Costs of induced responses in plants

Induced responses to herbivores in plants are thought to be a form of adaptive phenotypic plasticity, whereby plants save metabolic costs by expressing defenses only when they are necessary. Although costs of being inducible have never been examined, there have been many studies of costs of induced responses, with approaches ranging from the manipulation of induction with herbivores or wounding, and more recently, with chemical elicitors. While some early studies reported no significant costs of induced responses, evidence for significant costs of induced responses in both wild and agricultural species has recently increased in the literature. Recent studies have made it clear that benefits of induced responses in the field in the presence of herbivores can counterbalance costs of induced responses seen in the absence of herbivores. Moreover, as has been shown for constitutive resistance, costs of induced responses may vary with environmental conditions, including the presence of competitors and altered resource availability. Ecological costs of induced responses may include increased susceptibility to untargeted herbivores, either through the altered attraction of specific herbivores or due to defense pathway cross-talk, but the actual fitness consequences of such ecological costs have been little studied. Mutant and transgenic plants altered in induced responses are increasingly being identified or produced, as are specific elicitors of direct and indirect defenses. Their use, coupled with the increasing ability to analyze global gene expression in plants, will advance studies on the specificity, magnitude, and mechanisms of costs of induced responses.