Arrangement of microtubules and microfilaments during oriented secondary wall formation

Abstract

Introduction. Microtubules have long been hypothesized to function as track-like elements, directing cellular components (Hepler & Palevitz 1974). In higher plants, cortical microtubules (cmt) have been hypothesized to function in this manner in the orientation of secondary wall microfibrils (MF) (reviewed by Heath, 1974). Involvement in such a process requires that the cmt be close to the structures being moved, be parallel to the direction of orientation and be of sufficient length to function as guide elements. In radish root hairs, sub-pl cmt occur along the length of the hair, paralleling the secondary wall MF. Root hair elongation occurs via tip growth, which results in simultaneous deposition of primary, randomly arranged MF (first 25 μm of root hair) and secondary, oriented MF (distances greater than 25 μm, Fig. 1).Results. Longitudinal serial sections show that cmt start and terminate randomly throughout the length of the hair, but are absent in the tip region (0-3 μm from tip). Microtubules are rarely found more than 500 nm from the pl. Bridges between cmts and the pl are evident, Fig. 4. In the region between 20 and 60 μm from the tip, 50% of the cmt are less than 1 μm long.