Non-volatile metabolites mediating plant interactions with insect herbivores.

Non-volatile metabolites constitute the bulk of plant biomass. From the perspective of plant-insect interactions, these structurally diverse compounds including nutritious core metabolites and defensive specialized metabolites. In this review, we synthesize current literature on multiple scales of plant-insect interactions mediated by non-volatile metabolites. At the molecular level, functional genetics studies have revealed a large collection of receptors targeting plant non-volatile metabolites in model insect species and agricultural pests. By contrast, examples of plant receptors of insect-derived molecules remain sparse. For insect herbivores, plant non-volatile metabolites function beyond the dichotomy of core metabolite being nutrients and specialized metabolites being defensive compounds. Insect feeding tends to elicit evolutionarily conserved regulation of plant specialized metabolism, whereas its effect on plant core metabolism varies widely based the interacting species. Finally, several recent studies have demonstrated that non-volatile metabolites can mediate tripartite communication on the community scale, facilitated by physical connections established by direct root-to-root communication, parasitic plants, arbuscular mycorrhizae, and the rhizosphere microbiome. Recent advances in both plant and insect molecular biology, will facilitate further research on the role of non-volatile metabolites in mediating plant-insect interactions.