Oomycete and fungi deliver a variety of effector proteins into plant host cells to suppress defense responses and enable successful colonization. Some of these effectors are secreted into host apoplast where they target plant enzymes to inhibit their activities. Others are secreted into the host cytoplasm and nucleus where they manipulate host immunity. The mechanisms involved in transport of oomycete and fungal effectors and host defense manipulation remain unclear. One key approach to unraveling effector function is to find their host cell targets and derive hypotheses about the effector mode of action. Phytophthora infestans offers an excellent system for applying this strategy. The objective of this proposal is to determine the target proteins of the recently identified family of P. infestans effectors to elucidate their biological function. Our hypothesis is based on the fact that P. infestans secretes a diverse set of proteins that target host proteins to suppress host responses and facilitate infection. This hypothesis is supported by preliminary data that demonstrated that P. infestans RXLR type efector protein, AVR3a, is delivered into the host cytoplasm and suppresses hypersensitive cell death mediated by another Phytophthora protein INF1. In this proposal, we will focus on (i) identification of host interactors of P. infestans cytoplasm and nuclear localized effector proteins; (ii) identification of the functions of host effector targets using a VIGS approach in tomato plants. Considering that none of the host translocated fungal and oomycte effectors have been assigned a precise biological function, the proposed research will provide a significant contribution to understanding how filamentous pathogen effectors promote disease. Also, this proposal will provide insights into key molecular processes regulating susceptibility to an economically important pathogen.