Can plants hear? In a study, vibrations prompt some to boost their defenses
Specimens responded to audio of caterpillar chewing
식물은 송충이가 자기 잎을 먹고 있는 소리를 들으면 잎의 맛을 쓰게 하는 화학물질을 생성해
더 많은 잎이 먹히지 않게 대응한다. 동물과 달리 뇌와 신경계가 없지만 식물은 잎의 세포막에 가해지는 압박감에 단백질이 반응하는 방법으로 (송충이가 잎을 쏠아 먹는)소리를 감지할 수 있다.
끈끈이 주걱(일명 파리잡이 풀)이 주거안에 곤충이 들어오면 문을 닫아 먹어 치우고, 미모사는 잎에 접촉이 오면 오무라 드는 예 처럼 식물 중에는 물리적 동작이 가해지면 그에 반응하는 경우는 이미 알려져 있다. 또 식물은 빛과 온도에 항상 반응하고 있다. 하지만 식물이 소리를 감지한다는 것은 이번 실험에서 처음 밝혀졌다.
진화론적인 설명은 식물이 소리 (특히 자신의 잎을 먹어 해치는 애벌레의 소리)에 이처럼 반응하는 것이 생존을 위한 것, 적자생존의 원리에 따른 것이라는 풀이가 나온다.
식물이 소리를 감지하는 것이 인류 사회에는 어떤 의미가 있을까?
그 소리를 이용해 벼나 밀 등 곡물의 해충을 제거하기 위해 비용들고 독성있는 살충제를 사용하지 않고도 곡물 자체가 해충에 대한 자기 방어용 화학물질을 생성케 할 수있다면 건강에 좋은 식량의 증산을 도모할 수 있다는 것.
(동물 식물 포함해 모든 생물의 조상은 하나? 거기서 서로 다른 진화의 길을 밟아와...)
Through audio and chemical analysis, a University of Missouri study reveals that plants respond to sound with more defense if they detect a threat. (Roger Meissen, Nathan Hurst)
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 like 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 while a mustard plant might not respond the first time it encounters a hungry caterpillar, the next time it will up 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, they 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.
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; playing wind noises or insect mating calls did not trigger the same chemical boost.
A cabbage butterfly caterpillar eats an Arabidopsis plant. On an adjacent leaf, a piece of reflective tape helps record vibrations. (Roger Meissen)
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 each other 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.
University of Missouri-Columbia researchers Heidi Appel and Rex Cocroft found that plants respond to the sounds that caterpillars make when eating nibbling on their leaves. (Roger Meissen)
“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 Corcroft.
Even though some 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 were able to 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, but there was also a correlation between concentration and how strong the vibrations were. If the leaf moved a greater amount during playback, they saw more of the chemical being produced by the plant.
To see if a plant would react to any type of sound, the researchers tried playing a leafhopper mating call or blowing wind. In response to these, it did not appear to put up extra defenses.
Telewski, a tree expert who investigates perception of mechanical stimuli in plants, believes this work showcases a possible evolutionary advantage of perceiving sound: “I’m very impressed with the study — it’s very nice.”
He wonders if other plants not being attacked could pick up on the vibration as an auditory SOS-type signal, since 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,” said Braam said.
By Meeri Kim July 6, 2014 (The Washington Post)
Kim is a freelance science journalist based in Philadelphia.
