Cellulose study may improve biofuels

Cellulose, a fibrous molecule found in all plants, is the most abundant biological material on Earth. It is also a favored subject of renewable, plant-based biofuels research. Despite overwhelming interest, scientists know relatively little about how plant cells synthesize individual cellulose fibers.

However, recent work by scientists at Stanford and the Carnegie Institution's Department of Plant Biology describes the first real-time observations of cellulose fiber formation. The research, published in the April 20 online issue of Science Express, provides the first clear evidence for a functional connection between synthesis of the cell wall and an array of protein fibers, called microtubules, that help to shape growing plant cells from the inside.

"The more we understand about cellulose, the easier it will be to modify it," said Chris Somerville, professor of biological sciences at Stanford and director of Carnegie's Plant Biology Department. "Plant cell walls are nanoscale assemblies that have been very difficult to understand because of the high degree of structural complexity. Being able to watch the assembly of the cell wall at the single-molecule level is very enabling, because we can infer hidden properties of the walls by watching them assemble. With this knowledge, we are one step closer to designing energy-rich biofuel crops and improved fiber crops."

Cellulose fibers make up a significant portion of the dry weight of most plants. Because the fibers can be broken down into glucose, which can then be converted into ethanol and other biofuels, there are huge incentives to learn more about how plants produce and modify the molecule. Cellulose is also the main constituent of cotton, paper, wood and animal feeds, such as hay.

Somerville, along with Carnegie staff associate David Ehrhardt and Stanford biological sciences graduate student Alex Paredez, engineered plants to produce a fluorescent version of cellulose synthase, the enzyme that creates cellulose fibers. They also included a fluorescent version of tubulin, the protein from which microtubules are built. Using an imaging technique that can track the motion of single fluorescent molecules, the researchers found that cellulose synthase moves along "tracks" defined by the microtubules. When the microtubule tracks were disrupted with specific drugs, the cellulose synthase molecules kept moving but followed different, less directed patterns.

"Many scientists had suspected a relationship between cellulose synthase and microtubules, but the exact nature of the interaction was hard to pinpoint," Ehrhardt said. "The microtubules act as more than a general scaffold for organizing the cell wall. Individual elements of the microtubule array appear to actively direct the pattern of the cellulose fibers. This work should set a long-standing discussion to rest."

Added Somerville: "Now that we can visualize cellulose synthesis, we may be able to identify the factors that control how much cellulose a plant accumulates, or the length or width of cellulose fibrils. We might be able to modify those factors to produce a range of new biomaterials with improved properties, such as stronger paper. We might also be able to develop more environmentally benign biomass-processing methods for making paper or biofuels."

The study was supported in part by the U.S. Department of Energy and the National Science Foundation.

Matthew E. Wright is a science writer with the Carnegie Institution in Washington, D.C.