3025. Nervous System-Like Signaling in Plant Defense

By JoAnn Klein, The New York Times, September 13, 2018

Caterpillar bites triggers defense signals.

Plants have no eyes, no ears, no mouth and no hands. They do not have a brain or a nervous system. Muscles? Forget them. They’re stuck where they started, soaking up the sun and sucking up nutrients from the soil. And yet, when something comes around to eat them, they sense it.

And they fight back.

How is this possible?

“You’ve got to think like a vegetable now,” says Simon Gilroy, a botanist who studies how plants sense and respond to their environments at the University of Wisconsin-Madison.

“Plants are not green animals,” Dr. Gilroy says. “Plants are different, but sometimes they’re remarkably similar to how animals operate.”

To reveal the secret workings of a plant’s threat communication system for a study published Thursday in Science, Masatsugu Toyota (now a professor at Saitama University in Japan) and other researchers in Dr. Gilroy’s lab sent in munching caterpillars like in the video above. They also slashed leaves with scissors.

Calcium spreads to distant leaves when another leaf on the plant is cut.

They applied glutamate, an important neurotransmitter that helps neurons communicate in animals.

How a plant responded when glutamate was applied to one of its leaves.

In these and about a dozen other videos, they used a glowing, green protein to trace calcium and accompanying chemical and electrical messages in the plant. And they watched beneath a microscope as warnings transited through the leafy green appendages, revealing that plants aren’t as passive as they seem.

The messages start at the point of attack, where glutamate initiates a wave of calcium that propagates through the plant’s veins, or plumbing system. The deluge turns on stress hormones and genetic switches that open plant arsenals and prepare the plant to ward off attackers — with no thought or movement.

Like animals, plants are eukaryotes — multicellular organisms — that split from a common ancestor called Luca billions of years ago. To survive, we all sense threats, relay messages about them within our bodies or tissues and respond to these challenges. Our actions vary, adapted for the lifestyles we maintain in different environments, but much of our basic cellular machinery is the same. Biology kept it that way: If it ain’t broke, don’t fix it.

One mechanism our cells share is fluctuating levels of calcium ions, which carry an electrical charge. In humans, this charge assists in controlling when your neurons fire messages. Changes in calcium ions make your heart beat or your muscles contract so you can get up and leave when something threatens you.

Plants, obviously, can’t run away. But researchers knew that genes that make receptors kind of like those for glutamate initiate electrical signals that travel through plants after being wounded. They turn on genes elsewhere in the plant, allowing them to respond.

With the help of glutamate, calcium ions can flow, carrying their signal through channels that open like floodgates when glutamate fits into these special receptor spaces, like keys in locks. These channels aren’t quite the same as those in the mammalian nervous system, but they look very similar and probably worked similarly. They led Dr. Gilroy and his team to look into calcium ion flow.

To make the action visible, the researchers engineered Arabidopsis plants, botany’s lab rat, to make a protein originally from jellyfish that glows green under a microscope. This sensor, in this case, shines brighter when calcium levels increase.

They also made plants that lack the glutamate-like receptor. In these, the fluorescent signal was weak:

When plants were engineered to lack glutamate receptors, they barely registered any calcium waves.

The real surprise was the speed. The plant reacted within a few seconds and transferred information from leaf to leaf in a couple of minutes — as long as they were connected through the vascular system. This is slower than your nervous system, but “for a plant biologist, that is booking it,” Dr. Gilroy said.

The plant also seemed to be able to sense the amount of damage, because when they crushed a leaf, the plant responded all over.:

Wherever the calcium touched, the plant produced jasmonic acid, a defense and stress hormone, which they believe turns on genes that somehow activate a plant’s chemical and physical defenses.

Methyl-jasmonate, a product of jasmonic acid, for instance, floats through the air like a jasmine-scented perfume. But for insects, it can be unappealing or disrupt digestion and deter diners from return visits. Physical defenses may harden a plant’s cell walls, too, making them tough to eat.

“The authors add many pieces to the puzzle of how a localized wound triggers widespread defenses in distal leaves,” said Ted Farmer, a botanist at University of Lausanne in Switzerland who described the electrical wound signal in plants.

But much is still a mystery.

“We’re trying to understand what the machinery is that makes the whole system work,” Dr. Gilroy said.

What isn’t so mysterious is that plants and animals have a lot of the same problems. And while humans can deal with threats, plants can too.

“They may even have to be better than us at sensing the environment because they don’t have the luxury of getting up and leaving,” Dr. Gilroy said.