RALF peptides regulate herbivore-induced defense responses in Arabidopsis leaves

Plants employ small secreted peptides to mediate intercellular and intertissue communication. Among these peptides, rapid alkalinization factors (RALFs) participate in diverse signaling pathways, yet their roles in herbivore-induced defense responses remain poorly understood. In this study, we investigated the roles of RALF peptides in Arabidopsis thaliana leaves damaged by Spodoptera litura larvae. RNA sequencing analysis revealed that the expression of RALF1 and RALF27 was induced following herbivory. Functional analyses further showed that ralf1 and ralf27 mutants exhibited increased susceptibility to herbivore damage and reduced expression of defense-related genes, including DEFL206 and a cupin family gene, with the effects being more pronounced in ralf27 mutants than in ralf1 mutants. In contrast, exogenous application of the RALF27 peptide enhanced resistance to S. litura. Although RALF peptides have been reported to interact with malectin-like receptor kinases such as THESEUS1 and FERONIA, these receptors may not play major roles in RALF-mediated signaling under herbivore attack. Furthermore, expression analyses suggested that RALF1 and RALF27 may contribute to short-distance intra-leaf defense signaling during herbivory. These findings identify RALF peptides as regulators of herbivore-induced defense signaling.