The secreted peptide IRP1 functions as a phytocytokine in rice immunity.

Small signaling peptides play important roles in various plant processes, but information regarding their involvement in plant immunity is limited. We previously identified a novel small secreted protein in rice, named immune response peptide 1 (IRP1). Here, we studied IRP1 functions in rice immunity. Rice plants overexpressing IRP1 enhanced resistance to the virulent rice blast fungus. Application of IRP1 peptide to rice suspension cells triggered the expression of IRP1 itself and the defense gene PAL1. RNA-seq results revealed that 84% of genes upregulated by IRP1 peptide were also induced by a microbe-associated molecular pattern(MAMP) chitin, including 13 OsWRKY transcription factors, indicating that IRP1 and chitin share a similar signaling pathway. Co-treatment with chitin and IRP1 elevated the expression level of PAL1 and OsWRKYs in an additive manner. The increased chitin concentration arrested the induction of IRP1 and PAL1 expressions by IRP1, but cannot affect IRP1-triggered MAPK activation. Collectively, our findings indicate that IRP1 functions as a phytocyokine in rice immunity regulating MAPKs and OsWRKYs that could amplify chitin and other signaling pathways, and provide new insights into how MAMPs and phytocyokines cooperatively regulate rice immunity.