Plants are not exposed to herbivores without defenses. When an insect feeds on a leaf, thereby wounding it and releasing oral secretions, a signaling cascade is elicited in the plant, usually starting with a rapid increase in the amount of the plant hormone jasmonic acid and its active isoleucine conjugate. Jasmonic acid regulates various reactions in plants, including defenses against herbivores and responses to environmental stress.
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Source: Phys.org