Plants ‘listen’ for danger

The sound of a leaf-munching caterpillar triggers chemical defenses in plants

caterpillar chewing

A caterpillar takes a bite out of a plant leaf. On the neighboring leaf, a piece of reflective tape bounces back a laser beam used to measure vibrations from the bug’s chewing.

Roger Meissen/University of Missouri

Check out this video to learn how researchers determined that plants “hear” and respond to insect pests.

It’s hard to tell if plants grow faster with classic rock or prefer Haydn to hip hop. But what is clear: Plants can “hear” their predators. Tiny mustard plants react to the sounds of leaf-munching caterpillars by making defense chemicals. And a new study shows that this makes the foliage a turnoff to the predator.

Heidi Appel was a teen when The Secret Life of Plants hit the stores in 1973. This New York Times bestselling book devoted a chapter to music’s effect on plant growth. A self-described “nature nut,” young Appel devoured the book. Afterward, she wondered why plants would respond to music and other such unnatural stimuli. “I was fascinated, but skeptical,” she recalls.

Appel is now a plant biologist at the University of Missouri in Columbia. Seven years ago, she struck up an eye-opening conversation with colleague Rex Cocroft. He studies leafhoppers. These insects signal each other by sending vibrations through plant leaves. Cocroft had used sensitive microphones to pick up the vibrations. But recording those “drumming” sounds while the bugs were feeding proved a real nuisance, he told Appel. “When an insect starts to eat the leaf, I can’t hear a thing,” he said. “It’s just deafening.”

Then came the “Aha!” moment, Appel recalls. “We looked at each other and said, almost simultaneously, ‘Do you think plants might use insect feeding as a source of information?’” After all, when a hungry caterpillar encounters a leaf that tastes yucky, it just leaves in search of a yummier one. So it would be important for one part of a plant to recognize danger and let the rest of the plant know as soon as possible, Appel reasoned.

She and Cocroft designed an experiment to see if plants do in fact “listen” for predators. They placed a piece of reflective tape onto a plant leaf. Then they put a hungry caterpillar on a neighboring leaf. Now they trained a laser beam onto the tape. If the tape moved even slightly, the signal from the laser light reflected by the tape would vary too. And how quickly it varied with time provided a measure of the leaf’s movement.

The scientists worked with Arabidopsis, a weedy mustard plant. Over the years, it has become the “lab rat” for plant experiments. As caterpillars feasted on the plant’s leaves, Appel and Cocroft recorded the vibrations picked up by their laser. Then, they recreated these crunching vibrations for another plant. A tiny speaker shook the new plant’s leaves just a little bit. Each leaf moved up and down only about one ten-thousandth of an inch (2.54 micrometers). A leaf shook “in just the same way it does when a caterpillar is feeding, but there’s no caterpillar,” Appel explains.

In the final step, the researchers placed actual caterpillars onto plants that had listened to the feeding sounds. Other plants, called controls, had received the silent treatment.Two days later, the scientists removed the bugs and analyzed several leaves from each plant. Compared to control plants, those that had earlier heard the chewing sounds had made about 30 percent more defense chemicals.

“We were absolutely shocked,” says Appel — “and delighted, of course. It was a longshot, and we weren’t sure if plants could do this.” She and Cocroft report their findings in the August issue of Oecologia.

They wondered, at that point, if the plants only responded this way to chewing vibrations. Would wind movements or other non-eating sounds also provoke the plant to turn on its chemical defenses?

To find out, the scientists repeated the experiment. This time, they subjected the Arabidopsis plants to the silent treatment or one of three recorded sounds. Some plants received the same feeding vibrations used in the first experiment. Other plants experienced the sound of a gentle wind. “In that case, you could actually see the leaves move,” Appel says. For the third group of plants, the researchers played a mating song from Cocroft’s audio collection of leafhopper calls. Only the caterpillar crunching sounds triggered an outpouring of defense chemicals from the plants.

Frank Telewski is a plant biologist at Michigan State University in East Lansing.  There, he studies how plants respond to wind and other mechanical stresses. “In almost every science fair I’ve ever judged, some high school student is testing if plants grow better to rock ‘n’ roll or classical music,” he quips. But the new Missouri study is the first “to clearly show a plant responding to a sound that’s related to its survival — the chewing vibration of an insect pest,” says Telewski. “If a plant can perceive danger and respond biochemically to up its defenses, that’s a key advantage.” It also makes perfect sense, he says, for “plants to be responding to sound.”

In the future, Appel says her team will see if other plants also sense and make untasty chemicals in response to prey. And they’ll test different kinds of insects. They also want to identify which vibrations the plant responds to. For example, are low-frequency or high-frequency sounds more threatening? To find out, “Rex can chop out portions of the signal and deliver pieces of it,” Appel says.

“A lot of basic biology has unexpected effects down the road,” Appel says. For instance, agricultural researchers may one day apply what’s learned in these experiments to control plant pests. Weed killers were developed from a fundamental understanding of how plants respond to hormones. Maybe scary noises are the next step in helping plants protect themselves.

Power Words

Arabidopsis thaliana    A weedy mustard plant frequently used to study the growth and behavior of plants. It is so commonly used by plant scientists that it has come to be called the “lab rat” of green-plant studies.

biochemical       A compound made and used within living things.

biology  The study of living things. The scientists who study them are known as biologists.

chemical      A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.

control     A part of an experiment where nothing changes. The control is essential to scientific experiments. It shows that any new effect must be due to only the part of the test that a researcher has altered. For example, if scientists were testing different types of fertilizer in a garden, they would want one section of to remain unfertilized, as the control. Its area would show how plants in this garden grow under normal conditions. And that give scientists something against which they can compare their experimental data.

defense  (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)

frequency  The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

hormone  A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. 

laser  A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, and in surgery.

Oecologia      A research journal whose name means ecology. It publishes new research findings in areas of plant and animal ecology.

predator  (adjective: predatory) A creature that preys on other animals for most or all of its food.

prey  Animal species eaten by others.

vibrate  To rhythmically shake or to move continuously and rapidly back and forth.

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