Saitama, Japan: Plants lack the nerves and muscles that allow animals to move quickly. Mimosa pudica, commonly referred to as the touch-me-not, shame, or sensitive plant, moves its leaves by immediately flexing the motor organ “pulvinus” in response to touch and wound. This spectacular leaf movement has been studied since the era of Charles Darwin. However, the long-distance signaling molecules that trigger the rapid leaf movements and the physiological role of this movement remain unexplored.
The team, led by Professor Masatsugu Toyota (Saitama University, Japan), revealed which signals in Mimosa pudica travel long distances and trigger fast movements, and why mimosa pudica moves its leaves instantly.
The research will be published in Nature Communications on November 14. Takuma Hagihara led the work as a graduate student in Toyota’s lab, collaborating with researchers from Hasebe’s lab at the National Institute for Basic Biology, Japan.
“To clarify the long-range signals and physiological functions of rapid leaf movements, we created transgenic ‘fluorescent’ and ‘immobile’ Mimosa pudica,” says Toyota. The videos show that fluorescence bursts propagate rapidly through the leaves and trigger leaf movements (videos #1 and #2). The fluorescent light tracks cytosolic calcium in real time.
“Mimosa pudica closes its leaves just 0.1 seconds after arrival of the ca2+ Signals in the motor organ pulvini,” adds Toyota (video #3).
Previous studies have shown that electrical signals such as an action potential are crucial for the rapid leaf movements in Mimosa Pudica.
“We have developed a simultaneous recording system for cytosolic Ca2+ and electrical signals to uncover the spatio-temporal relationship between these signals,” says Toyota. When twisting the leaf, the ca2+ and electrical signals propagated systematically at similar speeds and passed through the recording site at a similar time (video #4). Therefore, the Fern-Ca2+ and electrical signals have been spatiotemporally coupled in Mimosa pudica.
The pre-treatment of Mimosa Pudica leaves with the Ca2+ Channel Inhibitors, The3+ and verapamil, and the ca2+ Cheating agent, 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 because of its spectacular movements,” says Toyota. “Although there are many hypotheses about the physiological functions of the rapid leaf movements, it is not scientifically understood why Mimosa pudica moves its leaves.”
Using the CRISPR/Cas9 genome editing technique, Toyota’s team of scientists created an “immotile” elp1b mutant that lacked the motor organ pulvini. They compared wild-type motile Mimosa pudica to 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, leaf movements and behavior of a grasshopper on the leaf under the microscope. When the locust fed, the leaflets moved sequentially parallel to the propagation of the Ca2+ signals and then the locust stopped feeding and moved away (videos #5 and #6).
“We finally got evidence that rapid motion based on the spread of Ca2+ and electrical signals protect mimosa pudica from insect attacks,” says Toyota. “Plants possess various communication systems that are not normally visible; Seeing is believing,” he adds.
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