Look Whos Been Talking

Toki Oshima drawing
 

By Teresa L. Johnson

Just like those creepy talking trees in “The Wizard of Oz,” the plants in your garden are talking to each other. Scientists have now identified ways that plants communicate with each other and with other species.

“Plants have sensory systems. They perceive a lot of things, they process a lot of things,” said Harsh Bais, a professor of plant and soil sciences at the University of Delaware, in a phone interview.

“They have to do this for their own benefit because plants are sessile – they can’t move – so they literally have to defend their ground. The way they do that is through chemical signaling.”

Plants make signaling chemicals when they’re hungry, or thirsty, or when pests attack. These chemicals accumulate in the stems, flowers, seeds and roots of plants and actively send messages during plant communication. The other side of the communication equation – receiving – relies on receptors, specialized proteins that straddle the membrane of a plant cell, with a “receiver” in the exterior of the cell environment and a “speaker” in the interior regions of the cell. The receptors “hear” the chemical signal and relay its message to the appropriate parts of the plant cell.

A chemical signal might promote growth or stimulate reproduction, but more often than not it initiates defense. For example, when a caterpillar starts eating a tomato plant, the plant senses the damage and immediately calls for help. It releases a chemical into the air that attracts a particular type of wasp that comes in and lays eggs inside the caterpillar. The eggs quickly hatch and the wasp larvae eat the caterpillar from the inside out, all coordinated by chemical signals and receptors.

The idea that plants could communicate sprouted more than 30 years ago when former University of Washington scientist David Rhoades’ research showed that neighboring willow trees could protect each other from caterpillars by communicating with one another, a phenomenon dubbed “talking trees” by the press. The term gained popularity among scoffing scientists when they couldn’t replicate Rhoades’ experimental results. Rhoades was laughed out of science (he ran a bed and breakfast until he passed away, about a decade ago), and the entire field of study was stigmatized.

New research with better designed, replicable experiments has vindicated Rhoades, and most plant experts now widely accept the fact that plants communicate. The consensus ends there, however. The fight among scientists today is the definition of “communication.” Much of the debate centers on whether a signal invariably induces a response in the receiver, whether the signal necessarily benefits the sender, and whether the receiver has “free will” to ignore the signal.

On the surface, Susan Dudley’s reasons for studying plants are much simpler.

“They’re pretty!” Dudley said, laughing. “And I have no guilt about working on them,” she added during a phone interview.

Dudley is a biology professor at McMaster University in Canada, and much of her work has centered on the communication styles of Cakile edentula, or sea rocket, a scraggly-looking relative of arugula that loves dry, sunny coastlines, and of Impatiens capensis, better known as “touch-me-not,” a jewel-toned bloomer more at home in moist bottomlands and ditches.

Dudley said that regardless of how it’s defined, communication happens all the time in nature, and most plant communication occurs among closely related plants, or “kin.” She explained that most plants, like most animals, reproduce sexually. Plant siblings – offspring produced from seeds that come from the same mother – share many of the same genes and tend to look out for each other. Dudley said these family chats prevent costly and unnecessary competition among plants from the same gene pool by providing a venue for discussions about resource allocation.

“Like the rest of nature,” said Dudley, “plants aren’t kinder or nicer. They have their own self-interest or their relative’s at heart. They cooperate if it benefits them or their family.”

Plants have diverse vocabularies, and you could even call them multilingual. They can communicate with other species, such as the wasp, for example. Some scientists liken the chemicals that plants emit to words; a blend of chemicals is like a sentence. Every plant emits a unique blend, which constitutes its personal scent, or voice. Kin plants share dialects – the ratios of compounds in their scents – to collectively help them attract or deter pollinators or plant-eaters, respectively. In a sense, the complex cocktail of chemicals plants emit serves as a message board describing what’s happening in the environment of the plant.

Perhaps the most universally understood “word” in the plant vocabulary is jasmonic acid, a hormone, and its derivatives, loosely referred to as “jasmolites.” These compounds travel between the aboveground parts (shoots) and belowground parts (roots).

“There are lots of signals that work shoots-to-roots in intra-plant communication,” said Bais, “in a kind of weird analogy to our vascular system.”

Of course, aboveground and belowground parts of plants exist in vastly different environments.

“Plants have both public and private messaging systems,” said Bais.

