Once again, what is probably the most bizarre plant in the world amazes with its sophistication: the Venus flytrap has also adapted its trap system to the increased risk of bush fires in its homeland, researchers report. The sensory hairs, which react to being touched by flylegs and the like, also have a “fire alarm”: if there is a sudden increase in temperature, the special heat receptors in the hair are activated and trigger the traps to close. In this state, the sensitive system can survive a short wave of fire, the experiments show.
It even became a popular houseplant, because the Venus flytrap (Dionaea muscipula) can create excitement like no other plant: its red "mouths" snap shut when an insect steps on them or you tickle them with a feather. The researchers led by Rainer Hedrich from the University of Würzburg have already impressively documented how amazingly sophisticated this mechanism is: when flies and the like crawl into the trap, they come into contact with sensory hairs there. The subsequent reactions are then based on electrical impulses – similar to our nerves. In addition, the Venus flytrap can even count: in order to trigger the snapping, a prey animal has to touch the sensory hair twice within 30 seconds. This is how the carnivores prevent their expensive traps from wearing out due to false alarms.
Shut up if there's a fire
Today's resident of many a window sill originally comes from North and South Carolina. Their hunger for meat was born out of necessity: they grow on very nutrient-poor swampy soil. For them, flies and the like fulfill the function of fertilizer tablets, so to speak. However, as Hedrich and his colleague Shouguang Huang report, the plants have another challenge to contend with in their traditional habitat: grass grows there and dries up in summer. It is then often ignited by lightning strikes and a small firestorm then rushes through the landscape. The Venus flytrap even benefits from this, because then the disturbing undergrowth is removed and apparently the fire waves hardly seem to harm the plants. But how does the plant protect its folding traps with the sensitive sensory hair from the fire?
The researchers are now investigating this question experimentally: "In order to find out how the flytrap behaves when a covering of dry grass burns off, we transplanted plants with open snap traps from the greenhouse into the field and covered them with hay," reports Hedrich. "Then we lit the hay at one end and used a blower to force it to spread to the other end." It was found that the fire had caused all the plants to close their traps and many subsequently emerged unharmed. This was later confirmed: the traps opened again and were fully functional. "So the question arose as to whether, in the event of a fire, the traps might already be reacting to the heat wave in the run-up to the fire," says Hedrich.
Sensor triggers on rapid heating
A first attempt with a hot air gun confirmed this assumption: the sudden heating actually triggered the folding mechanism. Next, the scientists conducted heat experiments in the laboratory. Using a special device, they exposed the traps to different temperatures and at the same time examined the electrical voltage potentials. This showed that when a local temperature of 37 degrees Celsius is exceeded, an action potential is formed that spreads across both halves of the trap. "If the temperature increased further to 55 degrees Celsius, a second action potential was then triggered and the trap snapped shut," reports Shouguang.
What was special about it was that the trap's reaction at 37 and 55 degrees Celsius only started when the temperatures rose sharply - as in the case of a rapid heat wave in the wake of an approaching fire. If, on the other hand, the temperature rose only slowly, as on hot summer days, the traps did not react. Apparently, the plant has a complex heat sensor system. "In contrast to humans, however, in the carnivorous plant it does not start when a specific body temperature is exceeded, but reacts to the speed of the temperature change," says Hedrich.
As the researchers report, their investigations also showed that the heat receptors, like the touch sensors, are located in the sensory hairs of the traps. The temperature jumps cause the same calcium-dependent electrical events there as the touches. "To track the calcium signal, we used flytraps that carry a genetically encoded calcium sensor," says Hedrich. When the cellular calcium level increases, this sensor begins to fluoresce. “We were amazed that the sensory hair glowed first when it was hot. "This shows that the hair operates as a touch and heat sensor at the same time," explains Hedrich.
As the researchers emphasize, this is an extremely exciting phenomenon from a biological point of view: the results suggest that it is a kind of membrane-bound temperature sensor system that is previously unknown in plants. Therefore, the thermal sensitivity of what is perhaps the strangest plant in the world will continue to be the focus of the Würzburg researchers.
Source: University of Würzburg, specialist article: Current Biology, doi: 10.1016/j.cub.2023.07.069