Learning to Speak Shrub
By Elizabeth Preston
Illustration by Eric Nyquist
Entomologist Richard Karban knows how to get sagebrush talking. To start the conversation, he poses as a grasshopper or a chewing beetle—he uses scissors to cut leaves on one of the shrubs. Lopping off the leaves entirely won’t fool the plants. So he makes many snips around the edges and tips of the leaves—“a lot of little bites.”
A few months later, Karban, a professor at the University of California, Davis who studies plant defense communication, returns to the sagebrush and examines its leaves, many of which now have damage from real grasshoppers or beetles. However, within about two feet of the branches he clipped, leaves have been spared the worst ravages of the hungry insects. That’s because Karban’s cuttings convinced those damaged leaves they were under insect attack, so they sent chemical alarms into the air. Neighboring leaves intercepted and deciphered the code messages, and began prepping their own defenses against the bugs.
If plants seem silent to us, it’s only because we’re oblivious to their chatter—we are just beginning to tap into their cryptograms. Plants emit codes into the air all the time, helping them defend against insects and other threats, and in some instances serving as warnings to their neighbors. Moreover, plants can send “SOS” calls for rescue missions and summon predators to feed on insect invaders.
Plants speak in chemical codes—carbon-containing molecules called volatile organic compounds (VOCs). Characterized by the ease with which they enter the air, VOCs are a diverse group: plants alone make more than 30,000 varieties. Some VOCs produce familiar herbal or flower smells. Others are released only in response to a specific cue. Within seconds of being damaged, plants send out green leaf volatiles (GLVs), which we can detect too—for example, as the smell of a newly mown lawn.
Humans don’t get much more information from VOCs. But the waves of molecules produced by a plant carry packets of cryptic messages. And just like any transmitted signals, plant-o-grams can be received, decoded, eavesdropped on, and even scrambled.
Plants send out VOCs in response to physical damage or to chemicals in insects’ saliva, vomit, or egg-laying fluids. Insect bites can activate hormones within the plant, like jasmonic acid, ethylene, or salicylic acid, which increase the activity of the plant’s defense genes. These hormones can also be released as VOCs to alert the plant’s other leaves and branches as well as its neighboring vegetation community. In particular, Karban says, methyl jasmonate—a volatile form of jasmonic acid—seems to be “pretty potent.” He also found that such communication is more effective between genetically identical plants—those that come from the same parental bushes or shrubs. And when Karban placed plastic bags over clipped sagebrush leaves and tied them up to prevent VOCs from escaping, neither the rest of its own leaves nor its neighbors were able to crank up their resistance.
VOC messages may be meant for self or family, but plants of other species can sometimes intercept them. Sagebrush’s alerts can trigger defense responses in both tomato and tobacco bushes, although it’s not clear how many plants can hack into other species’ signals.
Moreover, plants may not always want their cries to be heard, scientists say. “It’s not really in the best interest of a plant to actually tell its neighbor it’s being attacked,” says Amy Trowbridge, a postdoctoral research fellow at Indiana University, Bloomington. Adjacent plants are competitors, and warning a neighbor helps that plant survive while the Good Samaritan may succumb to an insect invasion. So why do plants shout anyway? Part of it may be unavoidable: The “chemical weapons” plants use to fend off bugs may inevitably leak into the air because they’re volatile—so other vegetation evolved to eavesdrop. So did some predatory insects—they essentially tune in for dinner calls. Apple trees that are chewed by spider mites send out a message that attracts other mites that eat the attacking spider mites. When female sawflies lay their eggs inside the needles of a Scots pine, the tree’s VOCs summon parasitic eulophid wasps that kill the eggs. Similarly, tobacco plants that are being gnawed by budworms summon parasitic red-tailed wasps that lay their own eggs inside the caterpillars’ bodies, later to be eaten by larvae from the inside out.
While plants and insects evolved to exchange these chemical messages, humans are only beginning to break the code. “We don’t really know how these compounds are perceived,” Trowbridge says. Researchers don’t yet understand how the plants collect VOCs from the air and what the detectable concentration is. Nor do they know whether the molecules are absorbed straight through the surface of the leaf, or enter through its pores, called stomata. But they do know that the “listening” plants must not only receive but also decode the message in order to set off chemical defense reactions. “Just because a plant may take up a compound doesn’t actually mean anything,” Trowbridge says. If the intercepted signal can’t be decoded, it’s of no help.
Moreover, messages could be encoded into a combination of molecules. “The bouquet that’s released when you clip sagebrush contains literally hundreds of chemicals that you can measure,” Karban says. He gathers VOCs in plastic bags filled with special fibers that collect the compounds and analyzes them in a gas chromatograph. But, he says that “identifying the active ingredients is really difficult.” Chris Jeffrey, an organic chemist and chemical ecologist at the University of Nevada, Reno, thinks that to really crack plants’ cryptography, scientists need to decipher the chemistry of whole ecosystems at one time. “You’re detecting a very complex mixture of molecules,” he says, likening the phenomena to our own sense of smell. “It’s not a single molecule that results in a single response.”
Why should we bother to crack plant codes? For one thing, they help us understand how plants will react to environmental changes—like those expected with climate change. Scientists warn that climate change can scramble communications, and destabilize ecosystems. Some signals may be amplified while others are dampened or never detected.
“A lot of volatility is dependent on temperature,” Trowbridge says, so a warming planet may let VOCs enter the air more easily. Higher temperatures can also increase the activity of enzymes that manufacture VOCs. On the other hand, plants trying to survive droughts will squeeze their stomata shut to prevent water loss. Leaves with closed stomata take in less carbon dioxide, which they need to manufacture VOCs. With less VOC communications, plants might not detect alarm signals and become more vulnerable to insects, or completely succumb to them, Trowbridge speculates. But with too much VOC emission, plant populations might defend themselves too well—so insects may seek new food sources, destroying other plants species and changing ecosystems.
So the next time you are enjoying the silence in a garden, alone, remember that the silence is an illusion. There is a riot of shouting going on, if only you could hear it.
Elizabeth Preston is the editor of Muse, a magazine about science and ideas for kids, and author of Inkfish, a blog about science and cephalopods for everyone. She has also written for Slate and National Geographic.
This article first appeared in Nautilus magazine
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