It sounds like fiction from “The Lord of the Rings.” An enemy begins attacking a tree. The tree fends it off and sends out a warning message. Nearby trees set up their own defenses. The forest is saved.
Injured plants emit certain chemical compounds, which can infiltrate a healthy plant’s inner tissues and activate defenses from within its cells, the new research found. A better understanding of this mechanism could allow scientists and farmers to help fortify plants against insect attacks or drought long before they happen.
For the first time, researchers have been able to “visualize plant-to-plant communication,” said Masatsugu Toyota, senior author of the study, which was published Tuesday in Nature Communications. “We can probably hijack this system to inform the entire plant to activate different stress responses against a future threat or environmental threats, such as drought.”
The idea of “talking” trees started to take root in the 1980s. Two ecologists placed hundreds of caterpillars and webworms on the branches of willow and alder trees to observe how the trees would respond. They found the attacked trees began producing chemicals that made their leaves unappetizing and indigestible to deter insects.
But even more curious, the scientists discovered healthy trees of the same species, located 30 or 40 meters away and with no root connections to the damaged trees, also put up the same chemical defenses to prepare against an insect invasion. Another pair of scientists around that time found similar results when studying damaged sugar maple and poplar trees.
These early research teams had a budding thought: The trees sent chemical signals to one another through the air, known today as plant eavesdropping. Over the past four decades, scientists have observed this cell-to-cell communication in more than 30 plant species, including lima bean, tobacco, tomato, sage brush and flowering plants in the mustard family. But no one knew which compounds were important and how they were being sensed — until now.
“There was this kind of controversy in the field,” said André Kessler, a plant ecologist who was not involved in the research. “First, how those compounds in general are taken up [by the plant], and then how they are able to change the plant’s metabolism in response to perceiving them.”
Kessler said this study helped answer some of those long-standing questions.
Plants obviously don’t have ears and eyes, but past research shows they communicate with their surroundings by emitting chemicals known as volatile organic compounds, which we can smell. But just as people can speak so many words, plants can produce an array of these compounds for different purposes. Some are used to attract pollinators or as defense against predators.
However, one class of these compounds are emitted when a plant is injured: green leafy volatiles. These are emitted by, as the name suggests, pretty much every green plant with leaves, and are produced when a plant experiences physical damage. An example of this compound is the smell released from fresh-cut grass.
In the new study, Toyota and his colleagues manually crushed leaves and placed caterpillars on Arabidopsis mustard or tomato plants to trigger the emission of various green leafy volatiles. Then, they spread individual fumes to healthy plants to see if the plants would react.
To track the healthy plants’ responses, the team genetically modified the plants so calcium ions would fluoresce when activated inside individual cells. Calcium signaling is important for cellular functions in most living organisms on Earth, including humans. When an electrical signal is sent to our motor neurons, ion channels open and allow calcium to flood inside. This increase in calcium can trigger a neurotransmitter release, which results in a muscle contraction in a muscle cell.
Calcium signaling, Toyota said, plays a similar role in plants. Depending on the plant, it can trigger messages to close its leaves or digest an insect.
After testing many green leafy volatiles, the team found only two seemed to increase calcium ions inside cells. Additionally, they found calcium signaling first increased in guard cells forming the plant’s leaf pores, or stomata — an important finding, because it shows the compounds are absorbed into the plant’s inner tissues.
“They cannot just seep through the surface of the plant easily,” said Kessler, a professor at Cornell University. “They have to go through the stomata, [which] allow the plant to actually breathe carbon dioxide in and oxygen out for photosynthesis.”
The calcium signaling, Toyota said, is like a switch to turn on the defense responses from the plant. After signaling increased, the team found the plant increased the production of certain gene expressions for protection. For example, Toyota said the plant may start producing certain proteins to inhibit insects from munching on them, giving the insects diarrhea.
“If the plant has lots of these genes, they are now very strong against the insect herbivory,” Toyota said.
With this new understanding, researchers say plants could be immunized against threats and stressors before they even happen — the equivalent of giving a plant a vaccine. For instance, exposing healthy plants to insect-ridden plants or the associated green leafy volatiles could boost their genetic defenses, so farmers use less pesticides, Kessler said. The revelation could also help make plants more resilient during a drought, signaling the plants to retain more water.
“If you have a plant early in its life exposed to drought, it will actually handle drought better than a plant that was not exposed to that,” Kessler said. “This is also a result of the plant’s metabolism totally changed.”
The advancement has planted many seeds for future research, Toyota said. For instance, researchers “have no idea” why only two specific green leafy volatiles could enter the stomata and trigger the calcium signaling. The next step is to identify the various receptors in the plants, which may be specific to the chemical structure of the two compounds.
In response to insect attacks, plants can also produce specific responses based on the species of the herbivore feeding on it, an impressive behavior that Kessler is further studying.
“If that plant can mount an adaptive response … this is the definition of intelligence,” Kessler said. “If you understand these kinds of things and how plants do it, it gets you onto a level that questions how we understand the world.”
This article is part of Hidden Planet, a column that explores wondrous, unexpected and offbeat science of our planet and beyond.
