Video: A grasshopper triggers the folding of the leaflets of a mimosa. Using fluorescence images, researchers were able to show the role of calcium ions in signal transmission. © Masatsugu Toyota/Saitama University
On the trail of proverbial sensitivity: Using fluorescence markers, researchers have literally shed light on the mechanisms that lead to the rapid folding of the mimosa's pinnae. The system is therefore based on the signaling function of calcium ions and changes in the electrical potential. After a stimulus, the signals spread through the plant and cause the movements in the leaf joints. The researchers were also able to clarify the purpose of the effect: grasshoppers ate only half as much from folded mimosa leaves as from artificially immobilized versions.
It is famous for its sensitivity and its amazingly fast reaction times for plants: If you touch a mimosa (Mimosa pudica), within seconds it folds the pinnate subunits of its leaves together so that only thin structures remain. Similarly rapid movements are known from the catching organs of carnivorous plants such as the Venus flytrap. But the mimosa has nothing to do with these rabid plants - it is a peaceful representative of the legumes. Although the basic functions of their movement system are known and there are also assumptions about its purpose, there have been no more detailed studies on it so far. That is why a Japanese team of scientists has now examined the prominent plant using modern research methods.
Earlier investigations had provided indications that electrical potential changes and calcium ions (Ca2+) have a signaling function in the movement system. In order to follow this trail, the scientists have developed a special line of research into the mimosa: they carry a gene that is responsible for the production of a sensor. It is a protein that causes a glowing fluorescent effect when the concentration of calcium ions in tissue increases. In this way, the researchers explain that these ions can be tracked in real time using the fluorescent light recorded with special cameras. At the same time, they also measured the potential changes via electrodes on the leaves.
signals on the trail
The simultaneous detection of increases in the concentration of calcium ions in the cell fluid and the electrical signals now documented: After stimulating a leaf, the increase in calcium ion concentrations and the changes in potential propagate systemically with a similar speed - they are spatially and temporally coupled. The impulse spreads from the stimulus point at about 1.31 millimeters per second. He then meets the so-called pulvini. These are articulated elements at the base of each pair of leaflets.
The signal then triggers a water shift in the cells of these pulvini, which leads to pressure changes in the joints, the researchers explain. As a result, the leaflets bend and close together with the counterpart on the other side, which also reacts. The signal then transmits to subsequent pulvini. The calcium ions apparently play a fundamental role in the system, as additional results made clear: When the researchers treated the leaves of their test plants with substances that impeded the functions of the ions, there was no signal transduction and leaf movements also stopped. However, the functional principle has still not been finally clarified, the researchers write: The molecular mechanisms responsible for the changes in the concentration of calcium ions and the electrical signals remain unclear.
Protection against insect damage
In the second part of the study, the team then pursued a more practical question: What purpose does the movement system serve for the plant? The reaction was already believed to irritate pests or spoil their appetite. In order to check to what extent this is true, the scientists again developed a special line of research into mimosa. In this case, they had switched off a gene that is necessary for the pulvini's locomotor function. This mutant therefore maintains the spread position of its leaflets after challenge. The scientists placed grasshoppers on these test plants and on "normal" controls and recorded how much leaf mass the insects consumed.
The researchers report that the control plants that reacted to the pests by closing their leaves only lost half as much leaf mass as the immobilized mimosa. "The bottom line is that we have now been able to show that the rapid movements based on the propagation of calcium ions and electrical signals protect the mimosa from insect attacks," says senior author Masatsugu Toyota from Saitama University.
Source: Saitama University, professional article: Nature Communications, doi: 10.1038/s41467-022-34106-x