A fine structural analysis of auxin-induced elongation of cucumber hypocotyls, and the effects of calcium antagonists and ionophores

The fine structure of epidermal cells, particularly in relation to dictyosomes, has been examined in different regions of dark-grown cucumber hypocotyls and in response to auxin treatment, using both dot overlay and image analysis techniques. The most noticeable change in cell structure along the hypocotyl is the increase in vacuolar volume. The volume fraction occupied by dictyosomes and secretory vesicles also increased, whereas that for mitochondria remained relatively constant. During auxin treatment, the volume fraction for dictyosomes showed an increase after 30 min followed by a fall, whereas that occupied by secretory vesicles fell steadily over 90 min. The number of cisternae per dictyosome showed some increase after 2 h of auxin treatment, although the increase in dictyosomal material with cell expansion was largely accounted for by an increase in the number of dictyosomes. Auxin-stimulated elongation growth of the hypocotyls was inhibited by a range of calcium antagonists, chelators and ionophores. The most marked inhibitions were observed with calcium chloride, the chelator chlortetracycline and the ionophores verapamil, nigericin and monensin. Linear transducer experiments showed that these compounds generally caused an immediate reduction in the rate of growth. Fine structural observations carried out on epidermal cells showed the most obvious effects with monensin and nigericin which caused dictyosomes and secretory vesicles to swell. EGTA and LaCl3 caused secretory vesicles to accumulate around dictyosomes, while the ionophore A23187 had little effect. The results suggest that the concentration of Ca2+ in the cytoplasm may be critical for cell elongation. Compounds which chelate Ca2+ appear to be more effective inhibitors of growth in the initial acid-induced phase, whereas those which affect ionic gradients are more disruptive in the second phase.