Plants may not have feelings, says Heidi Appel, senior research scientist in the Division of Plant Sciences at the University of Missouri, but they can detect light, odors and vibrations. "They can distinguish the vibrations of a caterpillar feeding from wind and other noises in their environment and get defenses ready to go," she says.
Appel says the implications of her study, "Plants respond to leaf vibrations caused by insect herbivore chewing," could lead to more targeted and efficient uses of pesticides in the future.
David Leite: You’ve been doing some very interesting research recently with plants. Your basic tenet, if I’m paraphrasing correctly, is that plants know when they’re being attacked and they react. Is that correct?
Heidi Appel: That’s correct. There are a bunch of plant scientists all over the world who have been contributing to this kind of study. What our work did was show for the first time that plants have a very fine-tuned ability to tell insects apart.
DL: How do they do that?
HA: When plants get wounded, it’s very much like when we get wounded. There are wound signals generated at the site of damage. Those signals are transported to other parts of the plant. Plants respond locally by trying to seal up the damage site right away, because they can get attacked by pathogens and get infections, just like we do when we get cut.
DL: Do they do anything environmentally? Do they send out any signals when they’re attacked?
HA: They do. There are changes that happen inside the leaves that you can measure, but there are also things that happen outside the leaves. The plants are releasing volatiles into the environment; that’s the smell of fresh-mown hay or even your yard when you mow it.
Those volatiles are detected by two other kinds of things that help out the plants. One is that the enemies of the insects eating the plants use those volatiles as a way to know that there is food in that plant for them. Plants that have been undamaged but are nearby, close enough to receive that volatile signal, will get their defenses primed and ready to go as well.
DL: It’s a whole cycle of protection?
HA: Yes, there are things going on underground. That’s not the focus of my research, but others have shown that there’s a whole network of communications below ground between plant roots that are mediated mostly by fungi that form associations with roots of plants.
DL: Is it a stretch to say that plants have feelings?
HA: I think so. Plants have senses certainly, but feelings involve emotions, which is higher-cognitive-level processing. I don’t think they do it.
DL: But there’s definitely an action and reaction system that’s going on that’s highly evolved.
HA: Yes, as highly evolved as ours. It’s just that the systems they use to sense things usually aren’t in specialized organs like ours.
They can see things; everyone knows that from watching their seedling grow toward the light on the kitchen windowsill. That’s because they can detect light. They can detect odors; I just gave you an example of that with odors being released by damaged plants and calling in helpful insects. They can also detect sound, vibrations. Previous work I’ve published with Rex Cocroft [a professor of biological sciences at the University of Missouri] showed that they can distinguish the vibrations of a caterpillar feeding from wind and other noises in their environment and get defenses ready to go.
DL: In that research, from what I read, if there was a recording of that particular sound or vibration that was played to plants versus plants that did not have that recording, there was a different reaction?
HA: Yes, there was because when those plants were subsequently attacked by insects, the ones that had received the feeding vibrations made more defenses.
DL: Tell me a little bit more about these defenses and how they’re used by the plant world, and also how they’re used by us as people.
HA: A lot of people will be very familiar with a lot of these plant chemicals, because they’re the things that we use in herbs and spices and in natural medicines. For example, in the plant I work with, the defense is largely in mustard oils. That’s the thing that gives mustard its kick.
DL: Those are some of the defenses that are kicked out into the environment?
HA: The plants make more of them in their leaves. Then, when the caterpillars come to eat them, the levels get high enough that they get turned away from eating the plant. It’s kind of like having too much mustard on your hotdog.
DL: Are there any practical agricultural uses for your research?
HA: We hope there are down the line. When plant breeders go to develop pest-resistant plants, it’s important for them to know how plants detect that they are being attacked and what they can do about it. This information helps them and their breeding programs. Also, the ability of plants to detect vibrations of insect feeding and those vibrations transmitted throughout the plant means that we might be able to develop sensors that can be used in the field to target pesticide use in a more efficient and thus less costly and detrimental way.
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