In nature, a balance between cost and benefit is achieved in several different ways. When a threat is common, plants produce constitutive defenses—defenses that are always present. When a threat is uncommon, however, there is an advantage to mounting a defense only when the threat is encountered. A defense that is activated only when the plant senses the threat is an inducible defense. An inducible defense can be either an increase in the level of an already present (constitutive) defense or the production of a new type of defense. The ability to sense and respond to the presence of herbivores allows plants to use resources efficiently.

When a plant is attacked by herbivores, it begins to express a new set of genes. Many of these genes increase the production of chemical defenses, while others stimulate the mechanical reinforcement of cell walls. In addition, herbivore damage triggers the synthesis of jasmonic acid, a signal that is transmitted through the phloem. Exposure to jasmonic acid induces the transcription of defensive genes even in parts of the plant untouched by herbivores. A version of the compound called methyl-jasmonic acid is highly volatile and can move through the air to reach portions of the plant that are not actively importing carbohydrates through the phloem.

Quick Check 3 What would happen to a plant in which jasmonic acid synthesis is blocked?