Position and cell type-dependent microtubule reorientation characterizes the early response of the Arabidopsis root epidermis to ethylene

The involvement of cortical microtubules in the control of plant cell expansion was studied in the Arabidopsis root epidermis. In the zone of fast elongation microtubules were transverse to the root axis in all epidermal cells. However when cells entered the differentiation zone cell type-specific microtubule reorientation took place. In the trichoblasts that were then approximately 130 µm long and formed the root hair bulge, the microtubules switched to a random distribution. In the adjoining atrichoblasts microtubules adopted a slightly oblique orientation. In more proximal parts of the differentiation zone atrichoblast microtubules were found in a more oblique and finally in a longitudinal orientation. Upon exposure to ethylene or 1-aminocyclopropane-1-carboxylic acid (ACC – the precursor of ethylene) at a saturating dose, cell elongation abruptly stopped. From then on trichoblast cells reached only a length of about 35 µm, and developed root hairs. Cortical microtubules changed orientation within 10 min. In trichoblasts they adopted the typical random orientation, in atrichoblasts however, they took up a longitudinal orientation. Microtubule reorientation was complete within 60 min. The possible role of microtubules in the control of cell elongation is discussed.