Plants can sense and react to temperature changes, harsh winds and even human touch. But can they hear?
They have no specialized structure to perceive sound as we do, but a new study has found that plants can discern the sound of predators through tiny vibrations of their leaves — and beef up their defenses in response.
It is similar to how our own immune systems work — an initial experience with insects or bacteria can help plants defend themselves better in future attacks by the same predator. So although a mustard plant might not respond the first time it encounters a hungry caterpillar, the next time it will boost the concentration of defense chemicals in its system that turn its once-delicious leaves into an unsavory, toxic meal.
Now, biologists from the University of Missouri have found that this readying process, called "priming," can be triggered by sound alone. For one group of plants, the scientists carefully mimicked what a plant would "hear" in a real attack by vibrating a single leaf with the sound of a caterpillar chewing. The other group was left in silence.
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When later faced with a real caterpillar, the plants that heard chewing noises produced a greater amount of insecticide-like chemicals than the silence group. They also seemed able to pick out those vibrations signaling danger; the playing of wind noises or insects' mating calls did not trigger the same chemical boost.
Although the mechanism of how plants can discern sounds is not known, a deeper investigation could lead to advances in agriculture and natural crop resistance — as opposed to spraying costly and harmful pesticides.
"We can imagine applications of this where plants could be treated with sound or genetically engineered to respond to certain sounds that would be useful for agriculture," said study author and biologist Heidi Appel.
The study was published online Tuesday in the journal Oecologia.
Despite not having brains or nervous systems in the traditional sense, plants are surprisingly sophisticated. They can communicate with one another and signal impending danger to their neighbors by releasing chemicals into the air. Plants constantly react to their environment — not only light and temperature changes, but also physical stimuli. Two famous examples are the Venus' flytrap, which snaps shut when an unsuspecting bug contacts one of its trigger hairs, and the touch-me-not plant (Mimosa pudica), which shrinks and closes its leaves upon even a slight touch.
"Plants certainly have the capacity to feel mechanical loads," said plant biologist Frank Telewski, who was not involved in the research. "They can respond to gravity, wind, ice or an abundance of fruit."
But trying to prove that plants can sense sound has been difficult.
"There is a long history of people interested in whether plants could hear sound, and that usually involved sounds that are very salient to us — music or tones of pure sound — just to see if plants would react," said study author and biologist Reginald Cocroft.
Even though some people swear that a soothing voice or classical music works wonders for their greenery, the scientific evidence is spotty. Experts believe that music in particular is too complex and varied to be able to use in a controlled study.
When pure tones are played, some experiments have seen changes in plant growth, germination or gene expression. For instance, one recent study showed that young roots of corn will grow toward an auditory source playing continuous tones and even responded better to certain frequencies.
But what would be the evolutionary advantage of responding to such stimuli?
One argument against plants perceiving sound is that being able to pick up on the music of Beethoven or a solid note has no bearing on a plant's well-being — but the leaf-chomping of a nearby insect certainly does.
"None of the sounds used before are things that are ecologically relevant sounds in the plant environment," Appel said.
Although it has not been proved, the suspicion is that plants can perceive sound through proteins that respond to pressure found within their cell membranes. Sound waves cause their leaves to vibrate ever so slightly, causing the plant to respond accordingly.
Because chewing insects produce high-amplitude vibrations that travel rapidly to other parts of a plant, the researchers could record the fine movement of a leaf during a caterpillar feeding episode using a laser tracking system. They then played back the recording to a group of 22 Arabidopsis plants, related to mustard and cabbage, that had not been exposed to caterpillars before.
Appel then placed real caterpillars on the leaves of the group to feed. After waiting a day or two for the plants to mount their defenses, she measured the chemistry of their leaves for insecticide-like chemicals called glucosinolates — the same substance that gives mustard its kick. If eaten in large doses, however, it becomes toxic.
Not only was the concentration of glucosinolates higher than a control group's, but there was also a correlation between concentration and the strength of the vibrations. If the leaf moved a greater amount in playback, the scientist saw more of the chemical being produced by the plant.
To see whether a plant would react to any type of sound, the researchers tried playing a leafhopper mating call or blowing wind. In response to these, the plant did not appear to put up extra defenses.
Telewski, a tree expert who investigates perception of mechanical stimuli in plants, said this work showcases a possible evolutionary advantage of perceiving sound: "I'm very impressed with the study — it's very nice."
He wonders whether other plants not being attacked could pick up on the vibration as an auditory SOS-type signal, because plants have been known to use airborne chemical signals in the same way. If the alarm can spread efficiently through a field, say, sound could potentially be harnessed in agriculture to ward off predators.
"It might be practical to see how loud you would have to play speakers in a field to get plants geared up to fight against an insect," he said. "This might be one way to fight off an insect attack without spending a lot of money on pesticides."
Biochemist Janet Braam, who was also not involved in the study, finds the results intriguing.
"Testing whether similar results are obtained for other plant-insect interactions will be important next steps to understand how broadly applicable this phenomenon may be," she said.