Scientists have studied how plant “muscles” quickly fold a mimosa leaf

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Call them plant engines. Or plant muscles. The mimosa plant’s tiny bulges of specialized cells can fold their feathery leaves together in seconds, then relax — and do it again.

A new look at these bulges on the plant Mimosa pudica revealed more details about how the leaf manages its extremely rapid folding, says biomechanist David Sleboda of the University of California, Irvine. “I think these particular organs are really cool because their movement is reversible,” he says. “When people see the reversible movement of plants, they more closely resemble the movement of animals.”

Scientists have already worked out the basic chemistry that powers the mimosa’s little engine, or pulvinus, he and his colleagues write in a paper due Feb. 6 Current Biology . When a deer’s hoof or something else scary pushes a leaf, potassium ions and some other ions move from one part of the pulvin to another. Water follows the ion tick. Cells that lose water swell and sag, while cells on the other side swell. Deformations in many pulvini cause the halves of the feathery leaf to fold together like an invisible hand that neatly closes a book.

Drawing of a folded mimosa leaf in side view showing the muscle-like structures of the pulvinus (orange), where chemistry creates temporary micro-flushes of water that allow some cells to inflate and others to sag, thus pushing parts of the leaf into place. DA SLEBODA ET AL / CURRENT BIOLOGY 2023

Instead of studying the chemistry, Sleboda and his colleagues looked at the microscopic structural details in the pulvinus cells that help create such useful distortions, he reported Jan. 7 at the annual meeting of the Society for Integrative and Comparative Biology in Austin, Texas. One of the features that make plant muscle cells swell more efficiently is the strengthening of microscopic fibrils. They work like corsets, preventing cells from protruding in all directions. Instead, the corset directs most of the swelling along an axis that bends the leaf halves.

In addition, the pulvinus cells, which must expand rapidly, have what appear to be folds of tissue that expand easily to allow water to enter, as well as special highly porous areas called pit fields. The pits look like water could easily seep through them in a leaf-tickling emergency. The cell structure itself appears to be specialized for expansion and contraction. A cross-section of the pulvina reveals a pattern “similar to concertina bellows,” Sleboda said.

Enlarged image of matted fibers called fibrils of the mimosa pulvinus plant.
Matt fibers called fibrils (enlarged) on some sides of the cell in the tiny muscle pulvin of the mimosa plant can make the cell more useful. When the chemical composition of the muscle cells changes and water enters the cell, the fibers reduce swelling in useless directions. Instead, the cell inflates more in directions that are likely to facilitate useful movements, such as folding. DA SLEBODA ET AL / CURRENT BIOLOGY 2023
Magnified image of the muscle-like cells of the pulvinus, which resemble empty honeycombs.  Some interior surfaces have opaque gray dots.
Opening for inspection some muscle-like cells of the pulvinus, like looking into empty honeycombs, you can discover another feature that promotes the movement of water. Some interior surfaces have particularly porous spots that look like ghostly gray dots. Called pit fields, they accelerate the surge of water through the cell wall as the cell changes shape to move its share of the load. DA SLEBODA ET AL / CURRENT BIOLOGY 2023

Widespread M. pudica , or sensitive plant, is one of the most famous leaf benders. However, clusters of other plants from the same family, the legumes, also move their leaves, says botanist Tainara Policarpo Mendez of the Universidade Estadual Paulista in Botucatu, Brazil. Some relatives close quickly, like M. pudica , but much slower. But she also thinks about why she leaves close at all. People have suggested a number of benefits: preventing animals from grazing on the plant, which suddenly becomes more stick-like, or even helping the plant lose less heat on very cold nights.

Sleboda can also refuse the proposed hypotheses, but is skeptical about them. “There’s not a ton of research,” he says. This, however, suits him. “I like the fact that sensitive plants close their leaves,” he says, “that we don’t know why they do it.”

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