Joanne E. Burn

Functional analysis of the cellulose synthase genes CesA1, CesA2, and CesA3 in Arabidopsis.

Polysaccharide analyses of mutants link several of the glycosyltransferases encoded by the 10 CesA genes of Arabidopsis to cellulose synthesis. Features of those mutant phenotypes point to particular genes depositing cellulose predominantly in either primary or secondary walls. We used transformation with antisense constructs to investigate the functions of CesA2(AthA) and CesA3 (AthB), genes for which reduced synthesis mutants are not yet available. Plants expressing antisense CesA1 (RSW1) provided a comparison with a gene whose mutant phenotype (Rsw1−) points mainly to a primary wall role. The antisense phenotypes of CesA1 and CesA3were closely similar and correlated with reduced expression of the target gene. Reductions in cell length rather than cell number underlay the shorter bolts and stamen filaments. Surprisingly, seedling roots were unaffected in both CesA1 and CesA3antisense plants. In keeping with the mild phenotype compared with Rsw1−, reductions in total cellulose levels in antisenseCesA1 and CesA3 plants were at the borderline of significance. We conclude that CesA3, likeCesA1, is required for deposition of primary wall cellulose. To test whether there were important functional differences between the two, we overexpressed CesA3 inrsw1 but were unable to complement that mutants defect in CesA1. The function of CesA2 was less obvious, but, consistent with a role in primary wall deposition, the rate of stem elongation was reduced in antisense plants growing rapidly at 31°C.

Morphology of rsw1, a cellulose deficient mutant of Arabidopsis thaliana

SummaryTherswl mutant ofArabidopsis thaliana is mutated in a gene encoding a cellulose synthase catalytic subunit. Mutant seedlings produce almost as much cellulose as the wild type at 21 °C but only about half as much as the wild type at 31 °C. We used this conditional phenotype to investigate how reduced cellulose production affects growth and morphogenesis in various parts of the plant. Roots swell in all tissues at 31 °C, and temperature changes can repeatedly switch them between swollen and slender growth patterns. Dark-grown hypocotyls also swell, whereas cotyledons and rosette leaf blades are smaller, their surfaces are more irregular and their petioles shorter. Leaf trichomes swell and branch abnormally. Plants readily initiate inflorescences at 31 °C which have shorter but not fatter bolts and stomata which bulge above the uneven surface of internodes. Bolts carry the normal number of flowers, but their stigmas protrude beyond the shortened sepals and petals. Anthers dehisce normally, but self-fertilisation is reduced because the stigma is well above the anthers. Anther filaments are short and show a crumpled surface. Viable pollen develops, but female reproductive competence and postpollination development are severely impaired. We conclude that theRSW1 gene is important for cellulose synthesis in many parts of the plant and that reduced cellulose synthesis suppresses organ expansion rather than organ initiation, causes radial swelling only in the root and hypocotyl, but makes the surfaces of many organs uneven. We discuss some possible reasons to explain why different organs vary in their responses. The morphological changes suggest that RSW1 contributes cellulose to primary walls but do not yet exclude a role during secondary-wall deposition.

Cellulose synthesis: mutational analysis and genomic perspectives using Arabidopsis thaliana

Abstract. Cellulose microfibrils containing crystalline β-1,4-glucan provide the major structural framework in higher-plant cell walls. Genetic analyses of Arabidopsis thaliana now link specific genes to plant cellulose production just as was achieved some years earlier with bacteria. Cellulose-deficient mutants have defects in several members of one family within a complex glycosyltransferase superfamily and in one member of a small family of membrane-bound endo-1,4-β-glucanases. The mutants also accumulate a readily extractable β-1,4-glucan that has short chains which, in at least one case, are lipid linked. Cellulose could be made by direct extension of the glucan chain by the glycosyltransferase or, as the mutant suggests, by an indirect route which makes lipid-linked oligosaccharides. Models discussed incorporate the known enzymes and lipo-glucan and raise the possibility that different CesA glycosyltransferases may catalyse different steps.

Molecular Biology of Cellulose Biosynthesis

Cellulose, the world’s most abundant biopolymer, is an integral part of human society and fundamental to plant morphogenesis. Humans exploit its abundance and properties in the form of wood (for fuel, construction and as a source of fibre for making paper and processing into man-made fibres such as rayon); as cotton and other fibres for textiles, ropes etc; and as a valuable part of our diet in the form of insoluble fibre. Cellulose is the major constituent of most plant cell walls, accounting for the great majority of production, but bacteria, tunicates and other organisms produce much smaller amounts. The global distribution of plants makes cellulose a significant carbon sink and a renewable energy source. Remarkably for a material that is so abundant, chemically simple, biologically central and economically important, the mechanisms by which bacteria and particularly plants synthesise it remain enigmatic.