Plants “Hear” Herbivores Coming—and Prepare

A pair of snails attach themselves to a branch of a dried black mustard stalk on a hillside in Huntington Beach, California. Photograph by Sam Gangwer, The Orange County Register/
A pair of snails attach themselves to a branch of a dried black mustard stalk on a hillside in Huntington Beach, California. Photograph by Sam Gangwer, The Orange County Register/

By Kate Horowitz

At the first whiff of a mountain lion, a mouse might scamper away, a rabbit might freeze, and a stray dog might prepare for a fight. These instinctive responses, so crucial to survival, are surprisingly common: New research shows that even plants can benefit from “eavesdropping” on chemical cues from their attackers.

Imagine that the mountain lion in this scenario is a common garden snail, and the scrappy stray dog is a mustard plant. Ridiculous? Not to John Orrock, who studies predator-prey and herbivore-plant interactions at the University of Wisconsin. Recent experiments by Orrock and botanist Simon Gilroy show that black mustard plants can “hear” the chemical signals of approaching plant-eaters—and prepare to fight back.

“One of the things that makes plants so ecologically interesting is that they can’t run away,” says Orrock. Forced by their immobility to get creative, plants have evolved a whole host of spectacular defenses—from thorns and poison darts to awful-tasting chemical juices—against would-be predators.

But clever weapons and armor come at a high price.

“If you’re investing in chemical defenses,” says Orrock, “that’s energy that you could be putting into growth or reproduction instead.”

In other words, a healthy plant is a well-rounded plant: one that doesn’t spend too much time on one particular activity. What this means, however, is that the plants must somehow prioritize and turn on their chemical defenses only when it’s necessary. But how do they know when that is?

Chemical Detection

Scientists have known since the 1960s that many animals, from minnows to bullfrogs to snowshoe hares, can detect their predators’ chemical cues. These cues, called kairomones, give prey animals a head start so they can flee, fight, or freeze. In the plant kingdom, previous studies have shown that individual maple trees and sagebrush plants can sense  chemical distress calls given off by neighboring plants that have been damaged, which inspires them to turn on their own chemical defenses. Exposing a plant that’s been previously attacked by caterpillars to more caterpillar spit can also inspire the plant to defend itself, but what about plants that have never been attacked?

(Related: “Plants ‘Listen’ to the Good Vibes of Other Plants.”)

Orrock and Gilroy used water mixed with snail mucus to slime the soil at the base of a number of black mustard plants. (Because snails secrete a slimy trail of mucus everywhere they go, the presence of mucus is a sure sign to plants that a snail is nearby.) The team then offered the slime-exposed plants to hungry snails, along with a control group of un-slimed plants. As expected, the plants that had been “warned” by the simulated threat had already armed their defenses, and the snails left them alone. The oblivious plants in the control group weren’t so lucky.

To see if the mere presence of another snail’s mucus was enough to make a snail lose its appetite, the scientists also served two samples of store-bought cabbage: one treated with slime, and one un-slimed. The snails chowed down on both samples with equal gusto. Clearly, the living black mustard plants had something the dead cabbage didn’t: defense.

As amazing as that sounds, there’s more: When time passed and no attack took place, the plants let down their guards. By the time the plants were served up for snail snacks, those that had been slimed in the distant past had become tasty again. Those specimens that had been slimed more recently, however, were still ready for a fight. Additionally, the more times a plant had been slimed previously, the more likely it was to charge up its nasty-tasting chemicals when threatened. “It’s about the probability of risk,” says Orrock. “Much like animals, plants are using this information in a very sophisticated way.”

The team presented their report last week at the annual meeting of the Ecological Society of America, and said they’re eager to dig deeper into their findings. Upcoming experiments will investigate the molecular and physiological whys and hows.

“This study is another fascinating example of just how perceptive plants are of the environment,” says Janet Braam, head of the Braam Lab for Plant Responses to Environment at Rice University. “Use of kairomone-sensing to gain advantage in defense could be a powerful way for plants to thrive against mobile attackers.”

When it comes to kairomones and plant defense, Orrock says, “We’re really just getting started.”


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