during P. infestans infection. I investigated the role of P. infestans effector proteins in nonhost resistance and cell death. I showed that INF2A and INF2B induced HR-like cell death in Nicotiana and that INF2A-induced cell death requires the ubiquitin-ligase associated protein SGT1. Consequently, I proposed a role for these genes in avirulence or incompatibility in Nicotiana. I investigated the involvement of cell death in P. infestans-host interactions. I characterized PINPP1.1 and the CRN family and speculated that they represent classes of virulence genes. Comparisons between PINPP1.1, CRN2 and INF1 induced cell death revealed that at least two multiple signaling pathways exist. These differences could reflect functional roles for the CRNs and PINPP1.1 during pathogenesis. Overall I conclude that P. infestans-plant associations are characterized by extensive cellular reprogramming. Characterizations of P. infestans effector genes and understanding the perturbations they facilitate in plants will greatly enhance our understanding of P. infestans virulence and host specificity. |
