Reactive oxygen species (ROS) play a vital role in plant immune response,but the genes involved in the regu lation of ROS are scantily reported.Phytophthora pathogens produce a large number of effectors to promote infection,but the modes of action adopted are largely unknown.Here,we report that RxLR207 could activate ROS-mediated cell death in Nicotiana benthamiana and was essential for virulence of P.capsici.We found that this effector targeted BPA1 (binding partner of ACD11) and four members of BPLs (BPA1-Like proteins) inArabidopsis,and the bpa1 and bpl mutants had enhanced ROS accumulation and cell death under biotic or abiotic stresses.Furthermore,we showed that BPA1 and several BPLs functioned redundantly in plant immunity to P.capsici.We discovered that BPA1 and all six BPLs interacted with ACD11,and stabilization of ACD11 was impaired in the bpa 1,bpl2,bpl3,and bpl4 mutants.RxLR207 could promote the degradation of BPA1,BPL1,BPL2,and BPL4 to disrupt ACD11 stabilization in a 26S proteasome-dependent manner.Taken together,these findings indicate the important roles ofArabidopsis BPA1 and its homologs in ROS homeostasis and defense response,highlighting the usefulness of a pathogen effector-directed approach as a promising strategy for the discovery of novel plant immune regulators.