Phytophthora sojae, a hemibiotrophic oomycete, is the causal pathogen of stem and root rot disease of soybean (Glycine max [L.] Merr). The annual soybean yield loss suffered from this disease is valued at about 300 million dollars. A series of single Rps genes protect soybean from the root and stem rot disease. In the last two decades Rps1-k has been the most stable and widely used Phytophthora resistance gene in the major soybean-producing regions of the United States;Two classes of CC-NB-LRR-type Rps1-k genes were isolated from the Rps1-k locus. Representative members from each gene class were expressed in transgenic soybean plants. Analyses of independent R0, R1, R2 and R3 progeny populations suggested that both gene classes confer Phytophthora resistance in soybean. Sequence of the entire Rps1-k locus indicates that it is composed of mostly repetitive sequences, including simple repeat sequences, tandem repeats and retroelements. Gene content in the Rsp1-k region appears to be very poor, with only a few full-length genes including the Rps1-k genes;A yeast two-hybrid system was applied in identifying signal transduction factors involved in the expression of Phytophthora resistance. Various baits derived from Rps1-k-2 were applied in the yeast two-hybrid screens. Thirteen putative Rps1-k-2-interacting proteins (RIP) were identified based on in vivo interactions in yeast and in vitro pull down assays. In vivo interactions in soybean cells determined by FRET analysis and preliminary RNA interference (RNAi) experiments in transformed cotyledons further confirmed that several candidate Rps1-k-2-interactors including a 26S proteasome AAA-ATPase subunit RPT5a, a receptor kinase, an unknown expressed protein and a type II metacaspase are promising Rps1-k-2-interactors. We further characterized RIP13, the type II metacaspase that showed to interact with the N-terminal 144 amino acids of Rps1-k-2. RIP13 is rapidly induced upon infection with an avirulent P. sojae race. Large-scale RNAi experiments suggest that RIP13 play a critic role in Rps1-k-2-mediated resistance. This study implicates that the functional role of RIP13 may be analogous to the mammalian effector casapses, and Rsp1-k-2 may act as an 'adapter protein' like Apaf-1 in plant hypersensitive response-related cell death machinery.