BACKGROUND: Plants seem like the most peaceful of organisms, but they nonetheless have their own form of defense mechanisms: they emit toxins or volatile chemicals in response to plant-eating insects. For the first time, researchers at the US Department of Agriculture's Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, Florida, have identified how plants known when they are under attack. Ultimately, their work could lead to the development and genetic manipulation of plants with improved protection against pests.
THE BEST DEFENSE: Proteins already present in the plants are eaten by the attacking insects. Digesting the proteins, the insects convert this food into a new type of chemical, which is in turn secreted back onto plants in later feedings. The plants recognize these secretions as a type of 'SOS' signal, and launch their telltale defensive chemistry. Although researchers have long known that some plants can distinguish between different insect attackers, they had not been able to fully describe all the potential interactions. There are, after all, at least four million kinds of insects and 230,000 flowering plant species, and the plant defense signals can occur at trace levels that are too small to easily detect or isolate. Prior research showed that bacteria and fungi, for example, can trigger a variety of chemical warning signals in plants, which respond by increasing hormones to regulate defensive responses. But until now, researchers didn't know which chemical signals act as alarms, or how. It turns out that the attacking insect's digestive enzymes may interact with the plant, generating a defensive response.
THE KEY INGREDIENT: The USDA scientists spent three years analyzing the biochemical response of the cowpea, a type of legume favored by an insect known as the fall armyworm. They conducted over 10,000 leaf bioassays, collecting just a few microliters at a time until they amassed a full liter of caterpillar secretions. They identified a specific class of small peptide 'elicitors', or plant defense signals that help plants react to insect attack. One in particular, an 11-amino acid peptide called inceptin, plays a pivotal warning role in cowpea plants under attack by armyworms. The USDA researchers also identified two related but less abundant peptide fragments that provoke similar defense responses in cowpea, and a third with no apparent effect. Furthermore, they showed that inceptin and related peptides spark a cascade of phytohormone increases in cowpea to trigger its defenses, and identified critical features of the protein's structure that enable it to function as a plant defense signal.