Scientists have known for a long time that plants are able respond to sound, and new research suggests the ability may help a plant defend itself when threatened by predators.
Researchers at the University of Missouri, using audio and chemical analysis, have found plants can respond to sounds that feeding caterpillars make and respond by creating more defenses.
"We found that feeding vibrations signal changes in the plant cells' metabolism, creating more defensive chemicals that can repel attacks from caterpillars," MU plant scientist Heidi Appel says.
Appel and her collaborator, Rex Cocroft, placed caterpillars on Arabidopsis, a flowering plant related to mustard and cabbage, and used lasers to measure the vibrations of leaves being consumed by the caterpillar.
Then they played back the recordings of the vibrations caused by the munching caterpillar to one group of plants, but not to another.
When caterpillars were later placed on both groups, the plants that had been exposed to the feeding vibration recordings were found to have produced higher amounts of mustard oils, a natural caterpillar repellent, the researchers reported in the journal Oecologia.
"What is remarkable is that the plants exposed to different vibrations, including those made by a gentle wind or different insect sounds that share some acoustic features with caterpillar feeding vibrations, did not increase their chemical defenses," Cocroft said. "This indicates that the plants are able to distinguish feeding vibrations from other common sources of environmental vibration."
And those vibrations do not have to be very strong for the plant to sense them, he said.
"There were feeding vibrations that got a strong response from plants that vibrated the leaf up and down by less than one ten-thousandth of an inch," he said.
While the precise mechanism employed by plants to detect such vibrations hasn't been determined, the researchers say they suspect it is accomplished using mechanoreceptors, kinds of proteins found in both plant and animal cells that provide a response to distortion or pressure.
Although plants have evolved a number of defense strategies against pests that allows them to generate appropriate protective responses, the ability to sense sounds may be one of the most effective, the researchers said.
"Plants have many ways to detect insect attack, but feeding vibrations are likely the fastest way for distant parts of the plant to perceive the attack and begin to increase their defenses," Cocroft said.
The findings could have practical applications, the researchers said.
"Caterpillars react to [a plant's] chemical defense by crawling away, so using vibrations to enhance plant defenses could be useful to agriculture," Appel said.
