The Secret Life of Plants

It turns out that plants “talk” to each other, sending helpful messages—and issuing the occasional death threat—over their own underground internet. One Xavier University biologist is following the conversation.
Do plants “talk”?

Illustration by Briony Morrow-Cribbs

Just inside the door of Kathryn Morris’s office in Albers Hall at Xavier University is a very dead plant. It’s brown, wilted, and still sadly rooted in a large white pot on the floor. The biology professor takes no responsibility for it. “It was here when I moved in,” she says.

That would have been four years ago, when she joined the faculty at XU as an assistant professor of biology, following a post-doctoral residency at Berlin’s Freie Universität. Morris may be blasé about the fate of her doomed office specimen, but she is keenly interested in how plants communicate, and why. It’s an interest that leads her to discuss plants as if they are sentient beings: She’ll describe her process as “eavesdropping” on plants as they “chat,” or the conditions that make them “happy” or “play nicely.” It may seem like a strange way to conceive of flora, but then you’ve probably never heard plants talk to each other. Morris has.

A self-described “Air Force brat” who bounced through multiple addresses and schools, Morris found herself at Wright State University when her father was posted to Dayton’s Wright-Patterson Air Force Base in the 1990s. The environmental sciences caught her imagination as an undergraduate, and particularly the work of her advisor, Don Cipollini Jr., who specializes in the chemical ecology of plants. She earned her Ph.D. in 2008 with a dissertation on garlic mustard, a noxious invasive species brought to America by European pioneers. Garlic mustard muscles into new areas by pouring hostile chemicals into the soil—just the kind of bizarro plant drama that can inspire a life’s work.

Morris clasps her hands on her desk while she describes her research. Her brown, shoulder-length hair is pulled back from her face and wire-rimmed glasses magnify her brown eyes as she intently monitors her visitor’s tolerance for scientific jargon. “I talk fast,” she warns.

Morris began her work by running experiments to track how individual plants send signals to each other. The signals can manifest as reactions (aphids infest one plant in a meadow, and plants nearby launch defenses against aphids, even though they have not yet been attacked) or more protective, even deadlier, warnings (the dispersal of allelochemicals—poisons—from one plant to another). A plant can essentially issue a death threat, and follow through on it. And Morris’s research shows that these chemical “conversations” happen underground on a fungal superhighway.

“Anthropomorphizing—using terms like that—has its benefits for getting the point across,” Morris says. “It helps students connect.” Yet when she talks about plants sending signals and messages over complex subterranean communication systems, Morris insists the terminology is not just a metaphor. There really is a network allowing plants to connect, and it is built from tiny strands of fungi. Other researchers, looking at the sorts of fungal networks that Morris studies, have taken to calling it the “Wood-Wide Web.”

“The way we think of this fungal network is basically providing a sort of ‘information superhighway’ with direct connections between plants,” Morris says. “So it’s a useful analogy.”

Where humans send electronic data over metallic wires or glass fibers, plants transmit “infochemicals” across arrays built by underground fungi. The types of networks Morris is studying are known as common mycorrhizal networks—“common,” not because they are ordinary, but because multiple plants share them and their relationships are interdependent. The fungi in question survive by allying with a host plant (they get sugars and carbohydrates from the roots); the plants themselves get a kind of fertilizer-style boost from being attached to the fungi. The network, formed from a web of tiny threads called “hyphae,” is indeed wide, despite its invisibility to the human eye. “When you are gardening, these threads are there,” Morris says. “They’re everywhere. You’re just not able to see them without a microscope.”

Scientists have known about the symbiotic relationships between fungi and plants for decades, and some suspected that this intimate connection allowed plants to transmit certain signals to other plants nearby. But they weren’t clear on how it worked and hadn’t figured out how to track this silent, invisible communication system. Morris started with substances more associated with puberty than botany: hormones.

“When plants are attacked, they produce hormones,” Morris says. “[They] are dispersed throughout the plant and it reacts. Many of these compounds are released into the soil.” Scientists have been aware that defensive chemicals flow through the plant and leach into the surrounding soil since a Chinese research team published their findings on the subject in 2010 in the science journal PLOS ONE. They also know that once the plant under attack responds, neighboring plants get the message. The trick is figuring out exactly how a response is triggered in other plants. Morris wrote a 2011 paper in the same journal suggesting underground fungal networks as the main medium.

Researchers discounted soil transport for a long time, Morris says, because it is such a dicey pathway—or as she puts it, an “extraordinarily complex matrix” filled with many obstacles to chemical communication. How does the infochemical respond to water? Will microbes gobble it up? Will naturally occurring chemicals degrade it? In other words, if you need to send a love letter, do not send it by soil.

And yet something transmits the information pretty efficiently. Morris has a hunch it’s all those microscopic fungi. She and her colleagues believe that common mycorrhizal networks provide a clear communication channel, allowing infochemicals to zip around the impediments between plants. To prove that hypothesis, she’s done some crafty gardening.

She takes me down the hallway from her office, through a teaching laboratory, and into a sun-washed prep room. Despite a couple of scientific posters on the walls and the odd chemical safety sticker, any gardener would recognize this as a very nice potting shed. Arrayed along the inside wall are dozens of unorthodox H-shaped planting pots, homemade by Morris and two biology majors who worked on the experiment with her, recent graduate Kira Liggins and undergrad Rachel Fletcher. The pots, constructed last summer out of PVC pipe, are swaddled in duct tape and smeared with dried soil and the occasional desiccated leaf.

