‘Minds’ of plants

It might be easy to laugh off the suggestion that plants can talk. The notion isn’t necessarily a joke, however. Although their patterns may be hard to interpret, scientists say plants have a secret life that takes place apart from human interaction.

In their own language, plants communicate to insects, animals, other parts of their own bodies and neighboring plants. Learning more about these patterns could enable scientists to genetically engineer plants that can better defend themselves.

Defense mechanisms trigger much of plants’ communication. Many of these mechanisms are regulated by chemicals secreted by the plants that act as natural pesticides, says Clarence A. Ryan, professor of biochemistry at Washington State University in Pullman, Wash. Mr. Ryan, who holds a doctorate in chemistry, is a member of the American Society of Plant Biologists in Rockville. He also receives funding for research through the National Science Foundation in Arlington.

“This is plant communication,” Mr. Ryan says. “People will say talking involves voice or sound waves, but it depends on how you define talking.”

For instance, when insects chew a tomato plant’s leaves, genetic signals are triggered that tell the plant to release chemicals such as methyl jasmonate. Leaves on the plant that haven’t been devoured produce these chemicals, which interfere with the insects’ digestive systems. The chemicals make the bugs unable to break down the proteins in the plant. When the insects can’t properly digest, they become sick and die.

Further, the chemicals released often attract other insects — predators to the insects feeding on the plants. Therefore, the plants issue a twofold defense system against hungry insects.

Sometimes, the methyl jasmonate that has been released also is picked up by neighboring plants, which then know to release their own defense system. In these instances, however, the plants must be close enough so the wind can clearly carry the chemicals. The signal becomes weak when diluted in the air.

Plants also may release methyl jasmonate When defending against animals. They also produce other chemicals, such as terpenes, says Thomas Sharkey, professor of botany at the University of Wisconsin at Madison. He holds a doctorate in botany and plant pathology.

For example, a pine tree waits to release terpene in emergency situations, such as when a deer might be chewing on its branches. The terpene doesn’t taste good, and it makes the deer retreat. Unfortunately, plants can’t have their defenses up all the time because it takes too much energy.

“If they don’t have to make these compounds, they can grow faster and make more seeds,” Mr. Sharkey says. “If there are no insects or animals around, they are better off not creating it.”

Occasionally, plants compete for water and nutrients in the soil, says John Yoder, professor of genetics at University of California at Davis, a land grant school that receives money from the U.S. Department of Agriculture to study agricultural interests.

When certain plants fight over turf, they release allelochemicals, which are toxins. For instance, creosote bushes in the desert release the chemical to prevent competition from other kinds of plants seeking water. The classic example of this is seen in walnut trees, which are known for keeping all other plants away from their land.

“It has been known for centuries that you can’t grow anything under walnut trees,” Mr. Yoder says. “What they’re basically saying is, ‘Get away from me. Get out of my space.’”

While many plants are warded off by the release of chemicals, parasitic plants, which make up about 3 percent of plants, often use those very toxins to identify the location of possible hosts. In fact, parasitic plants are attracted by certain types of allelochemicals. For instance, the parasitic plant striga rampantly kills corn when the corn releases these toxins, especially in Africa.

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