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New research shows that goldenrod plants demonstrate a form of intelligence by adapting their responses to herbivores based on the presence of neighboring plants and environmental cues, challenging traditional definitions of intelligence.

Goldenrod can perceive other nearby plants without ever touching them by sensing the ratios of far red light reflected from the leaves. When goldenrod is eaten by herbivores, it adapts its response based on whether or not another plant is nearby. Is this kind of flexible, real-time adaptive response a sign of intelligence in plants?

It’s not an easy question to answer, but Andre Kessler, a chemical ecologist, makes a case for plant intelligence in a recent journal article Plant signaling and behavior.

Determining intelligence in plants

“There are more than 70 definitions being published for intelligence, and there is no agreement on what it is, even within a given field,” said Kessler, a professor in the Department of Ecology and Evolutionary Biology in the College of Agriculture and Life Sciences.

Many people believe that intelligence requires a central nervous system, with electrical signals acting as an information processing medium. Some plant biologists equate the vascular systems of plants with the central nervous system and suggest that some sort of centralized entity within the plant allows them to process information and respond. But Kessler strongly disagrees with this idea.

Goldenrod plant.

“There is no good evidence for any of the homologies with the nervous system, although we clearly see electrical signals in plants, but the question is how important is this signaling to the plant’s ability to process environmental signals?” he said.

To make their case for plant intelligence, Kessler and co-author Michael Muller, a doctoral student in his lab, narrowed their definition down to its most basic elements: “The ability to solve problems based on information you get from the environment toward a specific goal.” , Kessler said.

As a case study, Kessler points to his earlier research examining goldenrod and its responses when eaten by pests. When leaf beetle larvae eat goldenrod leaves, the plant emits a chemical that informs the insect that the plant is damaged and a poor food source. These airborne chemicals, called volatile organic compounds (VOCs), are also taken up by neighboring goldenrod plants, prompting them to produce their own defenses against the beetle larvae. In this way, goldenrods move the herbivores onto neighbors and spread the damage.

Experiments and observations

In a 2022 article in the journal plants, Kessler and co-author Alexander Chautá, Ph.D. ’21, conducted experiments to show that goldenrod can also perceive higher ratios of far-red light reflected from the leaves of neighboring plants. When there are neighbors and goldenrods are eaten by beetles, they invest more in tolerating the herbivore by growing faster, but also start producing defensive compounds that help the plants fight off insect pests. When there are no neighbors, the plants do not resort to accelerated growth when eaten and the chemical responses to herbivores are significantly different, although they still tolerate fairly large amounts of herbivores.

“That would fit our definition of intelligence,” Kessler said. “Depending on the information it receives from the environment, the plant changes its default behavior.”

The neighboring goldenrod also shows intelligence when it perceives VOCs that signal the presence of a pest. “Volatile emissions coming from a neighbor predict future herbivory,” Kessler said. “They can use a cue from the environment to predict a future situation and then act on it.”

Applying the concept of intelligence to plants could inspire new hypotheses about the mechanisms and functions of plant chemical communication, while also changing people’s thinking about what intelligence actually means, Kessler said.

The last idea is timely, because AI is a current topic of interest. For example, he said, artificial intelligence doesn’t solve problems toward a goal, at least not yet. “Artificial intelligence, by our definition of intelligence, is not even intelligent,” he said. Instead, it is based on the patterns it identifies in the information it accesses.

One idea that interested Kessler came from mathematicians in the 1920s who suggested that perhaps plants function more like hives. In this case, each cell works as a separate bee, and the whole plant is analogous to a hive.

“This means that the brain in the plant is the whole plant without the need for central coordination,” Kessler said.

Instead of electrical signaling, there is chemical signaling throughout the superorganism. Studies by other researchers have shown that each plant cell has broad light spectrum perception and sensor molecules to detect very specific volatile compounds coming from neighboring plants.

“They can sniff out the environment very precisely; every single cell can do it, as far as we know,” he said. Cells may be specialized, but they all sense the same things and communicate through chemical signaling to trigger a collective response in growth or metabolism. “This idea is very attractive to me,” he said.

Reference: “Induced Herbivore Resistance and the Smart Plant” by Andre Kessler and Michael B. Muller, 30 Apr. 2024, Plant signaling and behavior.
DOI: 10.1080/15592324.2024.2345985

The paper was supported by a grant from the New Phytologist Fund.