Challenging Conventional Wisdom: Reconstructing Primary Cell Wall Models
For millennia, plants have provided human society with food and protection. Recently, plant biomass, plant material not utilized as either food or feed, began serving as an additional renewable energy source. It is being converted into both solid and liquid fuels, and biogas. Plants hold great promise as an important resource of large-scale bio-renewable energy that will reduce our dependency on fossil fuel if technical obstacles to its use could be overcome.
To take on that obstacle, scientists at the Center for Lignocellulose Structure and Formation (CLSF), a DOE Energy Frontier Research Center, are developing a detailed understanding of the plant cell wall. A better understanding of the plant cell wall structure at the molecular level will form the foundation for significant advancement in utilizing plant biomass in sustainable bio-energy applications.
Plant cell walls are particularly resistant to deconstruction. Determining key structural interactions can provide new insights into choosing effective targets for digestion by enzymes. Enzymes are substances in plants and animals that accelerate chemical reactions. The enzymes can access the fermentable sugar-based molecules, called polysaccharides, in plant cell walls. The polysaccharides are the building blocks for fuels.
The primary, or growing, plant cell wall is mainly composed of polysaccharides. The polysaccharides are built from sugar molecules bonded together. How those polysaccharide components are organized within the cell wall—to provide strength but still allow flexibility for cell expansion and growth—is a key question at CLSF. An essential step to understanding the architecture of primary cell walls is the construction of a reliable molecular model. The model must account for the mechanisms of cell wall growth and strength.
For more than two decades, researchers relied on the tethered-network model. In this model, three polysaccharides work together. Rigid cellulose fibers, called microfibrils, are tethered together by xyloglucan to form an interconnected network. This glued-together network is embedded in another phase formed by pectins. In this model, xyloglucan is an essential structural component of the wall, like glue, and a major wall-loosening target. Recently, the team at CLSF conducted solid-state nuclear magnetic resonance studies, along with other biomechanical and genetic studies. The results have weighed against predictions of the tethered-network model.
The CLSF team studied the structurally and compositionally varied primary walls. Their results show that there are only minor xyloglucan-cellulose interactions in the primary wall of the model plant, contradicting the tethered-network model. A biomechanical assay also supports their result and indicates the existing cell wall model needs reconstruction.
The cellulose-xyloglucan junction behaves as a mechanical hot spot between separate cellulose microfibrils. The digestion of cell walls with xyloglucan-cutting enzymes did not cause a reduction in wall strength or the induction of cell wall extension. However, digestion with the enzyme that breaks down both cellulose and a minor component of xyloglucan did introduce significant creep. Creep demonstrates the amount of cell wall loosening by measuring the slow and irreversible extension that occurs when a growing cell wall is held at a constant force. The team’s result indicates that only a relatively minor fraction of xyloglucan is involved in wall mechanics. The team’s results led to the construction of the biomechanical hotspot model in which cellulose microfibrils are linked together at limited junctions that are the targets of wall-loosening enzymes. Based on their results, the scientists proposed new concepts of interactions within the primary cell wall.
That’s not the end, of course. Now, the CLSF scientists are continuing to conduct studies on the structural details and interactions of the cell walls. Their work will produce a better understanding of the structures and solidify the idea of using plant biomass as a sustainable energy source.
The Center for Lignocellulose Structure and Formation, an Energy Frontier Research Center funded by the U.S. Department of Energy Office Science, Office of Basic Energy Sciences, funded both referenced articles.
Cosgrove DJ. 2014. “Re-constructing Our Models of Cellulose and Primary Cell Wall Assembly.” Current Opinion in Plant Biology 22:122-131. DOI: 10.1016/j.pbi.2014.11.001
Wang T, P Phyo, and M Hong. 2016. “Multidimensional Solid-State NMR Spectroscopy of Plant Cell Walls.” Solid State Nuclear Magnetic Resonance 78:56-63. DOI: 10.1016/j.ssnmr.2016.08.001