Costs and benefits of induced resistance to herbivores and pathogens in plants.

While once considered noise in the background of constitutive resistance, the phenomenon of induced resistance to herbivores and pathogens is now firmly accepted as an integral component of plant defence. As for other forms of adaptive phenotypic plasticity, fitness costs and benefits of induced resistance are presumed to exist and may be realized in numerous ways; with their relative importance depending on when, where, and how induced resistance is deployed. Carefully controlled studies in which induced resistance has been manipulated in the absence of attack have provided some of the best evidence for direct costs of resistance in plants. Similar studies in the presence of attackers have provided evidence for fitness benefits, but fitness benefits of induced resistance have been rarely measured directly, especially in long-lived plants. The growing interest in placing induced resistance in a broader context has revealed many potential ecological costs and benefits that are dependent on the environment, but their fitness effects have been little examined. Induced indirect resistance (that is, resistance mediated by interactions with the third trophic level) has been an increasingly popular topic that comes with a unique set of potential costs and benefits. Further progress in our understanding of induced resistance will benefit from an increased synthesis of the literature from both agronomic and ecological perspectives, the former containing more pathogen-oriented studies in agricultural crops and the latter containing more examples of induced resistance to herbivores in wild plants. Such a melding of information will also better inform applications of induced resistance in agriculture.