by Brian Gunning
Cellulose is the most abundant bio-polymer on Earth, of enormous commercial value in fibre industries. Plants deposit long chains of cellulose in their cell walls, providing mechanical strength and also determining the shape of the cells according to the directions in which the strands are laid down. But how plant cells make it was a long-standing mystery.
Many years of work with the electron microscope in the Plant Cell Biology Group in the Research School of Biology confirmed that the orientation of the cellulose is controlled by long proteinaceous “microtubules” lying just inside the surface membrane of each cell. Somehow they organise the way the molecular strands of cellulose are spun out from the cell surface to take their place in the cell wall and thereby governing how the cell will enlarge.
Richard Williamson and his co-workers pioneered studies that led to the first identification of a gene encoding an enzyme that catalyses synthesis of plant cellulose in vivo. They started from previous knowledge of microtubules and cell-shaping, searching for mutations that alter both the microtubule cytoskeleton and the way the cells expand. They went on to sort through the mutants they found in order to distinguish the cellulose synthesis machinery from the orientation control by microtubules. One mutant in the plant Arabidopsis was especially exciting: in the zone where cells normally elongate into cylindrical shapes at the tip of a growing root, its microtubules were unaffected but its cells were swollen and the cellulose content was reduced: they called it rsw1, for “radial swelling”. The methods available at that time (1997-8) were not as advanced as at present, and progress from characterising the mutant to cloning the gene that was responsible was very difficult, but they finally obtained the complete gene sequence, finding that the mutation involved substitution of just one amino acid, and proving conclusively that the rsw1 gene encodes a catalytic subunit of cellulose synthase. It was the first to be characterised fully in plants.
Williamson’s discovery was hailed as a landmark advance in the field. It opened the door to identification of previously unknown components of a cellular apparatus that operates at the most basic level of morphogenesis, determining how plants develop characteristic forms, and enabled great advance in knowledge of how the world’s most abundant biological substance is made.