It “remembers” the first touch of a sensory hair – if a second touch occurs within 30 seconds, the Venus flytrap snaps shut. How can the plant “remember” the first stimulus without a nervous system? Researchers have now literally shed light on this question through genetically modified versions of the carnivores: A fluorescent marker makes it clear that calcium ions are released when touched and form memory via a kind of clock function.
While other plants peacefully stretch their leaves into the light, they hunt for insects: the Venus flytrap (Dionaea muscipula) is probably one of the most bizarre plants in the world. In order to supply itself with additional nutrients in its barren habitat in North America, it has leaf tips that have been converted into trap-like traps. When prey crawls into these bizarre structures, they close in a flash and then break down the prey with digestive juices. Researchers have been investigating the mechanisms that make this fascinating system possible for 200 years – but there are still secrets to uncover.
Retention without a nervous system
With the naked eye it can be seen that the traps are triggered by small sensory hairs. Three of them sit on the reddish inside of each of the two flaps. In order to start the mechanism, a prey must touch these triggers twice within 30 seconds. This prevents the organs from becoming tired due to frequent “false alarms”. So it seems clear: If the second stimulus is delayed, the first action potential is deleted from the short-term memory of the trap. But how is this complex memory possible? An international team of researchers has now succeeded in proving that plant memory is based on the release of calcium ions, which mediate a kind of clock function.
The new insights were made possible by the development of a process for the production of genetically modified Venus flytraps. The researchers succeeded in developing lines of the plants that carry a gene that ensures the production of a calcium sensor in the traps. It is a protein that causes a glowing fluorescence effect when the calcium concentration in the tissue increases. “The production of these test plants was the decisive step towards testing our hypothesis of the calcium clock,” explains co-author Rainer Hedrich from the University of Würzburg, who has already uncovered some of the secrets of the fascinating plant in recent years.
A calcium clock forms the memory
The experiments with the sensor-equipped Venus flytraps showed that if a sensory hair is touched, a glowing wave spreads over the entire trap. This thus documents that the stimulus leads to a sudden increase in the calcium level in the cells. As is typical for a wave, it subsides: after a minute, the calcium content approaches the starting level again, the experiments show. If a second stimulus follows within 30 seconds of the first stimulus, calcium ions are released again, in addition to the ones already present. As a result, a threshold is exceeded, which sets calcium-dependent processes in motion, which in turn close the trap, the scientists explain.
“The electrical excitation of the trap cells is translated into an increase in the concentration of calcium. This means that the passing action potential is stored in the electrically excited trap cells. If another action potential occurs, its calcium value is added to the first signal. “Our findings thus show that short-term memory and the ability to count to two are actually based on the calcium clock,” summarizes Hedrich.
How do the traps count?
However, it is known from previous research results that the Venus flytrap can count not only to two, but to five. It does this when the insect is already trapped but can still move in the trap. Only when the victim touches the sensory hair again, the further “eating processes” are set in motion: From the fifth electrical stimulation, digestive enzymes are released to decompose the prey, and transport proteins are also generated through which the plant absorbs the nutrients can.
The researchers now want to clarify in further studies to what extent the calcium clock is also responsible for this further counting. Below is the question of whether other cells come into action with signal number three, four or five than before with one and two, which triggered the folding mechanism. “We also want to know how the various calcium-dependent processes are triggered after the respective calcium threshold has been exceeded,” says Hedrich.
Source: National Institutes of Natural Sciences, University of Würzburg, specialist article: Nature Plants, doi: 10.1038 / s41477-020-00773-1