In plants such as Populus spp., increases in defensive proteins and phenolics occur in response to wounding or herbivore attack. This response is triggered by jasmonic acid, a plant-produced signal of attack. Jasmonic acid and some of its close derivatives, such as methylester jasmo-nate or «s-jasmone, also attract aphid predators and parasitoids; jasmonic compounds also elicit the production of salicylate, a secondary phenolic (Figure 3). Salicylate is converted to methyl salicylate (oil of winter-green), a volatile compound used by plants as an airborne signal of wounding. Many insect herbivores can detect and are repelled by methyl salicylate. Plants near the wounded plant emitting methyl salicylate can detect the signal and are triggered to bolster their own chemical defenses.
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