Stretching and Loosening

A region of an onion cell wall was imaged at several time points using atomic force microscopy, resulting in the image at right showing microfibrils in purple. Simultaneous nanomechanical mapping revealed changes in tensile stresses, which were borne by both microfibrils and the matrix in which they were embedded, as shown at left for microfibrils (red: high tension, blue: low tension; scale bar: 500 nanometers).

Tian Zhang

The cell walls of plants are made up of tiny fibers three to five nanometers wide, which are embedded in a network of polysaccharides, a type of carbohydrate. When plants (and their cells) grow, these fibers, called microfibrils, must move and rearrange in order for the cell wall to expand. Visualizing the dynamics of this process has been a challenge, since we don’t yet have the technology to watch cell walls expand at such a small scale in living plants. However, researchers can mimic cell wall expansion by stretching thin sections of plant cell walls under a microscope. A new study led by biologists Daniel Cosgrove and Tian Zhang at Penn State University shows that the microfibrils move in different ways depending on what method is used to stretch cell walls.

Cosgrove and his team employed atomic force microscopy (AFM), which uses a needle to probe the surface of the material being analyzed in order to create a high-resolution image of the surface. Their study is the first report of AFM being used to observe microfibril movements in expanding cell walls. An advantage to using AFM is that the cell walls do not have to be dried, as is required for other types of high-power microscopy. Keeping cell walls hydrated allows them to behave more like they would in a living plant.

Cosgrove and his team mimicked growth by stretching the cell walls both mechanically, by clamping and physically pulling on cell walls, and chemically, using an enzyme called endoglucanase that relaxes the microfibrils and causes them to expand. This second method is thought to more closely mimic the effects of the hormone auxin, which induces wall loosening and growth in living plant cells.

By tracking the positions of specific microfibrils, the researchers found that the movement patterns in response to the enzyme differed from the movement patterns resulting from mechanical stretching. The different responses elicited by these techniques suggest that traditional methods of stretching cell walls mechanically to mimic growth may not provide an accurate representation of cell wall growth in nature. Indeed, the researchers propose that models of cell wall growth need to be updated to include both elastic stretching and hormone-mediated loosening. (Nature Plants)