The evolutionary context for herbivore-induced plant volatiles: beyond the 'cry for help'

Attacks by herbivores elicit changes in the bouquet of volatiles released by plants. These herbivore-induced plant volatiles (HIPVs) have been interpreted as being indirect defenses. However, given that no studies have yet investigated whether HIPVs benefit the fitness of a plant, their defensive function remains to be established. Moreover, herbivores, pathogens, pollinators and competitors also respond to HIPVs and, in addition, neighbouring plants in native populations also emit volatiles that provide a background odour. These considerations enrich the evolutionary context of HIPVs and complicate predictions about their adaptive value. Molecular advances in our understanding of HIPV signaling and biosynthesis is enabling the creation of HIPV-'mute' and possibly HIPV-'deaf' plants. As we discuss here, such plants could be used for unbiased examination of the fitness value of HIPV emissions under natural conditions.