The late blight pathogen Phytophthora infestans is a major threat for potato and tomato crops. It was responsible for the Irish Potato Famine in the mid 1840’s and continues to cause dramatic losses nowadays. This oomycete spreads rapidly and shows remarkable adaptability in overcoming plant defenses. To establish its biotrophic life style, P. infestans secretes proteins called effectors that function in the apoplast or in the cytoplasm of the plant cell. AvrBlb2 belongs to the RxLR class of P. infestans effectors that are translocated inside the host cell and are hypothesized to reprogram cellular processes. The AvrBlb2 family of effectors is recognized by RpiBlb2 resistance proteins from potato and render the pathogen avirulent. We recently conducted a co-immunoprecipitation approach to identify plant targets of AvrBlb2 effector. Our preliminary data indicate that transient silencing of one Candidate Plant Target (CPT) causes enhanced susceptibility. Our proposed research aims at understanding the role CPT plays in plant defense and how AvrBlb2 alters CPT to suppress plant resistance mechanisms. We will analyze tobacco CPT transgenic plants and modulate CPT expression in plants carrying the Rpiblb2 resistance gene to test the role of this protein in defense toward P. infestans. We will conduct a structural analysis of CPT, with and without the presence of AvrBlb2, to complement the structural analysis of the AvrBlb2 protein that has been initiated in our lab. These data will serve to elucidate how AvrBlb2 interacts with and alters CPT. Finally targeted mutagenesis approach on the CPT protein will be used to test the proposed mechanisms of action. This project is expected to lead to the first report of a virulence function for a eukaryotic plant pathogen effector with a specific target. In addition, knowledge of the host targets of pathogen effectors will create new opportunities for durable disease resistance engineering in cultivated plants.