"Plants make good sentinels because they can't run away," says chemical ecologist Jack Schultz, who is developing sentinel plant technology under a Department of Defense grant. "The trick is to design plants that respond in particular ways to particular stimuli, and to amplify these responses so they can be detected readily."

U.S. soldiers walk down a trail in a war zone. One of them pulls out a hand-held electronic device and points it at a native plant. The readings on the device indicate the plant was exposed to nerve gas sometime in the last 48 hours, allowing the soldiers to don protective gear before they suffer a lethal dose.

Although such a device does not yet exist, it’s not as far-fetched as it may sound. As concerns grow over the threat of bioterrorism and weapons of mass destruction, University researchers are working on an early warning system—the figurative canary in the mineshaft—that could be as unobtrusive and ubiquitous as plants in a landscape.

Under a three-year, $3.5-million grant from the Defense Advanced Research Projects Agency, Penn State scientists are laying the groundwork for genetically engineering plants that can detect and signal the presence of many harmful chemical or biological agents.

“Plants make good sentinels because they can’t run away,” says chemical ecologist Jack Schultz, professor of entomology. “Because they are rooted in their environment, plants must respond dynamically to environmental changes. Many of these responses can be observed or measured, such as changes in color, shape, or growth habit, or the emitting of volatiles into the air.

“In simple terms, if you don’t fertilize your houseplant, it may not grow well and may change color,” Schultz explains. “The plant is reporting the conditions it’s experiencing in its soil. In essence, it’s telling you, ‘Feed me.’ The trick is to design plants that respond in particular ways to particular stimuli, and to amplify these responses so they can be detected readily.”

The key to manipulating plant response to environmental stimuli is to understand the role of certain genes, says Ramesh Raina, former assistant professor of biology in the Eberly College of Science, now an associate professor of biology at Syracuse University.

“Plants and animals detect and respond to a range of things—including microbes, insects, chemicals, and hormones—via cellular proteins,” Raina says. “These proteins, called receptor-like kinases, or RLKs, have a sensing domain outside the cell membrane that binds molecules in the environment. This binding sends a signal inside the cell to the response domain, known as the kinase, which then turns on genes that trigger a response.”

To study this system, Schultz and Raina are using Arabidopsis, a small flowering plant from the mustard family that grows around the world and is widely used as a model organism in plant biology. “Arabidopsis is the most studied plant on Earth and the only one for which the entire genomic sequence is publicly available,” says Raina.

The problem is that while there are more than 600 known RLKs in Arabidopsis, scientists understand the functions of fewer than 10. “For most of these receptors, we don’t know what they sense,” says Raina, “and once they’ve sensed, we don’t know what response they trigger.”

To solve these mysteries, scientists in Raina’s laboratory are using recombinant DNA technology to fuse the receptor, or sensing part, of these proteins to the kinase, or response part, of another protein that can induce visible responses. As a result, the researchers hope to develop plants that respond to all environmental stimuli in a predetermined and visual way.

“When our work is complete, we’ll have a ‘kit’ of several hundred plant lines, each that will sense different things but will respond the same way,” Raina says. “In this case, if they sense an environmental stimulus, they will fluoresce, or glow, green.”

Schultz then will take these plant lines and treat them with various agents. “By exposing these plants to different stimuli and looking for the response, we can determine what sensor proteins are responsible for sensing what agents,” he says. “The ultimate goal is to develop ‘plug-and-play’ kits that can be inserted into a variety of plants to act as sentinels in various situations.”

Such sentinel plants have several possible uses. They might be able to sense and warn of the presence of chemical warfare agents or animal pathogens, such as anthrax. Other plants might be designed to detect and signal the presence of explosives in soil, which would aid in locating and removing land mines. “Land mines are leaky, and the soil around them contains products of TNT decomposition,” says Schultz. “Engineering plants that can detect mines is of great interest, both to the military and to humanitarian groups.”

The technology also holds promise for agriculture. If researchers can learn more about how plants sense and respond to insects, diseases, poor soils, drought, and other environmental challenges, they may be able to develop plants that can “tell” them where and when these problems exist.

“This work has enormous implications for precision agriculture,” Schultz explains. “Imagine a tractor with a sensor on the front that picks up plants’ chemical signals as it crosses the field. If the sensor detects plant response to Colorado potato beetle in one part of the field, it directs pesticide spray only to that area, while leaving the rest of the field untouched.”

Beyond the potential practical applications, the scientists are motivated by simple curiosity about how plants sense environmental stimuli. “The plant biology world is intensely interested in the results of basic research such as this,” says Raina.

—Chuck Gill