Researchers said experiments showed that the Venus flytrap's closure is initiated by a rapid softening of the cell walls in the outer layer of the plant's trap, which is a highly modified leaf divided into two hinged lobes that resemble jaws with teeth.
For more than a century, the prevailing hypothesis had been that the trap's closure was driven by a rapid redistribution of water within the leaf, with water moving between cells to swell one side of the leaf. The new research points to a different biological mechanism.
"One of the most iconic plants in the world can still surprise us. After more than a century of research, we are still discovering fundamentally new things about how the Venus flytrap works," said physicist Yoël Forterre of the French research agency CNRS and Aix-Marseille University, senior author of the study published on Thursday in the journal Science.
The Venus flytrap is a small carnivorous plant native to a limited region of North Carolina and South Carolina in the United States. Like many carnivorous plants, it grows in nutrient-poor environments and supplements its nutrition by capturing and digesting insects.
In experiments conducted in Marseille, the researchers used high-speed imaging, mechanical measurements by indentation of the plant's outer layer and mechanical modeling. They also measured water transport within the plant tissue to rule out that as the mechanism at play.
"The plant uses specialized trigger hairs located on the inner surface of the trap. When an insect touches these hairs twice within a short period of time, the trap closes. Closure can occur in as little as one tenth of a second," Forterre said.
"Our hypothesis is that the trap is already mechanically loaded before triggering, much like a spring. When the trap is stimulated, the cell walls of the outer epidermal layer rapidly soften by roughly 30 to 40%, meaning that the cell wall becomes more flexible. This releases internal stresses stored in the tissue and causes the trap to bend and close. The softening develops within about one second," Forterre said.
When the trap snaps shut, the insect is sealed inside for digestion.
"By directly measuring the mechanics of the living trap as it responds, we pinned down the internal 'motor' that drives the leaf across its instability threshold and sets off the snap-buckling that closes it," said physicist and study lead author Jeongeun Ryu, who worked on the study as a postdoctoral researcher at the CNRS and Aix-Marseille University.
After the plant absorbs the nutrient-rich liquid produced by the digestive processes, the trap reopens, with the insect's empty exoskeleton left behind.
"What I find remarkable is that evolution often does not invent entirely new mechanisms, but rather reuses and refines existing ones. Plants are known to modify the mechanical properties of their cell walls during growth, but the Venus flytrap appears to push this mechanism to an extreme, using it on a timescale of about one second," Forterre said.
There are roughly 800 known species of carnivorous plants. They are not all closely related, indicating that flesh-eating evolved independently multiple times during plant evolution.
How the Venus flytrap snaps shut is a topic that has long interested scientists including Charles Darwin, the 19th century naturalist who advanced the theory of evolution by natural selection. The researchers see potential practical applications from their findings.
"To our knowledge, this is the first time such a rapid change in the mechanical properties of cell walls has been seen in a plant," Ryu said.
"It settles a question that goes back to Darwin - what drives one of the fastest movements in the plant kingdom - and points to a new way for a living thing to move: not by pumping fluid or simply collapsing, but by actively tuning the stiffness of its own material. That principle could eventually inspire soft robots or smart materials, though that remains a longer-term prospect," Ryu said.
