Bursts of calcium ions prompt sensitive plants to fold up in defense

Sensitive plants (Mimosa pudica) quickly fold their many leaflets when touched, burned, or wounded. But the mechanism behind how these plants swiftly turn demure—and why—isn’t clear. New research suggests that a flood of calcium ions kicks off the process to help these touch-me-not plants defend themselves against hungry insects (Nat. Commun. 2022, DOI: 10.1038/s41467-022-34106-x).

“People think that plants are static, insensitive, kind of boring,” says Masatsugu Toyota, a plant physiologist at Saitama University. But plants can sense touch instantaneously, and the sensitive plant shows “fantastic movement” without having neurons, brains, or muscles, he says.

Toyota, Mitsuyasu Hasebe of the National Institute for Basic Biology, and their colleagues wondered how sensitive plants send long-distance signals to trigger leaf folding. To look for clues, they spent about a year and a half engineering sensitive plants to produce a green fluorescent protein–based biosensor, which binds to Ca²⁺ to reveal its presence in plants’ cells. Then the team touched or cut these plants and watched what happened.

When the tip of a leaflet was snipped, the team witnessed a burst of light that traveled down the leaflet’s vein to an organ at its base called the pulvinus that acts like a motor. A Ca²⁺ boost in each pulvinus corresponded with electrical signals, which are also known to factor in folding.

To verify that an influx of Ca²⁺ into cells triggers the folding, the researchers treated leaves with a solution containing lanthanum ions, which block calcium channels, and saw that the plants no longer shied away from touch.

Scientists have suggested that the purpose of the leaves folding could be to startle insects or make the plant look like a smaller meal. To investigate, the team set grasshoppers loose on the lanthanum-treated M. pudica plants and ones genetically modified to turn off their pulvinus organs. These plants got nibbled much more than sensitive plants whose leaves could fold normally. Although it’s likely the leaves do not move fast enough to be startling, the findings suggest that moving leaves do prevent munching by insects.

The sensitive plant has a relatively complex genome, with more sets of chromosomes than typical model plants. “It’s been very difficult to do genetics with them and to make transgenic plants to where we can actually see what’s going on inside them,” says Edward E. Farmer, a plant biologist at the University of Lausanne who wasn’t part of the work. In this study, “they did it rather elegantly.”

There’s still a lot to learn about the process that sets these plants’ leaves in motion. Toyota’s team plans to search for the molecules that are downstream of the calcium signal and that control the water content of pulvinus cells, which determines if leaves extend or fold away.

The plants, which are native to Central and South America, have been studied since the time of Charles Darwin. “It’s a plant that’s been intriguing scientists in Europe, at least for literally hundreds of years,” Farmer says. “Now, we’re on the verge of starting to understand how it how it moves.”