COLUMBIA — MU biologist Heidi Appel says she and Rex Cocroft have been surprised by the international media interest in their recent research on plant response to sounds.
"It has a broader appeal than I imagined," said Appel, a senior research scientist in the Division of Plant Sciences. "It has reminded me of how people have a natural fascination with plants."
In July, the journal Oecologia published Appel and Cocroft's findings that plants not only can respond to sound but also can differentiate among sounds and mount chemical defenses to those that are harmful, such as a caterpillar chewing.
The Missourian sat down with Appel and Cocroft, a professor in the Division of Biological Sciences, to find out more about their research. Here are excerpts from the interview:
Q. What excites you guys about your research on plant vibrations?
Appel: For me, it's that plants have this additional sense we didn't know about. When I had heard years ago that plants can react to tones and music, I had the same reaction that most people did — "Well, that's curious" — but then the biologist in me wondered "Why?" We know why now.
Cocroft: I agree with that. Something I find exciting as well is that this opens up a new field. If you just look outside, you'll see plants ranging from very large to very small and living in all kinds of environments and interacting with all sorts of organisms.
We've shown that this one plant can respond to the vibrations produced by one herbivore. We suspect that the ability to respond to meaningful vibrations is going to turn out to be widespread in plants. So there's a whole world out there, potentially, of plant acoustic interactions with their environment that we are now poised to explore.
Q. Can you describe the process you used to discover that plants respond to the vibrations?
Cocroft: Maybe we should back up a bit and talk about how we started collaborating. ... Heidi and I work in very different fields, and within the university, everybody tends to be to some extent within their own world. However, in recent years there's an emphasis on trying to get people from different disciplines together.
Sometimes that works. The way it worked in this case was that Heidi and I met after a presentation, and we, the way scientists introduce each other, said, "Oh, what do you work on?"
Heidi was studying how plants recognize and defend themselves against herbivores. I was studying insects that live on plants and feed on plants, but studying their social behavior and communication and the way they use plant-borne vibrations to communicate and to listen for predators or prey.
It was one of those conversations that ended up being quite productive, because as we were talking we both began to wonder, if insects can use those vibrations to find out what's going on, can the plants use that information, too?
It seemed like a far-fetched idea, but we had the ability to test it — because I could measure and characterize the vibrations and reproduce them experimentally, and Heidi could …
Appel: Measure the reaction in the plants.
Cocroft: Why don't you pick it up from there?
Appel: The first thing we had to do was to record a bunch of different caterpillars creating vibrations on plants as they fed. This is something that Rex does in his lab, and we created a library of feeding vibrations. Another thing that Rex can do is to play back that vibration to a leaf in the absence of caterpillars.
Then the actual experiments were to play back the sound and to also play back silence, and then put the caterpillars on the plants and see how much chemical defense the plants would make. When they received the chewing vibrations, they would make more.
But we still didn't know whether that was going to be important in the plant's life, because, of course, the environment is filled with all kinds of vibrations that the plant would be experiencing.
So in the next experiment, we added the vibrations caused by a gentle wind and also from the vibrational song of a leafhopper.
Cocroft: A little-appreciated fact about insects is that the insects we are hearing at this time of year in Missouri, like crickets, are just a tiny fraction of the singing insects that are out there. Most of the insects that sing actually send their signals through plants, so we don't hear them. But you can hear them if you use a contact microphone to pick up those vibrations.
So plants have a whole world of often very strange signals traveling through them, and that's part of what I study in my lab.
We decided to take the song of one of these insects that sends its vibrations through the plant and see if the plant responds to that. Honestly, in the back of my mind was the idea that maybe the plant would respond.
The closest thing to human music that a plant would experience would be some of these insects' songs that change in frequency and have different rhythms and tones put together. But the plant, in this case, didn't respond.
So Heidi's point is that we showed not just that they could respond to caterpillar feeding vibrations but they could tell them apart from other vibrations.
One interesting thing about that selectivity is that the insect song had all the same frequencies as the chewing, so it couldn't just be that the plant was listening for one particular frequency — it has to have a more complicated way of telling apart different vibrations.
That intriguing observation will form the basis for some of the experiments we are doing next to figure out: What is it that the plant is listening for? How complex is its ability to perceive differences between vibrations? And does it use multiple features?
To summarize, two things that this experiment contributes: one is that the plants are extremely sensitive to vibration because the vibrations caused by the caterpillar moving the leaf are so small you can't see them. And the plants are also selective, that makes their abilities, in some ways, comparable to some animal sensory systems.
Q. Have you already started on the next portion of research?
Appel: Yes, in fact we are in the middle of an experiment right now in which we're asking whether plants whose mechanoreceptors are defective can still react with more defenses to feeding vibrations. (Mechanoreceptors are proteins in the membrane of cells that respond to pressure or movement.)
Cocroft: We're trying to figure out what the plant is using to detect these vibrations. Most plants have the ability to detect stresses and strains in the individual cells using proteins. …
Appel: Proteins are embedded in the membranes, and they respond to distortion.
Cocroft: The first step is that somehow they have to process that information.
Appel: Right, and take note of some and ignore others.
Cocroft: Another step is to see if this is more of a general phenomenon, as we suspect it is. Does it occur in other plants? And is this particular kind of plant responding to other vibrations like the ones produced when a caterpillar walks around a plant?
Q. Whenever you were playing the recordings, versus putting the actual caterpillar on the plant, how did they know, when the caterpillar came, that the sound means they are in danger, because before it was just a recording and they didn't get hurt.
Appel: It depends on whether you think their ability to detect and respond is something that is innate or something that has to be learned. Whether and how and when plants learn is a matter of controversy in the plant world right now, but we know that all organisms have innate responses to the environment.
One thing we didn't mention yet is that plants have a lot of ways to know they are being damaged. They have a wound response, for example, and there are things in the insect's saliva that triggers certain plant reactions. But all of those ways are much slower than vibration transmission.
Cocroft: The plant can respond locally to the damage, but another thing that plants do is, once any part of the plant is attacked, to signal other parts of the plant that it has been attacked and they should begin to defend themselves. So when we say it's fast …
Appel: It means faster to other parts of the plant.
Cocroft: The problem plants have in recognizing when one part of them is being eaten is very foreign to us. Plants don't have a central nervous system, so there has to be a way for one part of the plant to begin to defend itself when another part of the plant is attacked, and those vibrations do travel very quickly through the plant.
They are changed, and they do get quieter and quieter as well as you get farther away. But there is still the possibility for them to be an alerting mechanism for parts of the plant at a distance from the part that has been attacked. The role of the vibrations as an alerting mechanism may be that they're faster and more reliable than other signals the plant can use.
Q. Do you think that further research can tell us how the plants can tell the difference between a caterpillar and the wind blowing?
Cocroft: That is one of the things that we want to know.
Appel: One of the cool things that Rex can do is take a vibrational signal that is pretty complex, like our speech, and cut it into segments and pull out pieces and just play back a signal with parts missing, or add stuff on top of it.
Cocroft: A great thing about sound is that, with computers and software, we can manipulate it in many ways that allow us to do experiments that, we hope, will tell us, for example, how plants can tell the difference between wind vibrations and caterpillar chewing vibrations.
Supervising editor is Elizabeth Brixey.