Listening in on Plant Defenses

It’s enchanting to consider that classical music might help plants grow better, like something out of a fairy tale. A simple Google search shows that a lot of people are interested in it, from the throngs at Yahoo Answers to marijuana growers looking for an edge. Mythbusters tested it, with mixed results. Academic researchers have explored the effects of tones on plant growth, finding frequency-specific gene regulation and growth responses. But it remains unclear what evolutionary benefit sensitivity to sound could provide, and a solid understanding of what is sometimes called ‘plant bioacoustics’ eludes researchers.

In a widely-reported study released last year, two researchers over at the University of Missouri, Columbia tested the effects on plant defenses of the vibrations caused by a caterpillar chewing on a leaf. Although much of the reporting fell prey to the temptation to claim the plants “heard” the chewing and responded, the real answer is both more complicated and more interesting. I had the opportunity to attend a talk Drs. Appel and Cocroft gave at Washington University a few months ago where I learned more than I could have extracted from their paper, published in Oecologia, alone.

Sound waves are longitudinal. Insect vibrations are transverse

Dr. Cocroft studies insect communication, especially the ability of insects to find mates and prey by sensing the vibrations of other insects on a plant. Like sound, the information is encoded in vibrational waves passing through a substance. Instead of a pressure wave like sound that varies in the same direction of travel—a longitudinal wave—insect vibrations on plants are transverse waves, moving up and down like a wave on the ocean (see figure).

We could never hear these kinds of waves ourselves, but their frequency can be directly translated to sounds we can hear. Cocroft played a number of humming soundscapes recorded with a laser on a wild prairie—the result of hundreds or thousands of insects communicating silently on stalks of grass. A plant, Cocroft noted, is a great conductor for these vibrations, flexible yet strong. His field studies how insects benefit from communicating this way, but he joined forces with Appel to ask: Do plants respond to the vibrations of insect herbivores in an adaptive way?

One major defense that plants have against pests is producing noxious compounds to deter feeding. Appel and Cocroft hypothesized that Arabidopsis plants would produce more defense compounds if they were exposed to the vibrations of herbivorous insects before actually being attacked. This effect is called priming, and could help defend against a second wave of insect damage.

To test this, the researchers first used lasers to record the vibrations of caterpillars allowed to eat the leaves of Arabidopsis plants. To play the vibrations back to undamaged plants, Cocroft attached leaves to tiny pistons driven, essentially, by speakers, ones that could replicate the vibrations of an insect chewing. Then caterpillars were allowed to feed on either the leaf that was vibrated or another, untouched leaf.

Both vibrated and distant leaves responded more vigorously to caterpillar attack than leaves on untouched plants. The plants that were primed by recorded caterpillar vibrations produced more glucosinolates, or mustard oils, than those of unvibrated plants. This is evidence of an adaptive response to insect vibrations, but leaves open the possibility that any vibration encouraged plant defenses.

To see if the effect really was specific to the herbivorous caterpillars, Appel and Cocroft played back vibrations of harmless insects, wind, or caterpillars on different plants and again measured defense compounds—this time anthocyanins, responsible for the deep reds and purples of many plants. Only caterpillar vibrations could prime plants to increase their defense response to herbivory; wind and the neutral insects had no effect.

One important caveat: although the researchers looked for an effect of vibrations alone, they found none. Only vibrations plus actual insect feeding induced higher defenses; the plants were primed for future attack, but vibrations alone made no difference. Of course, a real insect is more than just its vibrations. Herbivore attack is a physical, chemical, and auditory assault, and plants likely respond to each stimulus in different ways.

But how are plants able to sense the vibrations of caterpillars, and even differentiate them from similar sounds in nature? It’s entirely unknown. A very good candidate is a diverse group of proteins bound together by their responsiveness to physical forces—mechanoreceptors. These proteins can signal within a cell in response to vibration or touch and are potentially behind the priming effect that Appel and Cocroft observed.

In fact, to test this, the Haswell lab is working with Appel and Cocroft to see if our favorite mechanosensitive ion channels are part of the vibrational-response pathway. I got to see Liz’s face pop up in the corner at the end of their presentation over on the medical campus as they told us that work was underway. We’ll just have to wait to find out.

[A version of this post first appeared on my lab's blog]

Reaching Across the Gap with Curiosity

"I think there is nothing so exciting as listening to someone think on the radio." — Jad Abumrad

On Wednesday, Jad Abumrad and Robert Krulwich of Radiolab fame presented at Washington University’s first Ampersand Week, a series of events celebrating the ‘and’ of Arts and Sciences, or the value of liberal arts education over exclusive specialization. A perfect choice for such a purpose, Radiolab draws on the composing background of Jad and the inventive science journalism of Robert to explore scientific topics with a humanistic lens. The event took place in Graham Chapel, the pews filled with students, faculty, staff, and the public for the free event.

