Symposium V: The Plasma Membrane: A Happening Place

Abs # 50004: Cellulose synthesis and alignment at the plasma membrane

Presenter:	Williamson, Richard E., richard@rsbs.anu.edu.au
Authors	Williamson, Richard E. (A)
Affiliations:	(A): Plant Cell Biology Group, Research School of Biological Sciences, Australian National University

The plasma membrane occupies a key place in cell wall assembly since it synthesises cellulose, the wall’s most important structural polysaccharide, as well as providing the target membrane with which vesicles fuse to deliver to the wall Golgi-synthesised polysaccharides and glycoproteins. Recent molecular analyses of cellulose-deficient mutants of Arabidopsis thaliana have identified many genes needed for cellulose synthesis. Reduced cellulose synthesis follows mutations in the putative glycosyltransferases of the CesA family, in the endo-1,4-beta-glucanases of the KOR family, in several enzymes of the N-glycosylation/quality control pathway and in KOB and COB, proteins of so far unknown function. CesA glycosyltransferases have attracted particular interest as candidates for the primary synthetic role. Mutant phenotypes suggest that sub-families of the ten CesAs contribute mainly to either primary or secondary walls but the severity of their phenotypes suggests that members of each sub-family may have specialised roles. The idea of specialisation is reinforced by CesA3’s inability to complement a CesA1 mutation even when overexpressed. Coprecipitation and two hybrid experiments suggest different CesAs physically interact, probably within the rosette terminal complexes where immunoelectron microscopy locates them. Lipid-linked glucans accumulating in herbicide-treated plants and mutants are possible synthetic intermediates but evidence for their role in normal metabolism is still lacking. The proper alignment of newly synthesised cellulose is as important as synthesis per se since it affects mechanical anisotropy in growing cell walls and critical mechanical properties in specialised fibre cells. Much evidence points to coalignment of microtubules beneath the plasma membrane and newly synthesised microfibrils at its outer surface. Surprisingly, however, recent analyses of mutants show that a CesA1 defect reducing cellulose synthesis misaligns microfibrils even though cortical microtubules remain intact whereas disruption of cortical microtubules leaves microfibril alignment unaffected. Chemically generated phenocopies show similar results. Further detailed analyses of the chemical and structural phenotypes of the numerous cellulose mutants now available will surely be as rewarding in the next few years as the molecular analyses of the last five years have been.