The plant world is a violent place. When munching caterpillars or grazing cattle set their sights on a luscious leaf, a plant can’t hightail it out of harm’s way. Instead, flora fight back with noxious chemicals. But what repels one critter may not work on the next hungry mouth, explains Heidi Appel, a senior research scientist in the Bond Life Sciences Center at the University of Missouri. She’s found that some plants can actually hear their attackers noshing away, discern which species is chowing down—and respond accordingly to each assailant.
That’s right: Plants can hear and feel touch, and Appel’s work is revealing that their sensory systems are far more complex than previously realized. “They are way more clever than we give them credit for,” she says. They also have their own way of seeing, she points out—just watch a seedling grow toward a light source—and smelling, too, in how they sense volatiles released as warning signals by other plants.
Besides its inherent curiosity value, the research could play an important role in conservation. According to the U.S. Department of Agriculture, farmers applied about 87 million pounds of insecticides to crops in 2007 (the most recent data available). And at least one type of those chemicals, neonicotinoids, are known to harm bees and monarch butterflies. Appel, who often collaborates with her husband, Jack Schultz, head of the Bond Life Sciences Center, hopes their work will help find ways to control insects without traditional insecticides.
“Our discovery that plants can tell their adversaries apart suggests that we might tailor their responses, for example, to ignore less important pests but respond more strongly to the serious ones,” Schultz says.
So how did they figure out that a plant can tell which bug is dining on it? They started with Arabidopsis (a member of the mustard family), two species of caterpillars, two species of aphids, and “caterpillar wranglers” (a.k.a. undergraduates). They loaded bugs onto plants and let them chew for a set amount of time. Appel, meanwhile, used a camel-hair paintbrush to mimic the insects' movement on a control group of plants to rule out responses to touch alone.
After the feeding frenzy, the team harvested the plants and analyzed gene expression in the various samples. Of the 28,000 genes in Arabidopsis, 2,778 genes responded, they reported in Frontiers in Plant Science earlier this year. Some of those genes trigger defense responses, while others regulate processes like root growth and water use.
They weren’t surprised to see that the aphids’ needle-like mouthparts activated different genes than the crescent-shaped caterpillar bites. But Appel says she was amazed at the subtle distinctions the plants could make: When fed on by two aphid species, the Arabidopsis shared less than 10 percent of their changes in gene expression. For caterpillars, the overlap was less than 25 percent. “It really shows that plants can tell the insects apart and respond in very different ways,” she says.
Appel’s hearing research is similarly striking. Plants have long been known to detect sound, but the reason for this ability has been a mystery. As Appel and Rex Cocroft, a University of Missouri animal communication expert, reported in Oecologia last summer, it turns out the skill might help plants identify an attack.
The duo discovered that the Arabidopsis actually hears the cabbage butterfly caterpillar chewing. The vibrations reverberating through leaves and stems trigger the release of mustard oil, a natural insect repellant. The researchers first recorded, with a cold laser, the extremely subtle vibrations caterpillars make while feeding, then played them back in the absence of the insects and found it generated the same chemical response. Neither the vibrations of a gentle breeze nor the chompings of another insect stimulated the release of mustard oil.
Next they’ll look at how other plants respond to predator sounds and try to pinpoint which aspects of the noises sends the plants into defense mode. On the flip side, Appel and Schultz are also examining how other insects trick plants into forming galls—abnormal growths that offer protection and nourishment to the bug as it matures.
Appel’s work is enough to make anyone look at plants in a new light (and maybe cringe the next time they accidentally step on one). They may be rooted in place, but they go down fighting. Some more than others: When asked what plants she has in her office, Appel answers, “Carnivorous ones. They’re the plant world’s sweet revenge.”
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