Stretching the limits of plasticity: Can a plant defend against both competitors and herbivores?

Plants exhibit numerous adaptive responses to competitors and herbivores mediated by hormones, including induction of the shade avoidance syndrome in response to neighbor shade, and induction of chemical defenses in response to herbivore damage. Although largely studied in isolation, interactions between plastic responses to competitors and herbivores can occur and may be mediated by external and internal resource availability, direct interactions between hormones controlling plastic responses, or other pleiotropic effects. Such interactions may have positive or negative consequences for plant fitness and could serve as an important ecological and evolutionary constraint on the expression of these responses. In this article, I illustrate the potential for plastic responses in plants to physiologically interact, with particular reference to Arabidopsis thaliana. I also suggest avenues for future study utilizing characterized plant material, tools to manipulate the expression of plastic responses, and ways to monitor physiological and ecological consequences that are available using this and other plant species.