Ecologically regulating functions of herbivore-induced plant volatiles

Plants could release herbivore-induced plant volatiles(HIPVs) into atmosphere or soil when they are damaged by herbivore insects.These volatiles serve as a language that plants use for their communication and interaction with the surrounding environment.HIPVs are mainly represented by terpenoids,fatty acid derivatives including lipoxygenase pathway products,benzenoids and phenylpropanoids,and various nitrogen and sulfur containing compounds.These chemical compounds have been demonstrated to act as indirect defenses to protect the emitting plant from herbivores and as direct defenses by repelling herbivores or deterring oviposition.That is,the signaling plant can recruit natural enemies of pests by the defensive strategy,thus minimizing further damage to it.Some factors including plant varieties,insect instars,and natural enemy's learning and experience,influence natural enemies to locate their hosts or prey.In fact,most plants are commonly attacked by more than one herbivore.Understanding on natural enemy responses to volatile blends induced by multi-species vs.single-species herbivore is essential to get more realistic situation in volatile composition in an ecological context.The chemical signals also mean that the plant has produced toxic metabolites and proteins,there are more enemies in the vicinity,and the food is scarce and falling,so HIPVs may serve as direct defenses by repelling herbivores or deterring oviposition.On the contrary,Herbivores might also exploit these chemical signals,either to locate suitable food plants or mating partners or to avoid competition,parasitoids,or predators.Generally,roots are attacked by as many,if not more herbivores than above-ground parts.Recent studies have shown that HIPVs emitted from below-ground plant parts could also attract natural enemies of root herbivores,and also to affect aboveground indirect defense and vice versa.Therefore,HIPVs have important ecological roles in intraplant or interplant communication.HIPVs as internal signals allow damaged tissues to warn their neighbors about the presence of herbivores,and trigger an increase in direct and indirect defenses in undamaged tissues.Simultaneously,the neighboring plants could "eavesdrop" the intraplant messages to elicit their own defenses.As an alternative mechanism,the chemical signal might not induce defenses immediately but rather sensitize or 'prime' the neighboring plants for a quick and intense response when damaged by herbivores.Recent published studies suggest that green leaf volatiles,terpenes,MeJA,and MeSA are good candidates for the real airborne signals.The mechanisms responsible for priming against herbivores and ecological function of priming are reviewed.In addition,the main ways that applied volatiles to control pests,their merits and demerits,and some considerable problems in the practical application are discussed.