A phytocytokine and its derived peptides in the frass of an insect elicit rice defenses

Upon recognizing elicitors derived from herbivores, many plants activate specific defenses. Most of the elicitors identified thus far are from the oral secretions and egg-laying fluids of herbivores; in contrast, herbivore fecal excreta have been sparsely studied in this context. In this study, we identified elicitors in the frass of the striped stem borer (SSB; Chilo suppressalis) larvae using a combination of molecular and chemical analyses, bioactivity tests and insect performance bioassays. Treating rice plants with SSB frass or a solution composed of SSB frass and buffer elicited mitogen-activated protein kinase (MPK) cascades and the jasmonic acid (JA)-signaling pathway. Moreover, the treatment induced both the expression of defense-related genes and the production of defensive compounds, and enhanced the resistance of rice plants to SSB. Heating SSB frass solution did not affect its induction activity, but eliminating proteins and peptides from the solution by adding proteinase K impaired its activity. Additional chemical analyses and bioassays revealed that the rice phytocytokine, immune response peptide 1 (IRP1), together with some of its derived peptides in SSB frass, induced the MPK cascades, JA biosynthesis, the expression of defense genes and the production of defensive compounds in rice. These results reveal an important role for the plant-derived fecal peptide phytocytokine IRP1 and some of its derived peptides in inducing defenses in rice against SSB.