Yes, plants can hear: A peek into the life of Dr. Heidi Appel

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Heidi Appel in front of the “Living Green Wall” at MU’s Bond Life Sciences Center. Her research hones in on the intersection between insects and plants. Photo by Marlee Ellison.

By Marlee Ellison

Nestled in a dim office in the Bond Life Sciences Center is a small tank of carnivorous plants–plants that consume insects. Heidi Appel tends to pitcher plants, “sticky traps” and Venus flytraps, their colors vibrant against damp soil. The fluorescent-green Venus flytraps lie in wait for an insect meal, stems extended and jaws slack.

“That’s my little shop of horrors,” says Appel, placing the lid back on the terrarium.

Appel, a senior research scientist at the University of Missouri’s Division of Plant Sciences, explains that some plants evolved to supplement their nutrition with insects.

Plants may not move around like members of the animal kingdom, but they have developed complex mechanisms to protect themselves from predators. Understanding how plants perceive, respond and adapt to their environment lies at the crux of her research.

In the summer of 2014, Appel and her team made tsunami waves around the internet with her “Arabivibe” project, which essentially found that plants have selective hearing.  The study went viral and reached thousands of eager readers. Why? Because it dispelled the widely held notion that plants can’t make sense of the world around them.

“We shouldn’t underestimate plants,” says Martin Heil, a colleague of Appel’s and an ecologist based in Mexico. He says that they’re often perceived as boring because they don’t share immediate similarities with animals. However, a lot of plant activity happens on a cellular level, so human beings can’t see with the naked eye. This is what Appel and many other plant scientists explore.

Caterpillars that have turned into butterflies live in these specialized containers in the lab. Video by Marlee Ellison.

Earthy beginnings

Appel hails from Waterford, Michigan, which she calls the “blue-collar ‘burbs” of Detroit. She spent many days camping with her parents and exploring the outdoors. “I can’t remember a time I wasn’t rummaging around outdoors and collecting things, looking at things,” she says. “I would collect buckets full of seeds and just run my hands through them. I would trace patterns on bark on paper. I loved the aesthetic aspects of nature.”

Appel was never particularly attracted to larger animals but was fascinated by the interaction between plants and insects. She became a vegetarian in college and naturally didn’t want to dissect flesh-and-blood animals in biology classes. “I realized that I didn’t feel guilty chopping up insects,” she says, laughing. Besides one understandable nightmare after her first giant grasshopper dissection, nothing peeved her about bugs.

Appel observed the interactions between plants and insects and found chemistry in the mix. “I kind of liked chemistry, and so the field of chemical ecology really appealed to me,” she says. Plant responses to insects turned out to be the perfect intersection.

She graduated from Oakland University with a Bachelor of General Studies* and later earned a Master of Science and a doctorate in biology from the University of Michigan. In 2007, she relocated to MU after her husband was recruited to the Bond Life Sciences Center. The Arabivibe project is based out of MU and stemmed from earlier lab research on Arabidopsis thaliana, or rockcress plants.

According to Appel, this particular species is an invaluable tool for plant science researchers. “It’s a plant that grows very quickly and easily indoors,” she says. Its genome is also sequenced, so the lab can order seeds with specific mutations.

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Arabidopsis thaliana, or rockcress, growing in the basement of MU’s Bond Life Sciences Center. Arabidopsis is useful to researchers because it’s easy to grow in the lab, and its genome has been sequenced, allowing scientists to look at the effects of specific genetic mutations. Photo by Marlee Ellison. 

In previous studies, Appel and her colleagues found that Arabidopsis activated genetic responses after critters like caterpillars and aphids preyed on it. 

By chance, Appel met Reginald “Rex” Cocroft, a bioacoustician at MU’s Division of Biological Sciences. Bioacoustics is the study of sounds made by living things as well as the sounds that affect them. The pair began working together on plant bioacoustics with teams of undergraduate and graduate students. “It was an idea collaboration from the start,” Appel says.*

Plant responses to sound were not a new discovery in the science world, but it was unclear if they could differentiate among them. This was what the Arabivibe researchers were looking for: Would the plants respond equally to every little outdoor vibration? Or would they know a caterpillar when they heard one?

The team found that Arabidopsis plants responded to chewing insects, but this wasn’t necessarily a surprise. More importantly, the researchers confirmed that the plants failed to respond to wind and insect song.

They also found that plants “learned” from past experiences. Some plants were first “primed” with a recording of a chewing caterpillar. When real-life caterpillars munched on them later, they created more mustard oil, a natural insect repellent. On the other hand, plants that hadn’t been primed didn’t create as much mustard oil. “This priming of chemical defenses from previous experience is widespread in plants,” she said, “but now we know it happens in response to vibrations, too.”*

These initial results gave the green light for the Arabivibe project. Appel coined the word “Arabivibe,” a cross between “Arabidopsis” and “vibration.”

“When we showed that the plant had this ‘selective hearing,’ as it were, it was goosebumpy,” Appel says. She cites receiving these results as her most memorable research experience.

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Alexis “Lex” Kollasch waters Arabidopsis plants in various stages of the growth cycle. She is working with the team on the newest stages of the Arabivibe project. Photo by Marlee Ellison.

