Phytophthora pathogens secrete hundreds of proteins, many of which play a role in infecting and colonizing host plants. One large class of secreted proteins comprises effectors that all share a N-terminal RXLR domain required for effector translocation into host cells. In contrast, their C-terminal domains show extensive sequence divergence and contain determinants for recognition by specific intracellular NB-LRR resistance (R) proteins. In the presence of the corresponding R protein the RXLR effector is recognized as avirulence (AVR) factor resulting in a hypersensitive response (HR) that blocks further growth of the pathogen. In the absence of cognate R proteins RXLR effectors promote virulence of the pathogen by suppressing defense responses. This project is aimed at unraveling the mode of action of RXLR effectors. We focus on Avr1 and IPI-O, the Avr counterparts of the R proteins R1 and Rpi-blb1, respectively. Via a yeast-2-hybrid screening we selected putative host targets of Avr1 and IPI-O in potato for functional analysis based on gene silencing, overexpression, in planta localization and co-immunoprecipitation.