To demonstrate that fungi are essential for communication between plants, Morris inserted, right in the center of the crossbar of the H, what she calls a cassette. It’s a simple device, just a square of plastic screen sandwiched between two layers of filmy mesh. This mesh looks like it might have been cut from a white plastic kitchen bag, but it has a soft, almost velvety texture because it is perforated by scores of tiny holes no bigger than 35 micrometers (a micrometer is .001 millimeters—think of a grain of sand; now shrink that by a factor of 30). The holes are important; they’re too tiny for plant roots to penetrate but just big enough for the microscopic threads of fungal hyphae to scoot through.

The cassettes sit firmly between the PVC pipes, but they are just loose enough to be moved. Morris has found that, left alone, the fungal threads will grow back and forth from both compartments of the H-shaped pot. However, moving the cassette just a few millimeters each day snaps the connection, just like cutting a network cable.

Last summer, Morris and her team filled the pots—around 60—with dirt and chopped up roots laced with fungus. Then the legs of each pot were planted with a corn plant and a tomato plant and the whole collection was hauled to the sunny roof of Xavier’s Lindner Family Physics Building. As the plants grew, their roots reached out and commingled with the nourishing fungus. Then those plants were cut down and new corn and tomato seeds planted into the established fungal network. Each day, someone on the research team climbed up to the roof to jiggle the cassettes in half of the pots.

For their experiment to work they had to find a way to inspire some conversation between the plants. In nature when something, such as bacteria, infects a plant, the victim pumps out hormones to defend itself. Some of these hormones are familiar and beneficial for humans. “The hormone that bacterial infections induce in plants is salicylic acid, which is basically aspirin,” Morris says. “Plants make a lot of it.”

Since the plant associates a flow of salicylic acid with a bacterial infection, the team didn’t have to infect the plant to get a response. All they had to do was drip a little salicylic acid into one leg of each pot, then wait and see what happened to the plants in the other leg.

As Morris anticipated, the hormonal signal reached across the cassette barrier only when fungal threads were allowed to grow through and connect to the plants in both legs of the pot. In those pots where the fungal network was disrupted every day by moving the cassette, the signal was a lot weaker.

The experiment is still ongoing, and no one has proven definitively how the signals are transmitted. But Morris has her suspicions. Through her experiments, she has deduced that the signal was not airborne (because the plants were covered after the hormone was applied). She thinks water could play a role, but she’s not sure how. She does know one thing, however: Only where the underground fungal network remained intact did a signal travel between the two compartments. Pay dirt.

Morris is still not exactly sure what chemical carried the signal through the network. In a paper she published in Trends in Plant Science, Morris asks scientists to look at several possible mechanisms. Do infochemicals move physically or biologically? Are they carried inside the fungal threads, or do they adhere to the surface? Perhaps the fungus merely carves a tunnel through the ground and the infochemicals flow through it.

“We have only begun to scratch the surface of our understanding of the mechanisms behind infochemical transfer via [these] networks,” Morris says, “and to appreciate its importance in natural systems.”

The 2009 movie Avatar featured a forest of interconnected plants that respond to invaders as a collective organism. Research in fungal networks provides the science for that science fiction. Scientists around the world are looking into the fungal superhighway to better understand how forests and fields function as communities, rather than as an agglomeration of independent plants. In her paper, Morris and three coauthors call on their colleagues to look into some of the outstanding questions surrounding how fungal networks operate.

Understanding these networks is about more than just good conversation; there’s a lot at stake if human advancements interfere too much with plant communication. Morris and company have observed a connection between fungal networks and flourishing plant communities. Just as humans gravitate to WiFi hotspots, plants seem to enjoy a good network connection as well. And modern monoculture farming—especially on the huge scale that we see now throughout the world—disrupts these fungal networks and the healthy plant life that they help cultivate. “If we artificially fertilize a whole area of soil so the plants don’t need to support the fungi anymore, then they stop doing it,” Morris says. In other words, fungi won’t grow there. Monoculture fields become a fungus-free environment and the plants lose all the potential benefits from that symbiotic relationship.

Which is, in part, why Morris’s experiments with corn and tomato plants have still larger implications. Beyond their role in providing nutrition and sending signals, fungal networks could potentially play a role in stalling the effects of global warming through a process of clumping called soil aggregation. When fungal threads aggregate the soil, it stabilizes, resists erosion, and promotes healthy plant growth. Aggregation also contributes to carbon sequestration, in which soil acts as a kind of sponge, absorbing excess carbon from Earth’s atmosphere.

“Well aggregated soils sequester more carbon than soils that are not,” Morris said. “So with climate change and people talking about more carbon taxes and being able to sell carbon credits and all of that, it’s nice to know exactly where the carbon is.”

As the world looks for better ways to cut back on carbon emissions and to promote sustainable agriculture, Morris thinks some answers might be found in less fertilizer and more investment in the fungal superhighway.

Long story short: Fungus might just help save the world—if we would stop interrupting.

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