In the real world, plants are routinely confronted with more than one threat. A plant may be attacked simultaneously by both a pathogen and an herbivore, or it might be in danger of being shaded by a fast-growing neighbor at the same time that it must defend its leaves from being eaten. Given that the resources a plant can draw on are limited, how do plants respond to multiple threats?

Often, the response to multiple threats suggests a trade-off. For example, a plant exposed to a pathogen may be less able to respond to a later herbivore attack. If tobacco plants are exposed to tobacco mosaic virus and systemic acquired resistance is activated, they become more susceptible to the hornworm caterpillar, Manduca sexta. Studies indicate that plants do not defend themselves as vigorously against subsequent herbivore attacks because of crosstalk between the signaling pathways for responding to biotrophic pathogens and those for responding to herbivores.

There can also be trade-offs between the threat of competition with neighboring plants and the threat of attack by herbivores. For example, when the phytochrome receptor in plants detects the presence of neighboring plants (Chapter 31), plants allocate more resources to growing tall, but they also produce fewer defensive chemicals in response to herbivore damage. Interactions among the signaling molecules associated with each of these processes underlie this trade-off.