The antennal lobe of a moth brain, where the first olfactory processing occurs in response to antennal neuron stimulation by odor molecules
IT SOUNDS MORE LIKE SCIENCE FICTION THAN SCIENCE: A caterpillar chews on a cotton leaf. The cotton plant, in response to the caterpillar’s attack, produces and releases an odor—a blend of chemical compounds—that attracts a species of wasp that’s a natural enemy of the caterpillar. The wasp detects the odor and follows it to the cotton plant, where it stings the caterpillar, injecting an egg into it. Later, what emerges from the caterpillar’s cocoon is not a moth but a new wasp. The plant has effectively intercepted the caterpillar’s attack by calling on the wasp.
This scenario, depicting a plant “calling in its friends,” is not sci-fi; rather, it’s an example of chemical ecology in action. In the early 1980s, Penn State entomologist Jack Schultz made an important discovery: Plants appeared to respond to attack by releasing volatile chemicals. That discovery provided the basis for a broad range of chemical ecology studies over the past two decades. “All plants produce a bewildering array of chemicals,” Schultz says, “and in much the same way that the body’s immune system responds to bacteria or other disease, plants recognize dangers and respond by changing their chemistry. Here at Penn State we have a handful of the top entomologists in the world who study this branch of chemical ecology, and together we’re trying to understand not only how plants perceive their environment and respond to it, but how we can develop plants that can tell us what’s happening to them so we can apply that information to agriculture.”
Schultz and research associate Heidi Appel study chemical interactions between plants and insects—in particular, how the chemistry of plants protects them against insect enemies. Schultz and Appel are curious about how plants produce these responses and how they “know” what’s attacking them.
“We’ve learned that plants can recognize different kinds of insects and produce appropriate defense responses,” Schultz says. “So if species A has attacked the plant, the odors released are different than if species B attacked the plant. We want to figure out how the plants tell things apart and how they coordinate their responses to produce the right defense to a particular enemy. And as ecologists we’re interested in the outcome. Does it matter to the plant that it did all this?”
For the past several years, Schultz and his colleagues have been developing “sentinel plants”—plants that are genetically modified to “report” what they are experiencing. “The very fact that a plant produces an odor in response to a specific enemy means that it ‘knows’ what attacked it,” he explains. “That means the plant must have genes encoding the receptors necessary to tell what did the damage and for maintaining that information all the way through the response process of producing a specific odor. To get a sentinel plant, we can redirect the plant’s genetic pathway to produce a response that we can see—for example, a color change.”