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Close-ups show how hairy fibers move when plants grow

Using a powerful microscope to examine plant cell walls has afforded researchers new insight into the process of plant cell growth at a level of detail previously unseen, a new study suggests.

The researchers, who report their findings in Nature Plants, used an atomic force microscope, which allowed them to take high-resolution images at the nanometer level. This enabled them to watch microfibrils—hair-like fibers made of cellulose that help form the cell walls—and how they responded when researchers stretched the walls in ways that mimic the strains of growth in natural conditions.

“With the help of the atomic force microscope, we can see for the first time the conformation of cellulose microfibrils under hydrated conditions, and how these microfibrils move under different stretching regimes,” says Daniel Cosgrove, professor of biology at Penn State.

“Plant cell-wall growth is the underlying mechanism by which really small seedlings can grow to really large trees…”

“What we saw was a very different pattern of motion than what happens when you loosen the cell wall in a way that mimics plant cell growth. The growth of the cell wall is not simply playing by the rules of normal cell wall mechanics, but in fact there’s another element there—cell wall loosening—that is changing the mechanical behavior of the cell wall into something different.”

Plant growth is guided by how the cell walls grow, according to Cosgrove. Microfibrils and a combination of other types of polysaccharides—or sugar polymers—slowly fill in the plant cell wall and separate from each other, increasing the walls’ surface size.

microfibril mapping illustration
An atomic force microscope allowed researchers a chance to take a much closer look at how microfibrils—hair-like fibers made of cellulose that help form the plant cell walls—moved in the wall. This close-up view may revise the understanding of how plant cell walls grow. (Illustration Credit: Patrick Mansell/Penn State)

“Plant cell-wall growth is the underlying mechanism by which really small seedlings can grow to really large trees,” says Cosgrove.

“It’s the cellular basis for the way leaves expand—and that’s important for photosynthesis—so that everything that we eat, is directly or indirectly dependent on this process.”

Plant cell walls have both elastic—or reversible-stretching—and plastic—or irreversible-stretching—properties that allow the walls to be flexible enough to expand with growth, but strong enough to remain intact. The findings revise previous models of cell wall growth that suggested the wall’s elasticity was the first step in cell wall growth.

“There has been a longstanding debate in the literature about whether the elastic properties of the cell wall are of primary importance for cell growth,” says Cosgrove.

“Some scientists consider the elastic properties as being indicative of the ability for the cell wall to grow. The more it can stretch, the more readily it will grow.”

How plants slow their growth under stress

In the past, researchers only used bulk measurements to test the elasticity of cell walls. “If you just measure elastic properties, then you’ll get an incomplete picture of the ability of that cell wall to grow and the pattern by which it grows,” says Cosgrove.

The researchers also said that microfibrils were once seen as passive reinforcing elements in the material that makes up the cell wall and that they simply underwent passive realignment as the wall grew, says Cosgrove. However, under the microscope, the microfibrils revealed more complex movements, he adds.

Technology—as in the microscope, and also a device that can pull on the cell wall, in this case, the cell wall of an onion plant—was critical in making these observations, according to Cosgrove.

Fiber ‘roadways’ drive how plants take shape

“We’ve never before seen microfibril motions at the nanoscale level,” he says. “And that’s key to understanding what’s happening molecularly in the cell wall.”

The researchers removed a 30 mm long and 5 mm wide strip from the outer layer of a white onion and placed it under the atomic-force microscope. To simulate wall stress and strain, the strips were mounted to a device that could stretch the skin. The microscope took time-lapsed photographs during the stretching process.

The US Department of Energy, the Center for LignoCellulose Structure and Formation, and the Huck Institutes of the Life Sciences supported this work.

Source: Penn State

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