Herbivore Elicitors in Rice Defense: New Insights for Sustainable Crop Protection

Abstract

Rice (Oryza sativa) serves as a staple food for more than half of the world's population, and its productivity is continually challenged by insect herbivory. Among the most destructive pests in Asia, the striped stem borer (SSB, Chilo suppressalis) is a lepidopteran larva that feeds endophytically within the rice stem. Its feeding behavior impairs the plant's vascular function, leading to hallmark symptoms such as ‘dead heart’ during the vegetative stage and ‘white head’ at grain filling, both of which result in significant yield losses (Yu et al. 2024). Despite decades of research into insect resistance, the molecular mechanisms by which rice perceives and responds to herbivore attack remain largely elusive.

In this issue, Jing et al. (2025) provide a substantial advance by identifying and characterizing the defense-inducing components in the oral secretion (OS) of SSB. Using a combination of biochemical fractionation and genetic analysis, the authors demonstrate that SSB OS contains elicitors capable of triggering robust defense responses in rice. Application of SSB OS to wounded leaves enhanced the phosphorylation levels of mitogen-activated protein kinases OsMPK3/4/6, elevated levels of phytohormones jasmonic acid (JA), JA-Ile, abscisic acid (ABA), and ethylene, and induced the activity of trypsin protease inhibitors and production of defense compounds such as some flavonoids, phenolamides and volatiles (Figure 1). These physiological and molecular changes cumulatively suppressed larval growth, indicating that rice can rapidly detect and respond to specific herbivore-derived cues. Jing et al. (2025) also show that although the elicitor activity of SSB OS was reduced after proteinase K treatment, it was not completely abolished, suggesting that both proteinaceous and non-proteinaceous components in OS contribute to defense induction. While fatty acid–amino acid conjugates (FACs) in oral secretions are established elicitors in other plant–insect systems, the active rice-eliciting fractions in SSB OS contained no detectable FACs. This indicates that rice may recognize a distinct class of herbivore-associated molecules other than FACs. Furthermore, defense responses were attenuated in rice mutants impaired in JA and ABA pathways (e.g., jar1 and aba2), confirming that these hormones are key regulators of the induced resistance. The study thus highlights the complexity of herbivore perception and defense activation in rice and sets the stage for identifying novel elicitor classes.

This study advances the evolving concept of herbivore-associated molecular patterns (HAMPs), which function analogously to pathogen-associated molecular patterns (PAMPs) in activating innate immunity in plants (Ali et al. 2024). While the PAMP paradigm is well-established in plant–pathogen interactions, the idea that plants can also recognize conserved molecular signatures from herbivores is gaining traction. Recognition of HAMPs typically leads to rapid activation of MAPK cascades, shifts in hormone signaling (JA, ABA, SA, ethylene), and transcriptional reprogramming that strengthens plant resistance (Deng et al. 2024). Interestingly, the molecular defense mechanisms triggered by HAMPs partially overlap with those initiated by beneficial microbes. For example, some volatile organic compounds from plant-growth-promoting rhizobacteria (PGPR) can trigger MAPK activation, hormonal changes, and early defense responses such as ROS bursts and immune priming (Goggin and Fischer 2024; Zhu et al. 2022). These parallels underscore the possibility that plant immunity may rely on a shared set of signaling hubs capable of integrating cues from diverse biotic sources, whether pathogenic, herbivorous, or mutualistic. In evolutionary terms, such convergence may reflect a cost-effective strategy for maintaining broad-spectrum defense capabilities using a limited repertoire of receptors and signaling modules.

From an applied perspective, the identification of elicitors in SSB OS offers exciting opportunities for sustainable agriculture. In the face of climate change, pesticide resistance, and environmental degradation, developing pest-resilient crops through ecological and molecular means is increasingly vital. The components in elicitor-active fractions described in this study could serve as templates for designing novel pest management strategies. For instance, synthetic mimics of these molecules might be used to ‘prime’ rice plants for enhanced defense, a concept that has gained traction in plant-pathogen systems. Alternatively, genetic engineering or marker-assisted selection could be used to enhance the expression or sensitivity of key receptors and signaling proteins involved in elicitor perception.

The discovery of both proteinaceous and non-proteinaceous elicitors emphasizes the need to explore a broader chemical space in the search for bioactive molecules. While peptides and proteins are classical elicitors, non-proteinaceous metabolites may be more stable, cost-effective, and easier to synthesize or apply in the field. The separation of OS fractions with distinct elicitor activity, as performed by Jing et al. (2025), paves the way for detailed structural and functional studies to identify the molecular identities of these signals. Future work should aim to determine the key elicitors and identify the rice receptors that perceive them. Are they detected by pattern recognition receptors (PRRs) at the plasma membrane, or by intracellular sensors such as nucleotide-binding leucine-rich repeat proteins (NLRs) (Lu and Tsuda 2021)? What downstream pathways are uniquely activated by these OS components, and how do they interact with other stress response networks? Addressing these questions will deepen our understanding of plant immunity and facilitate its application in crop protection. Moreover, these findings raise intriguing questions about the specificity and evolution of herbivore-plant interactions. Do other rice herbivores possess similar elicitor-active OS components, or are these features unique to SSB? Comparative studies across different insect species could help determine whether elicitor composition is phylogenetically conserved or shaped by co-evolutionary arms races with host plants.