New hope for old dreams: evidence that plant cellulose synthase genes have finally been identified.

It is common in the cellulose literature to read that cellulose is the most abundant macromolecule on earth. Authors perhaps hoped that the abundance of the polymer would compensate for the lack of progress in understanding the biochemistry and molecular biology of its synthesis in higher plants! Despite substantial effort, cellulose biogenesis has remained for decades one of the most enigmatic aspects of plant biochemistry, with only low activity observable in vitro, no synthase purified to homogeneity, and no relevant genes identified. Many scientists who tried to make progress eventually quit to concentrate on more productive and publishable research. However, the subject is of such critical importance to plant biology and global productivity that a few research groups worldwide persevered. Cellulose is important because it is an almost ubiquitous component of plant cell walls. The high molecular weight chains of the (1 -> 4)-f-D-glucan homopolymer form crystalline microfibrils of high tensile strength (Fig. la), and, in growing walls, the microfibril orientation constrains the direction of plant cell expansion. Therefore, cellulose synthesis is a fundamental determinant of plant growth rate (without it, plants cannot grow substantially) and morphogenesis (because plant cells do not migrate during development, direction of cell expansion determines plant form). Furthermore, cellulose is a substantial component of the biomass produced and decomposed in the biosphere because it is highly abundant in thick secondary walls of plant cells that conduct water and provide support, finally forming wood in trees. Cellulose also composes the secondary cell walls of cotton fibers, our most important natural fiber. Finally, cellulose biogenesis likely reflects many novel regulatory processes, since the cytoplasmic precursor UDP-glc is converted at the plasma membrane into insoluble extracytoplasmic microfibrils, which are often laid down in precise patterns. For all these reasons, it is essential to understand how the synthesis of cellulose is regulated. One of the research groups that persevered in the study of cellulose biogenesis was that of Deborah P. Delmer, who, along with her associate Yasushi Kawagoe, is a coauthor on the paper in this issue of the Proceedings identifying the first strong candidates for higher plant cellulose synthase genes (1). Other authors on the paper, Julie R. Pear, William E. Schreckengost, and David M. Stalker are from Calgene, a biotechnology company interested in the control and modification of cotton fiber development. Their approach was not biochemical, relying instead on random sequencing of plant cDNAs combined with insightful sequence analysis. This finding is noteworthy because it provides the first likely direct route to unraveling the cellular and molecular details of plant cellulose biogenesis.