Cellulose microfibril assembly and orientation: recent developments.

Abstract

A brief history of the literature dealing with cellulose microfibril assembly is presented, and a current summary of cellulose microfibril synthesizing complexes among eukaryotic cells is given. Terminal complexes not described before include the following: linear terminal complexes (TCs) with three rows in Eremosphaera, Microdictyon and Chaetomorpha; globular terminal complexes in Ophioglossum, Psilotum, Equisetum and Gingko. Cellulose microfibril assembly in Acetobacter xylinum is described very briefly and compared with the process among eukaryotic cells. Particular emphasis on structures that may be involved in the spatial control of cellulose synthesis is given. Among these are cytoplasmic structures such as microtubules and microfilaments. Microfilament structures are shown to clearly surround individual microtubules that lie adjacent to the plasma membrane. Using freeze-fracture techniques, these labile associations have been shown for the first time. Microfibril orientation may be mediated through an interaction of cortical microtubules in association with microfilaments. A review of Mueller and Brown's membrane flow model for microfibril orientation is presented. Cellulose terminal complex clustering and its role in gravitropic response is covered. Definitive membrane changes with TC clustering/disaggregation and intramembranous particle frequencies, occur within 12 min following gravistimulation. These differences are pronounced in the cells from upper and lower hemicylinders of rapidly frozen tissue, which was studied by the freeze-fracture method. A hypothesis for cellulose microfibril interaction in controlling the constraint of the growth axis is presented, and the supporting data for terminal complex clustering/disaggregation as well as fluorescent brightener inhibition of the gravitropic response support this hypothesis. The onset and regulation of cellulose microfibril assembly is presented for synchronized protoplasts generated by Boergesenia, using inhibitors of transcription and translation. These results suggest dynamic turnover of terminal complex subunits during the assembly of the cellulose microfibril. This study is concluded with a brief discussion of possible phylogenetic trends in the evolution of cellulose synthesis. A principal underlying theme is that the specific arrangement and consolidation of the terminal complex subunits determine to a large degree the size and shape of the microfibril, its crystallinity, as well as intramicrofibrillar associations. Three basic types of TCs appear among all eukaryotic cells studied so far: namely, the rosette, the globular and the linear complex.