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Cotton, paper and wood -- they're all made of the cellulose that plants use for strength and flexibility. But surprisingly, scientists do not know a lot about how plants actually make cellulose. Now research at the University of California, Davis, has shed light on a key step: how fibers get started.

Sitosterol, a fatty substance similar to cholesterol, combines with glucose to form a "primer" that gets the process started, according to research by Langcai Peng and colleagues at the °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis Section of Plant Biology.

"This adds a key step in understanding cellulose synthesis," said plant biologist Deborah Delmer, who is senior author on the paper.

Cellulose is made of chains of glucose molecules joined together. Researchers have identified a family of enzymes, called CesA, each of which can add new glucose blocks to existing chains of cellulose. But the CesA enzymes cannot take the first glucose molecule and begin a chain, Delmer said. They can only make an existing chain longer.

The sterol may help keep the first glucose unit near the cellulose-making machinery in the cell membrane. After the first step, the enzymes can make the chain longer by adding extra glucose units to the end away from the sterol.

The growing chain is pushed through the cell membrane to the outside of the cell. In some plants, such as cotton, extra cellulose provides the strength and other properties that make cotton fibers so valuable to the textile industry.

To study cellulose production, the researchers grew fibers from cotton plants in the laboratory. They also transferred genes into yeast, to find the minimum requirements for making cellulose.

The paper is published in the Jan. 4 issue of .