Cell Wall Pore Subunft Crystallization Subunit Cytoplasm

There is probably no major biochemical process in plants that is both so important and so poorly understood at the molecular level as cellulose synthesis. This is surprising, because the basic synthetic event is a simple polymerization of glucose residues from a substrate such as UDP-glucose to form the homopolymer p-l,4-~-glucan (for reviews on the structure and industrial uses of cellulose, see Kuga and Brown, 1991; French et al., 1993). However, this is a deceptively simple description, and the process is clearly very complex and requires many higher levels of organization. This complexity can be ascribed to a number of factors that relate to the forms and patterns in which cellulose is deposited in nature. First, the stereochemistry imposed by the p-1,4-glycosidic linkage creates a linear, extended glucan chain in which every other glucose residue is rotated -180O with respect to its neighbor (Figure 1). This means that cellobiose, and not glucose, is the basic repeating unit of the molecule and contrasts with other glucan polymers such as starch (a-l,4-glucan) or callose (p-l,3-gIucan; see Figure l), in which the disaccharide is not the repeating unit and the chains are not perfectly extended but assume less ordered, helical conf igu rations. The extended nature of the p-1P-glucan chain creates a situation in which chains can interact with each other in a very precise manner to form a rigid structure. Thus, cellulose in nature never occurs as a single chain but exists from the time of synthesis as a composite of many chains, called microfibrils (Figure 1). The chains associate very strongly via both intraand interchain hydrogen bonding between glucose residues in a manner so precise that microfibrillar cellulose is largely crystalline. Although this notion has been controversial for many years, the general consensus now is that, in the cellulose I crystal (that is, the form found in nature), the chains are aligned parallel to each other. This at least eliminates complications of the sort involved in the synthesis of antiparallel chain structures such as DNA, and it suggests that the mechanism of polymerization of adjacent chains probably proceeds in a