Trafficking of the Plant Cellulose Synthase Complex

Cellulose is the most abundant component of plant cell walls and its importance is well documented. In the primary cell wall it forms part of the load-bearing network that both controls and maintains cell shape, while allowing regulated cell expansion that is essential during growth. Cellulose is also the major component of secondary cell walls where it is required for mechanical strength for the plant (for review, see Cosgrove, 2005; Taylor, 2008). Most plant cell wall matrix polysaccharides are first synthesized within the cell in the Golgi and subsequently deposited into the wall by exocytosis (Moore and Staehelin, 1988; Reyes and Orellana, 2008). In contrast, cellulose is synthesized at the plasma membrane by a very large membrane-bound complex known as the cellulose synthase complex (CSC). The complex extrudes up to 36 individual cellulose chains that are bound together to form the cellulose microfibril. It is believed that the microfibril forms a rigid structure such that the energy from cellulose chain elongation pushes the CSC through the plane of the plasma membrane (Herth, 1980). Freeze fracture studies have shown the CSC to be a six-lobed rosette structure of 25 to 30 nm in diameter (Mueller and Brown, 1980; Herth, 1985), although more recent analysis suggests that it extends into the cytoplasm, reaching a maximum diameter of about 45 nm (Bowling and Brown, 2008). The large size of the CSC and the need to target it to specific sites in the plasma membrane represent a particular logistical problem for plant cells. Recent evidence from live cell imaging suggests CSC trafficking is both highly dynamic and that it does not easily fit into our understanding of the conventional secretory pathway. In this article we will focus on recent studies that have contributed to improving our understanding of the intracellular trafficking of the CSC. For other aspects of cellulose synthesis, including biochemistry, structure, and microfibril orientation readers are referred to several recent reviews (Somerville, 2006; Lloyd and Chan, 2008; Taylor, 2008).