How Venus Flytraps also Snap

Venus flytraps are known for the fact that their catching leaves close in a flash when unsuspecting prey touch highly sensitive trigger hairs twice in a row. A team of researchers from ETH Zurich and the University of Zurich has now discovered a new snap mechanism.
The force sensor of the microrobotic system deflects a sensory hair of a flytrap that is kept open by the sensors of the load cell. (Image: Hannes Vogler / UZH)

The Venus flytrap (Dionaea muscipula) is perhaps the most well-known carnivorous plant. It catches its prey, mostly spiders and insects, using a sophisticated trapping mechanism. Its distinct leaves have three highly sensitive trigger hairs on each lobe. These hairs react to even the slightest touches – e.g. when a fly crawls along the leaf – by sending out an electrical signal, which quickly spreads across the entire leaf. If two signals are triggered within a short time, the trap snaps within milliseconds.

New trigger for trapping mechanism

The physiological reactions on which this trapping mechanism is based have been studied for over 200 years. The consensus has been that every sufficiently strong touch of a trigger hair causes an electrical signal, and that two signals within 30 seconds result in the closing of the trap.

Researchers at ETH Zurich, together with colleagues from the Department of Plant and Microbial Biology at the University of Zurich (UZH) have now found another triggering mechanism. Contrary to popular belief, slowly touching a trigger hair only once can also cause two signals and thus lead to the snapping of the trap. The researchers report this in a joint study that just appeared in the journal PLOS Biology.

Using microrobotics and a model

First, the interdisciplinary team of researchers determined the forces needed to trigger the plant’s trapping mechanism. They did this by using highly sensitive sensors and high-precision microrobotic systems developed by the team of co-last author Bradley Nelson at the Institute of Robotics and Intelligent Systems at ETH Zurich. This enabled the scientists to deflect the trigger hairs to a precise angle at a pre-defined speed in order to measure the relevant forces. These experiments confirmed the previous theory. If the chosen parameters approximate the touch of regular prey, it takes two touches for the trap to snap.

From the collected data, the researchers at the ETH Institute for Building Materials developed a mathematical model to determine the range of angular deflection and velocity thresholds that activate the snapping mechanism. “Interestingly, the model showed that at slower angular velocities one touch resulted in two electrical signals, such that the trap ought to snap,” says co-last author Ueli Grossniklaus, director of the Department of Plant and Microbial Biology at UZH. The researchers were subsequently able to confirm the model’s prediction in experiments.

Deflection of a sensory hair as viewed from the two cameras with simultaneous indication of the position of the force sensor and the measured force. (Video: Jan Burri / ETH Zurich)

Catching slow prey

When open, the lobes of the Venus flytrap’s leaves are bent outwards and under strain – like a taut spring. The trigger signal leads to a minute change in the leaves’ curvature, which makes the trap snap instantaneously. The electrical signals are generated by ion channels in the cell membrane, which transport atoms out of and into the cell.

The researchers think that the ion channels stay open for as long as the membrane is mechanically stretched. If the deflection occurs slowly, the flow of ions is enough to trigger several signals, which causes the trap to close. The newly discovered triggering mechanism could be a way for the Venus flytrap to catch slow-moving prey, such as larvae or snails.