Phytohormonal signaling network and immune priming pertinence in plants to defend against insect herbivory

Over millions of years, the coevolution between plants and insect herbivores has led to the development of diverse defense mechanisms in response to herbivory. In response to herbivory, plants mount a defense response by the onset of specific metabolic mechanisms and hormonal pathways, including salicylic acid (SA), jasmonic acid (JA), ethylene (ET), cytokinin (CT), abscisic acid (ABA), auxin (AUX), and gibberellic acids (GAs). Indeed, plants employ antixenosis and antibiosis as defense mechanisms, triggering various morphological, biochemical, and behavioral responses that include the increase of trichomes, sclerophylly, callose deposition, and the production of defensive secondary metabolites to combat herbivory. This intricate network of phytohormonal signaling and their molecular interplay in response to herbivory damages, immune priming against herbivory induced by microbes and abiotic stress, systemic signaling to prime distant tissues for pre-emptive immune responses, and the transmission of immune memory to subsequent generations by plants are all adaptive mechanism, plants follow which are all essential areas covered in this comprehensive review. Additionally, the recognition of damage/herbivory-associated molecular patterns (D/HAMPs) and pathogen/microbe-associated molecular patterns (P/MAMPs) by pattern recognition receptors (PRRs) triggers intracellular signaling events that lead to pattern-triggered immunity (PTI). This occurs through pre-signaling events such as plasma transmembrane potential depolarization, ion efflux/influx, cytosolic calcium ([Ca2+] cyt) elevation, molecular pattern recognition, and defensive effector identification. These processes are crucial for the plant to mount a rapid and appropriate response to herbivory, which is discussed in detail. Furthermore, understanding these complex natural defense mechanisms that may navigate through new avenues is essential for developing sustainable crop production practices with minimal external inputs for an eco-friendly environment.