Investigations of Cellulose Biosynthesis

In the past few years, some impressive observations have been made by microscopists studying cells actively engaged in cellulose synthesis. It is now generally accepted by most workers that, in the bacterium A. xylinum, in most algae, and in all of the higher plants, cellulose is synthesized at the cell surface by an enzyme system localized in the plasma membrane. The notable exception to this conclusion concerns those algae which synthesize a cell wall composed of cellulosic scales such scales are synthesized intracellularly by way of the Golgi apparatus (see Ref. 57 and references cited therein). 

Terminal complexes have also been claimed to have been observed in freeze-fracture studies of higher-plant cells. However, at least to the inexperienced eye of this biochemist, the images seen are far less clear than those observed with the algae. Thus far, such complexes have been reported in cells of com roots,78,79 radish roots,80 and cotton 

A Model of Cellulose-Fibril Deposition During Secondary-wall Formation in Microsterias.7 [Each rosette is believed to form one 5-nm microfibril. A row of rosettes forms a set of 5-nm microfibrils which aggregate laterally to form the larger fibrils of the secondary wall. Above side view. The stippled area in the center of a rosette represents the presumptive site of microfibril formation, although details of its structure, composition, and enzymic activity remain unclear. Below surface view, with expanded, cross-sectional view of cellulose fibrils.] 

Ever since the discovery, by Ledbetter and Porter,88 of microtubules below the surface of the plasma membrane, suggestions have been made that these structures play some role in microfibril orientation. The suggestion arose because of two observations that (I) the orientation of microtubules has very frequently, but not always, been observed to be parallel to the orientation of the microfibrils most recently synthesized, and (2) agents, such as colchicine, that disrupt microtubules interfere with the orientation, but not the synthesis, of cellulose microfibrils. The literature pertaining to these studies has been well reviewed by Robinson,4 Schnepf and coworkers,89 Hepler and Palevitz,90 and Heath.91 In sum, the present evidence seems to favor some role for microtubules in orientation in some cases, such as the studies on guard cells by Palevitz and Hepler,92 and a series of papers on Oocystis by Robinson and coworkers,84,93-95 the case for micro- 

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