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.