Sentinel plants have potential applications for agriculture. If growers could detect a plant pest in the very early stages, before any visible signs are evident, they could greatly reduce the expense of applying pesticides. "Farmers could use information from sentinel plants to apply remedies in time- and space-specific ways," he says. "Rather than spraying an entire field for Colorado potato beetle, for example, a farmer could target only the affected areas of the potato field. The economic and environmental benefits of precision agriculture--treating only the part of the field with the pest--have been demonstrated over and over again."

Poplar trees, Schultz says, have the potential for being effective sentinel plants because they are very responsive to their environment. Based on what they've learned about poplar genes, Schultz and colleagues John Carlson and Chris Frost are exploring the idea of designing a poplar to report environmental stresses. "For example, we could have a poplar that reports ozone concentration in the atmosphere," he says. "Imagine the Los Angeles freeways lined with poplars that change color according to ozone levels."

The ability of plants to respond chemically to specific events could even be put to use in developing pest-resistant crops more cheaply and quickly. "Traditionally, people have bred plants for resistance to diseases and insects by putting them out there, saving the ones that didn't get killed or eaten, and crossing them with others until they came up with, for instance, a corn plant that produces lots of corn and doesn't get eaten," Schultz says. "If we knew that a set of genes produced certain defenses against x, y, and z, we could alter the plant's genetic composition and introduce those genes to make it resistant in a matter of months instead of years or decades."

Schultz and Appel also are studying how defensive chemicals in plants affect the soil after the plants die. When a plant dies, it eventually becomes litter, and the litter eventually becomes soil. New plants growing in that soil extract nutrients from it. "What happens to a plant's defensive chemicals after it dies, and how do those chemicals affect new plants growing in the soil?" Appel asks. " There's a certain set of defensive chemicals in plants that regulates bacterial decomposition. We're trying to find out how the chemistry of a plant's leaves determines the chemistry of the soil when the leaves turn into litter. This cycle is an important part of how the ecosystem functions."

In another laboratory, natural products chemist Jim Tumlinson works with pepper and tobacco plants, exposing them to different strains of bacteria. He has found that plants can tell the difference between bacteria that differ only by a couple of genes. The plants make very specific discriminations and respond by producing slightly different blends of odors. This could have practical applications in agriculture. For example, a plant could be infected by bacterium A, which is not much of a pest problem, as opposed to bacterium B, which is closely related but a terrible pest. Knowing which bacterium is affecting plants could be useful in determining whether any treatment is necessary.

"Plants have a fascinating array of chemical defenses," Tumlinson says. "They can release toxins, chemicals that deter insect feeding, chemicals that interfere with insects' digestion and metabolism, chemicals that attract the natural enemies of their attackers. What we'd really like to know now is, how do they know all this? We want to put all the pieces together so we can understand the system better and develop more effective, environmentally sound best-management practices."

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