The Site of Cellulose Synthesis

Abstract

beta-1, 4-Glucan (cellulose) synthetase activity (UDP-glucose: beta-1, 4-glucan-glucosyl transferase) present at cell surfaces of growing regions of Pisum sativum epicotyl was assayed by supplying UDP-(14)C-glucose directly to thin slices of tissue. Initial rates of glucosyl transfer under these conditions approached the rates of cellulose deposition observed in vivo in intact tissue at various stages of growth. Normal tissue homogenization procedures destroyed the high surface activity, although a small amount of residual activity (3-10% of total) could be detected in particulate fractions. In homogenates from elongating tissue, the residual activity was almost entirely associated with Golgi membrane. In homogenates of tissue which had ceased elongating, whether because of normal maturation or treatment with ethylene (or high levels of auxin), the activity was present in Golgi plus a membrane fraction rich in smooth endoplasmic reticulum vesicles. It is suggested that cellulose synthetase activity associated with these two organelles represents intracellular enzyme in transit to specific sites of cellulose synthesis and microfibrillar orientation at the cell surface.