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Summer 2018

Spotting Tiny Movements When a Plant's Cell Wall Expands

Unraveling how nanoscale fiber movements lead to plant growth potentially benefits biofuels

Mohamadamin Makarem

Microfibrils movement in elastic deformation. Reprinted with permission from Nature Springer: Zheng et al. (see "More Information") The arrows show curves and kinks of microfibrils induced by the wall’s extension. See how regions stretch and move.

Microfibrils movement in enzyme-induced creep. Reprinted with permission from Nature Springer: Zheng et al. (see "More Information") The arrows show curves and kinks of microfibrils induced by the wall’s extension. See how regions stretch and move.

For millennia, plants have been the source of food, clothes, and energy for humans. However, the road toward producing sustainable, clean biofuels from plants is facing many obstacles. The major hindrance on the way to obtaining sufficient renewable energy from plants is determining how to break down the complex plant cell wall into simple sugars that can be converted to biofuel.

Understanding how plant cell walls are made provides the basis for converting the primary component of cell walls -- complex carbohydrates -- into sugars. The Center for Lignocellulose Structure and Formation (CLSF) Energy Frontier Research Center (EFRC) is addressing a number of major questions such as how plants synthesize cellulose, how cellulose interacts with other cell wall polymers, and how these components are organized in the cell wall.

Plant cell wall structure. In growing plants, the cells are surrounded by a tough yet flexible box called the cell wall. This cell wall, which is typically 0.5 to 2 micrometers thick, is composed of the cellulose fibers known as microfibrils embedded in a concoction of polysaccharides and proteins. Polysaccharides are large molecules consisting of multiple sugars attached to each other. In the cell wall structure, the cellulose microfibrils are the strongest and stiffest component and, therefore, of particular mechanical significance. These microfibrils are bonded laterally into sheets, and these sheets are stacked to form a multilayered, composite-like structure.

Cell wall expansion. This tough composite material has to elongate and expand during plant cell growth. The high internal pressure inside growing plant cells (turgor pressure) stretches the cell wall and provides the physical energy for expanding the cell wall after it is “loosened.” A highly controlled loosening process is needed to avoid cell wall rupture due to expansion. Weakening or cutting specific linkages between polymers in the cell wall may be the key to selective wall loosening and expansion. Such loosening decreases the tensile stress in the plane of the wall in a controlled manner. This relaxes cell turgor pressure, and as the cell takes up water, the relaxed wall expands. The load-bearing linkages within the cell wall may be cut by specific enzymes, called endoglucanases.

Nanoscale movement of fibrils during growth. To identify how cellulose microfibrils behave during growth, researchers from CLSF developed a micro-stress testing machine that fit on the stage of an atomic force microscope. This apparatus allowed the imaging of the cell wall while it underwent a series of mechanical stretching events to characterize the change in the arrangement and movement of the cellulose microfibrils with different kinds of stretches.

Microfibril movements during elastic deformation, plastic deformation, and enzyme-induced creep were monitored. Elastic deformation means the cell wall can return to its initial length as the stretching force is removed, similar to how balloons -- when deflated -- go back to their initial size. By contrast, the cell wall remains stretched irreversibly after plastic deformation, like a stretched piece of chewing gum which cannot go back to its initial state. During these deformations, cell wall elongation was accompanied by shrinkage in width and resulted in passive reorientation of microfibrils toward the direction of elongation. Such passive reorientation was expected from the multinet growth hypothesis, a concept in which microfibrils are predicted to reorient in the direction of cell elongation during growth. In contrast to the results observed with elastic and plastic stretching, the wall extension induced by endoglucanase enzyme resulted in extension in both length and width. Moreover, no significant reorientation of microfibrils was seen by atomic force microscopy. These results show that microfibril movements are different in response to applied physical forces versus the action of wall loosening. This conclusion, in turn, means that simple models of cell wall growth based only on polymer mechanics and simulations are missing a key aspect of cell wall growth.

In addition, atomic force microscopy images show that groups of two to five microfibrils form bundles, and such bundles were not substantially disrupted by mechanical stretching. Also, microfibrils that were oriented transverse to the direction of passive stretching became bent or kinked by mechanical stretching. These kinks happen every 200 to 300 nanometers and were reduced after the endoglucanase was added to the cell wall, strengthening the idea that there are load-bearing junctions that are spaced at these intervals.

By mimicking the growth of the plant cell wall using a series of mechanical extensions combined with enzyme treatment, this study recorded nanoscale movement of microfibrils. The results obtained by the atomic force microscope shed light on how cellulose microfibrils move during cell wall growth and challenge the conventional models on the reorientation of microfibrils in plants during growth. By understanding the growth process of cells in plants, more informed strategies can be laid out to decompose cell wall polymers to biofuels in an efficient way.

Acknowledgments

This work was supported as part of the Center for Lignocellulose Structure and Formation, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences. D.V. was supported by the National Institutes of Health.

More Information

Zhang T, D Vavylonis, DM Durachko, and DJ Cosgrove. 2017. “Nanoscale Movements of Cellulose Microfibrils in Primary Cell Walls.” Nature Plants 3:17056. DOI: 10.1038/nplants.2017.56

About the author(s):

  • Mohamadamin Makarem is a Ph.D. candidate in chemical engineering at Pennsylvania State University. He is a member of the Center for Lignocellulose Structure and Formation, an Energy Frontier Research Center. His research focuses on studying the orientation and packing of cellulose microfibrils in plants using sum frequency generation vibrational spectroscopy.

Stretching Out to Understand the Bane of Biofuels: Cellulose Fibers

Tiny threads complicate biofuel production, but insights into how they move when a plant grows could simplify matters

As a plant grows, its cellulose fibers (its protective armor) stretch taller and wider. Image courtesy of Nathan Johnson, Pacific Northwest National Laboratory

It’s a common sight in preschools: little trays of potting soil are balanced on the windowsill with tiny green shoots sticking up. While fragile looking, these seedlings survive many little hands tugging and pulling. Tough fibers made of cellulose keep the plants in shape. These same fibers are a problem to scientists converting switchgrass and other plants to fuels. Researchers at the Center for Lignocellulose Structure and Formation (CLSF) are learning how these fibers behave during plant growth. They found that when stretched without adding any enzymes, such as by a toddler’s hand tugging on a leaf, the cellulose fibers line up in the same direction. But that’s not exactly what happens when a plant grows. Understanding cell wall growth can unveil what modifications in plant cells are needed to improve the productivity of cellulose fibers. Also, how the loosening of cell wall happens during growth can give us clues on finding better ways to untangle cellulose fibers from one another and decompose them for biofuel production. CLSF is led by Pennsylvania State University.

More Information

Zhang T, D Vavylonis, DM Durachko, and DJ Cosgrove. 2017. “Nanoscale Movements of Cellulose Microfibrils in Primary Cell Walls.” Nature Plants 3:17056. DOI: 10.1038/nplants.2017.56

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.