Importance of OsRac1 and RAI1 in signalling of nucleotide-binding site leucine-rich repeat protein-mediated resistance to rice blast disease

• Plants depend on Resistance (R) genes, most of which encode nucleotide-binding site leucine-rich repeat (NLR) proteins, for pathogen race-specific disease resistance. However, only a few immediate downstream targets of R proteins have been characterized, and the signalling pathways for R-protein-induced immunity are largely unknown. In rice (Oryza sativa), NLR proteins serve as important immune receptors in the response to rice blast disease caused by the fungus Magnaporthe oryzae. • We used site-directed mutagenesis to create an autoactive form of the NLR protein PID3 that confers blast resistance and used transgenic rice to test the resulting immunity and gene expression changes. • We identified OsRac1, a known GTPase, as a signalling molecule in PID3-mediated blast resistance, implicating OsRac1 as a possible common factor downstream of rice NLR proteins. We also identified RAI1, a transcriptional activator, as a PID3 interactor required for PID3-mediated blast resistance and showed that RAI1 expression is induced by PID3 via a process mediated by OsRac1. • This study describes a new signalling pathway for NLR protein-mediated blast resistance and shows that OsRac1 and RAI1 act together to play a critical role in this process.