This split-level existence requires different communication systems. Aboveground communication relies on airborne chemicals that can travel short or long distances (even miles) to convey messages from one part of a plant to another (say, leaf to leaf), or to entire fields or forests of plants. Unfortunately, this form of plant communication doesn’t occur in a vacuum. Some environmental factors, such as air pollution or humidity, can render a message “garbled” or completely indecipherable. And, because the chemical signals emitted by plants rely on wind and other airborne assistance, they have no control over where a message goes – regardless of the intended target. So if a plant is downwind of its allies and upwind of a threat, it’s likely nobody will hear the distress call and come to its rescue, potentially costing the plant its life.

Belowground communication relies primarily on soluble chemicals in the soil and occurs in a 1-millimeter-thick microbe-rich zone surrounding the roots called the rhizosphere. Here, in this dark, subterranean world, plants send out soluble chemicals from their roots to identify themselves as kin and to ward off predators. The microbes participate in the conversation by adding their own chemicals to the mix, influencing the overall health of the rhizosphere, much like the microbes in the human intestine influence human health.

And, as in the human intestine, diet (or, in the case of plants, fertilizer) influences the microbial mix in the rhizosphere. Organic fertilizers have a greater beneficial influence on microbial diversity than do synthetic chemical fertilizers, especially during the mildly warm months, due to a sort of “Goldilocks effect,” when soil temperatures are just right – between 60 and 85 F – and soil moisture tends to be optimal.

Bais has been studying plant communication for nearly 20 years. Much of his research has centered on Arabidopsis thaliana, or thale – a diminutive plant noted for its hardy nature and fast, prolific reproduction, and a close relative to Dudley’s sea rocket. Now Bais is exploring rice plants and the conversation occurring between them and their resident rhizosphere microbes. Since the rhizosphere is typically hidden belowground in soil, Bais studies the plants in hydroponic growing systems where, suspended in a watery solution of nutrients and microbes, the plant root systems are easily observed, and communication signals are readily interpreted.

Although Dudley’s work uses more traditional growing techniques, one thing is apparent: “They actually listen better to kin than strangers,” said Dudley. So if you plant two thale siblings in the same pot, they’ll negotiate a comfortable space within the soil and find ways to share the resources. But when plants encounter strangers belowground, they respond by becoming more competitive. Plant two non-siblings (even those of the same species) in the same pot and they crowd each other out, like bullies fighting over an airplane armrest.

If plants have evolved to communicate with each other and with insects, will they ever be able to communicate with humans? Some scientists believe they already do. Many of the stress-induced chemicals plants produce have nutritional benefits for humans. In the early 1990s, Paul Talalay, a researcher at the Johns Hopkins University, identified these naturally occurring protective chemicals in edible plants such as fruits and vegetables. Often referred to as phytochemicals, thousands of these beneficial substances have been identified, including resveratrol in red grapes, sulforaphane in broccoli and allicin in garlic.

“These chemicals raise the intrinsic mechanisms that cells use to protect themselves from the vicissitudes of life,” Talalay told me in an interview.

Simply put, phytochemicals stress a cell just enough to elicit a response that protects it from bigger stressors – the same ones that plague plants: nutritional deficiencies, ultraviolet radiation and foreign attack. In a type of “pay it forward” scenario, the stress experienced by one organism (the plant) benefits another (humans). Human cells “read” the chemical cues from plants and in turn launch a defense response that increases their chances of survival. Some scientists describe this phenomenon as a type of interspecies communication, made possible by humans’ shared evolutionary history with plants.

Still, some might wonder what scientists hope to gain by studying plant communication. Bais believes that manipulating the conversations in the rhizosphere can have far-reaching effects. He hopes to identify plant communication strategies that increase disease resistance of plants, thereby increasing production.

Similarly, Dudley said that by controlling the conversations between kin or non-kin plants, farmers might be able to boost production by building more cooperative plant communities that can better fight pests or survive drought together.

In the Technicolor world of Oz, trees not only talked, they threw apples. Although nobody’s throwing apples yet, plants actively engage in and respond to their environments. Exactly how they do that, in most cases, remains an enigma.

“It’s pretty clear plants are communicating,” Dudley said, “but in a mysterious – because it is unknown – sort of way. Nature is always more complicated than we expect it to be.”

About the author: Teresa L. Johnson is a freelance science writer and avid organic gardener with graduate degrees in public health and human nutrition. Her work draws on elements of phytochemistry, toxicology and nutritional biochemistry.

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