ELR is a true pattern recognition receptor that associates with elicitins from diverse Phytophthora species

The first layer of plant immunity against pathogens is mediated by cell surface pattern recognition receptors (PRRs) that recognize pathogen molecules in the apoplast. Several pairs of PRRs and their matching extracellular ligands have been described but, in many cases, actual evidence for ligand binding by the PRR is lacking. The receptor-like protein ELR from Solanum microdontum, which triggers cell death upon co-expression with elicitins of various Phytophthora species and enhances resistance to late blight caused by Phytophthora infestans, was previously identified as the elicitin receptor by forward genetic screenings employing the INF1 elicitin of P. infestans. In this study, we investigated whether ELR associates with INF1 and other elicitins that are secreted by diverse Phytophthora spp. We performed in planta and in vitro co-immunoprecipitation of ELR with several affinity-tagged elicitins, as well as in planta transient co-expression assays. We found that ELR physically interacts with the class I elicitins INF1 and ParA1, from P. infestans and Phytophthora parasitica, respectively, which is in line with their ability to cause cell death when co-expressed with ELR in potato. Together, we demonstrate that ELR is a genuine PRR that binds elicitins of Phytophthora species.