Molecular and Biochemical Mechanisms of Elicitors in Pest Resistance

Insect herbivores have a variety of life cycles and feeding habits, making them extremely diverse. With their host plants, they form close relationships and suppress their defense mechanisms. Molecular elicitors are the key bio-elements in the detection and recognition of attacking enemies in tissue consumption. Insect oral secretion, frass, and fluid of egg deposition contain biologically active molecules called herbivore-associated elicitors (HAEs) that are recognized by pattern-recognition receptors (PRRs). Many plants distinguish insect feeding from wounding by HAEs present in their oral secretions (OS) and induce local and/or systemic responses against arthropod feeding. PRRs perceive HAEs in the oral secretion of caterpillars in a species-specific manner to elicit exclusive defense responses. HAEs-PRRs interactions induce plant resistance by reprogramming plant metabolism and transcriptional machinery. Quantitative, timely, and coordinated plant response initiate early signaling events, including Ca²⁺, reactive oxygen species (ROS), and mitogen-activated protein kinases (MAPKs). However, in insect herbivory, little is known about the molecular basis of signal transduction and regulation of plant resistance. We discuss here how early signaling cascades converge into the accumulation of phytohormones that regulate downstream special metabolites against herbivores. In this review, we propose a hypothetical model of PPRs-HAEs-mediated-induced responses in plants and discuss how PRRs-HAEs interactions elicit short- and long-term induced defenses in plants. The understanding of PRRs-HAEs interactions will help to explore the fundamental molecular mechanisms of host manipulation and may generate prospects to develop novel pest-resistance strategies.