Scientists had known for years that plants were influenced by sound, but the Arabivibe project was the first of its kind to assert that plants respond to herbivores in a way that is “ecologically meaningful.”

The project is still going strong. On weekends, volunteers water the Arabidopsis plants, which are housed in special chambers that control light, temperature and humidity in order to stimulate the plants’ circadian rhythms. Open the door of one of these chambers, and you’re hit with a whiff of Home Depot’s garden section–the pleasant, earthy scent of soil. 

Alexis “Lex” Kollasch is one of these volunteer research assistants. She’s working with the team on the current round of the project and is currently raising a batch of Hübner caterpillars. When her first group of caterpillars grew large enough to use in the Arabivibe project, she used a cold laser to record their vibrations. “I liked the process of starting out with something small and watching it grow and actually become usable material,” Kollasch says. She also appreciates Appel’s genuine investment in her students, which encourages her to be open and productive.

Pieris rapae caterpillar munches on an Arabidopsis leaf.

Media swarm

Besides her students, Appel is also interested in the media. She’s a savvy communicator and knows how to talk to journalists, a skill that she says is often overlooked in the world of science. This is part of why the Arabivibe project went viral that summer.

After she learned that most science reporters get story ideas from personal connections instead of press releases, she emailed a reporter from The New York Times with her findings. Her attention-catching subject line? “Plants can hear.”

The team’s astounding findings were picked up by publications such as NPR and National Geographic. Even radio pundit Rush Limbaugh got his two cents in: “Some wacko scientist claims that certain plants maybe know and feel when a caterpillar is eating them,” he said on his show. Limbaugh also added that he didn’t have time to read an article about the study, and then chalked the study up to radical environmentalism. Most of her experiences with the media, however, were positive.

Appel took a foray into journalism herself and wrote an essay for The Conversation: Five things I learned when my research went viral.” Her post has racked up more than 20,000 page views.

The experience taught her a lot about the bridges between scientists, journalists and the public.

For Appel, dealing with the news media was an eye-opener. “I love the challenge of explaining what I do to whatever the particular audience is so that they can understand it best,” she says. Her experience with the news media gave her confidence to share her findings with others. She now teaches a science communication course at MU called “Finding the Story in Science.”

“Most scientists have blinders on,” she says. She says that if reluctant researchers learn how to explain their findings to wider audiences, they can harness a new world of impact.

From insects to wine

Appel is married to Jack Schultz, ecologist and director of the Bond Life Sciences Center. The duo co-direct the Schultz-Appel Chemical Ecology Lab, where they collaborate on studies. The latest one looks at galls, foreign structures that insects force plants to build. Once the gall is formed, the insect lives inside it for protection and feeds off of the plant.

The how and why of galls is still a mystery. “Nobody knows how they do it,” Schultz says. “It’s something that both of us have wanted to understand since we were young students, and this is another case where our curiosity converges.”

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Leaf galls induced by Phylloxera aphids on a species of American wild grape. Photo courtesy of Melanie Body.

Galls are a persistent agricultural pest that affect staple crops like grapes, rice and wheat.“If we can understand better how the insects are able to do this to the plants, we can perhaps interrupt their ability to do it,” Appel says. It’ll save farmers a lot of time and money.

A greener planet

It’s easy to think of plants as inanimate or stationary, life forms that are unmoving and relatively unchanging. As it turns out, plants are more similar to us than we think. However, the Arabivibe project is more than just a fascinating study–it has large implications for agriculture.

Scientists now understand that the feeding vibrations of insects are important in the Kingdom Plantae. And if Arabidopsis has this kind of response, other species are bound to as well, Appel thinks. The team is working to figure out what other plants respond to chewing vibrations, starting with rapeseed, the source of canola oil.

Another question remains. Without specialized hearing organs, how can plants hear the sound of a hungry caterpillar? “The goals of our current round of funding are to show that hearing is common in many different kinds of plants, probably all,” Appel says. Since plants don’t have ears like humans do, the lab needs to figure out how they can distinguish between vibrations. They’re wondering if plant mechanoreceptors, which respond to stimuli like touch and sound, are causing the reaction. The team is also trying to figure out what part of the vibration triggers a response in the plant. Cocroft is in charge of making “designer signals” to single out what portions are important.

No matter who you are, your wellbeing is connected to plants. “We are all feeding on plants,” says Heil. If scientists can understand how plants defend themselves, they may also be able to provide the answer to how to feed our skyrocketing population for the next few centuries while using less pesticides.

Plants should also be appreciated for the fact that they are an interesting, dynamic part of the world around us. “The major motivation of science is really curiosity and just to understand nature,” Heil says.

For this reason, Appel and her team have made an indelible mark in the world of plant science. We are now able to see the intricacies of plant and insect interaction through a new lens.

*CORRECTION: This article has been updated to reflect the research methods used in the Arabivibe project and that the partnership between Appel and Cocroft was a collaboration from the start. An earlier version of this article also misstated Appel’s bachelor’s degree field. She received a Bachelor in General Studies.

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