I was not sure what this presentation would entail. Would they present a live version of Radiolab? Or just introduce a series of archived podcast segments? The experience was somewhere between those two. Relying on existing tape, Jad and Robert discussed the production of Radiolab, the task of distilling technical knowledge from experts for a lay audience, and the musicality and intimacy of radio over other mediums.

Jad opened by acknowledging his mother, sitting in the front row, an obesity researcher here at Washington University, a professor in my program no less. I had no idea. On the large screen behind them, Jad put up a picture of his mother’s protein, what she studies every day, which helps bring fats into the cell. In fact, Jad grew up in a scientific home, with a medical doctor father and scientist mother, an environment that clearly influences his work to bridge the sciences and the arts.

The first segment began by peeling back the curtain on how a formal interview with a scientist becomes radio drama. Robert spoke with Cynthia Kenyon, a C. elegans researcher at UCSF, about two genes that control aging in the tiny worms. One is a hormone receptor, which, when activated, represses the activity of a transcription factor. When the transcription factor is allowed to function, it controls the expression of many separate genes that work together to increase the lifespan of the worms several fold. Jad played the unedited interview, demonstrating how even a media-savvy researcher stumbled to translate the molecular action of genes into something a non-scientist could grasp, and even care about. It was awkward and difficult to keep track of.
But as Jad pointed out, Cynthia’s explanation naturally gravitated to exciting, narrative verbs. Spring. Inhibit. Leap into action. The nouns suffered from alphabet soup—scientists rely heavily on acronyms and jargon for naming genes—and specialized phrases. Receptor. Transcription factor. DAF-2. Radiolab’s job, then, was what Jad called “noun replacement therapy.” Keep the substance, but swap technical language with vernacular.

The translated version: The Grim Reaper Gene (hormone receptor) cue evil laugh battles it out with the Fountain of Youth Gene (transcription factor) cue toddler giggles for control of the aging process. Beat up on the Grim Reaper (mutate it) painful groans and the baby is free to keep cells, and the animal, youthful, blowing spit bubbles as it does.

To some scientists, this kind of translation may seem simplistic. (Cynthia produced the gene nicknames, it was not a liberty taken by Radiolab.) Robert even phrased it as having to ask, “How stupid do you want to be?” Always there is a trade-off between accuracy and understandability. Always. “You’re somehow trying to find a way to stay in the middle,” Robert said. Choosing that point, and then finding that point, is the challenge a show like Radiolab contends with for every topic. But to avoid any kind of simplifying is to wall off scientific research to the ivory tower, something far more damaging than “noun replacement therapy.”

This translation is not foreign to most of us, maybe just lost. Jad recalled trying to bridge the gap with his mom to explain her work when he was a kid, dinner plates standing in for cells and the salt shaker for her protein, the iterative process of trying to get closer to the truth one curious question after another. Our interest in understanding something new, something difficult, is dampened by a culture that discourages looking stupid, but it can be encouraged as well. Jad and Robert try to use the power of stupid questions asked with genuine curiosity to recapture that sense of wonder. “Yes, but why?”

Robert said that if they approach a scientist with sincere curiosity, about 60 percent will spend the time to tell them what they need to know. I wish that number were higher, but I am surprised it is that high. I think they may have a self-selecting group of scientists more inclined to work with the media than most. But I could not say for sure.

Beyond translation, the hour-long presentation delved into the frenetic production of the show, with layers of music and noise and swirling audio energy, a style that aims for a composer’s musicality and an authentic struggle for new knowledge. As a technically naïve but huge fan of radio, I appreciated seeing the depth of production at the software level that goes into making one of my favorite shows. Although hard to miss in a show like Radiolab, I know that most audio production is successful when it goes unnoticed, but it is good to be reminded of the work that goes into these programs.

The floor open to questions, I waited in line at the mic to ask: How can scientists help reach back out to the journalists, or the public, who have reached toward us to help bridge these gaps? I did not get an answer to my question, but I did get a good answer to a good question.

Robert instead answered the why. Why should scientists care about communicating their work? He couched it in militaristic, epic terms—scientific inquiry is the product of intellectual freedom, a resource that is constantly endangered. To tell a story of the science we do is an enchantment, one that can draw people in and convince them that the freedom required for this kind of work is worth demanding and worth preserving. No less than the ability to perform honest work is at stake in the communication of our research.

Jad again put on screen the structure of his mother’s protein, her life’s work, to help illustrate his partner’s answer on the value of free inquiry. He then answered a question closer to my own. “The story of science is in most cases the story of ceaseless failure, which is really the story of everyone who walks the earth,” he said. Tell that human story of vulnerability, confusion, failure, and occasional bright points of insight and success, and anyone can be reached.