Indigestion's basis as a plant defense.

An invasion of insects ravaging alfalfa leaves is likely to be courting malnutrition. The plant quietly responds, in a matter of hours, by producing a chemical throughout that inhibits the insects' digestion. Related chemicals are found in large quantities in seeds of many legume species, where they may be part of a complex system boosting plant survival, biologists now report.

Scientists used to think of plants as generally defenseless against foragers, says Clarence A. Ryan of Washington State University in Pullman. But recent work demonstrates that plants can marshall a complex array of chemical countermeasures.

When a leaf of tomato, potato or alfalfa is injured by a chewing insect or by other mechanical means, a chemical message travels rapidly throughout the plant and induces the synthesis and accumulation of proteins that inhibit digestion in an animal's gut. That signal is a fragment of the plant cell wall; and the characterization of one of the digestion-inhibiting chemicals has just been reported. Called alfalfa trypsin inhibitor, it is a member of a family of protein-breakdown inhibitors that had previously been detected only in legume seeds.

Members of this set of inhibitors, called the Bowman-Birk family, make up a sizable fraction, about 5 percent, of the protein in the typical legume seed. They had been considered simply as storage proteins -- materials broken down for their components as the seed germinates into a plant. "But they might be there as a protective agent," Ryan says.

In laboratory experiments, the wound-induced inhibitors have serious effects on animal nutrition, Ryan says. Chicks fed inhibitor die. Rats and mice not only become malnourished because they cannot break down the protein they ingest, but their health is further undermined by an internal feedback system. Their pancreases enlarge to produce more and more protein-breakdown enzymes, which are ineffective in the presence of the wound-induced inhibitor.

In insects, scientists find that the inhibitors delay growth of larvae, allowing diseases and other predators to increase their toll. "Small larvae within a day or so will be in trouble," Ryan says. "Larger insects may take longer."

Although legumes (including alfalfa) and solanaceous plants (including potatoes and tomatoes) have similar wound-regulated defense systems, their inhibitors are distinct. Ryan now wants to analyze the underlying genes to determine whether the systems share an ancestor. "It is possible that similar gene-regulating systems are present in many plant families that activate genes coding for a variety of defense proteins," Ryan and colleagues Willis Brown of Washington State University and Koji Takio and Koiti Titani of the University of Washington in Seattle say in the April 23 BIOCHEMISTRY.

Ryan is also searching for the DNA "switchbox" that is regulated by wounding. He envisions using genetic engineering techniques to increase the amount of inhibitor produced by a plant or to introduce other genes, such as those encoding bacterial toxins, that would be produced only when the plant is under attack. The digestion inhibitors are broken down by heat, so an increase in their amounts would not be harmful to consumers of cooked food. In fact, Ryan says the inhibitors are themselves high-sulfur, high-lysine proteins, so they would build up the nutritive value of a plant.

"They could increase the quantity of the yield and also increase the quality," Ryan says. "That excites us very much."