Signaling molecule involved in rapid leaf movement

Plants do not possess nerves and muscles that enable rapid movement in animals. However, Mimosa pudica, commonly called touch-me-not, shame or sensitive plant, moves its leaves by bending the motor organ “pulvinus” immediately in response to touch and wound. Since the era of Charles Darwin, this spectacular leaf movement has been studied. However, the long-distance signaling molecules that trigger the rapid leaf movements and the physiological roles of this movement remain unexplored.

The research team revealed what signals travel long distances and trigger rapid movements in Mimosa pudica and why Mimosa pudica moves its leaves immediately. The research is published in Nature Communications.

“To clarify the long-distance signals and physiological functions of the rapid leaf movements, we created transgenic ‘fluorescent’ and ‘immotile’ Mimosa pudica,” says the author. The videos demonstrate that bursts of fluorescence travel rapidly throughout the leaves and trigger leaf movements. The fluorescent light tracks the cytosolic calcium in real time.

Mimosa pudica closes its leaves just 0.1 seconds after the arrival of the Ca2+ signals in the motor organ pulvini,” the author adds.

Previous studies have suggested that electrical signals, such as an action potential, are critical for the rapid leaf movements in Mimosa pudica.

“We developed a simultaneous recording system for the cytosolic Ca2+ and electrical signals to reveal the spatiotemporal relationship between these signals,” says the author. Upon wounding the leaf, the Ca2+ and electrical signals propagated systemically at similar speeds and passed through the recording site at a similar time. Therefore, the long-distance Ca2+ and electrical signals were spatiotemporally coupled in Mimosa pudica.

Pretreating Mimosa pudica leaves with the Ca2+ channel inhibitors, La3+ and verapamil, and the Ca2+ chelating reagent, EGTA, blocked both Ca2+/electrical signals and the leaf movements in response to the wound. These data support the idea that Ca2+ acts as a long-distance signaling molecule that triggers rapid leaf movements in Mimosa pudica.

Mimosa pudica is one of the most famous plants due to its spectacular movements,” says the author. “However, although there are many hypotheses for the physiological functions of the rapid leaf movements, why Mimosa pudica moves its leaves has not been scientifically elucidated.”

Using the CRISPR/Cas9 genome editing technique, the team of scientists created an “immotile” elp1b mutant lacking motor organ pulvini. They compared the wild-type motile Mimosa pudica and the genetically and pharmacologically immotile Mimosa pudica and discovered that herbivorous insects, such as grasshoppers, consumed these immotile leaves more than wild-type leaves.

They also visualized the Ca2+ signals, the leaf movements, and the behavior of a grasshopper on the leaf under a microscope. Upon feeding by the grasshopper, the leaflets moved sequentially in parallel with the propagation of the Ca2+ signals, and thereafter, the grasshopper stopped the feeding and moved away.

“We finally obtained evidence that rapid movements based on propagating Ca2+ and electrical signals protect Mimosa pudica from insect attacks,” says the author. “Plants possess various communication systems that are normally hidden from view; seeing is believing,” the author adds.