Electric ‘Muscle’ Found in Sensitive Plant

If ‘muscle’ is loosely defined as a system that powers movement,
the sensitive plant qualifies as a bodybuilder

– Mimosa: A plant that can flex its muscles with prowess! –

I had heard about the “sensitive plant” (Mimosa pudica) years before I saw one in Fiji in 1986. Now it is 36 years later, and to this day scientists have not figured out many things about how it works. The leaves fold up rapidly along the spine when touched, becoming like stems in just a tenth of a second.

Mimosa is one of very few plants, like the Venus flytrap, capable of rapid motion. It is believed that cells in certain locations undergo water efflux, producing a sudden loss of turgor pressure that results in motion. But what signals them to do this?

“Since the era of Charles Darwin,” a press release says, “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.”

Sensitive plant (Mimosa pudica). Left: before touch. Right: After being touched, the leaves quickly fold into tight bundles that resemble stems. Photos by David Coppedge.

Bursts of fluorescence caught on video reveal how and why the sensitive plant Mimosa pudica moves its leaves rapidly (Saitama University via Phys.org, 14 Nov 2022). Japanese researchers found that electrical signals travel long distances to trigger the reaction.

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 wounds….

“To clarify the long-distance signals and physiological functions of the rapid leaf movements, we created transgenic ‘fluorescent’ and ‘immotile’ Mimosa pudica,” says Toyota. 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,” Toyota adds.

Video clips embedded in the article show the bursts of fluorescence traveling down the vane, triggering the leaflets to close. The fluorescent proteins track the calcium signals moving down the leaf. These signals trigger the motor organs, called pulvini, at the base of the organs to respond. Since calcium ions create an electrical potential, it is accurate to say that the plant’s response is brought about by electricity. This is similar to what happens in our muscles.

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 Toyota. 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.

Does this phenomenon help the plant? One video clip shows a grasshopper eating the leaves losing interest when they close up.

The work was published open-access in the journal Nature Communications.

Hagihara et al, Calcium-mediated rapid movements defend against herbivorous insects in Mimosa pudica. Nature Communications, 14 November 2021.

The Darwin-free paper does not mention evolution or speculate on how this system evolved, other than to speculate that a few genes appear to have originated by whole-genome duplication. This does not explain the origin of the movement, the response of water efflux to the calcium signal, the placement of pulvini at the base of the leaves, the propagation of the signal, and the resetting of the original leaf shape after the threat has passed. If this electrical ‘muscle’ response had not already existed in the genes of some imagined ancestor, a whole-genome duplication by blind copy errors would not create the phenomenon.

So many wonders, so little time! God planned to keep us busy trying to figure out his wisdom.

In the paper, the authors elucidate some of the mysteries about this amazing plant.

Non-wounding stimuli, e.g., mechanical touch, cold shock, or electrical stimulation, applied to the M. pudica leaf, generate a rapid depolarization of the membrane potential, i.e., an action potential (AP), propagating toward the motor organ or pulvinus at the bases of leaflets, rachillae, and petioles. Wounding stimuli, e.g., cutting or burning, generate both APs and a subsequent long-lasting delayed depolarization, named a variation potential (VP). When the electrical signals arrive, the pulvinar cells of the contractile (extensor) side shrink due to water efflux (loss of turgor pressure), instantaneously folding up the leaflets and dropping the petiole downwards. Although numerous studies have assumed physiological roles of these rapid movements, e.g., being unnoticed against the dark ground, startling insects, exposing thorns, and giving the appearance of a less voluminous meal, clear evidence supporting these theories thus far does not exist. Furthermore, many gaps remain in our knowledge of the mechanisms underlying this rapid movement and the plant-wide signal network used to trigger it.