Both Proteins/Peptides and Non-Proteins/Peptides in Larval Oral Secretions of a Chewing Herbivore Function as Elicitors That Induce Plant Defenses.

Herbivore elicitors, most of which identified thus far are from the oral secretions (OS) of herbivores, are important in herbivore-induced plant defense. However, whether and how larval OS of the striped stem borer (SSB), Chilo suppressalis, a devastating pest on rice, induce plant defenses remains largely unknown. We found that treating plants with SSB OS enhanced levels of phosphorylated OsMPK3/4/6, JA, JA-Ile, ethylene, and ABA in rice, which subsequently induced the production of defense compounds, such as trypsin protease inhibitors, and decreased the growth of SSB larvae. Digestion by proteinase K decreased the induction activity of SSB OS on rice defense, but it still activated OsMPK6, JA, ABA, and ethylene pathways, and enhanced plant SSB resistance. Treating plants with SSB OS fractions that did not contain fatty acid-amino acid conjugates (FACs) activated OsMPK3/4/6, JA, ABA, and ethylene pathways, whereas treating plants with other OS fractions containing FACs did not. Bioassays revealed that impairing the JA or ABA signaling pathway in rice decreased plant SSB resistance. These results demonstrate that both proteins/peptides and non-proteins/peptides-but not FACs-in SSB OS play a central role in inducing rice defenses by activating signaling pathways mediated by JA, ABA, and ethylene.