Plants aren’t usually known for moving quickly. The Venus flytrap is an exception: when a prey lands on its blades, they snap shut in a matter of seconds. But how does the plant do this? A study has now discovered the mechanism using sophisticated mechanical and hydraulic measurements: According to this, the cells of the outer epidermis of the trapping leaves suddenly become soft and thereby release previously built-up elastic energy. This could also serve as inspiration for soft robotics.
The Venus flytrap (Dionaea muscipula) uses sweet nectar to attract insects to its trapping leaves. But as soon as the potential prey settles down and touches the fine tactile bristles on the inside of the trapping leaves twice in a row, the trap closes – at a speed that is one of the fastest in the plant kingdom. How the carnivorous plant is able to move so quickly has puzzled science for centuries.
Too fast for hydraulic mechanism
It is already known that the opened traps store elastic energy with their convex shape. When the trap is triggered, they collapse into a concave shape. But what enables the sudden release of energy? In order to uncover the secrets of the Venus flytrap, a team led by Jeongeun Ryu from the University of Aix-Marseille in France carried out numerous sophisticated experiments and measurements. The researchers sometimes cut the traps into narrow strips, sometimes they fixed them in an open state and measured changes at the level of individual cells.
“A long-standing hypothesis proposes that Venus flytrap closure is driven by an osmotically induced flow of water across the leaf thickness, creating differential expansion between the inner and outer surfaces of the trap,” they explain. “To test this mechanism, we directly measured the water transport within the trap leaves using a cell pressure probe.”
But as Ryu and her colleagues discovered, the water moves through the different cell layers much too slowly. If the Venus flytrap relied on this, it would need about 30 seconds for a trap to close. “The closure occurs too quickly to be explained by water transport,” the team reports. “This suggests an independent, non-hydraulic mechanism.”
Softening releases tension
And indeed: Further measurements revealed that the plant uses a different trick instead. As soon as the sensory bristles are touched twice, the cell walls on the outside of the trapping leaves become softer within a few seconds. This causes the outer layer to give slightly – like a bow suddenly loosened. The elastic tension stored in the trap is released and the leaves snap together. “This represents the fastest modulation of wall mechanics that has been described in plants to date,” report the researchers.
It is still unclear how exactly the signals are transmitted from the sensory bristles to the cells of the outer epidermis, so that they only become soft after the second touch. But from the researchers’ point of view, the current findings could also be relevant beyond biology: “Our discovery reveals a form of plant motility that is based on the dynamic adaptation of material properties, and thus provides clues for muscle-free, bio-inspired drive mechanisms,” they write. For example, materials used in soft robotics could be equipped with similar properties.
Source: Jeongeun Ryu (Aix-Marseille University, France) et al., Science, doi: 10.1126/science.aed5051