Cytoplasmic microtubules and fungal morphogenesis : Ultrastructural effects of methyl benzimidazole-2-ylcarbamate determined by freeze-substitution of hyphal tip cells

Abstract

The effects of methyl Benzimidazole-2-ylcarbamate (MBC), one of only a few agents that are active against microtubules of fungi, were analyzed at the ultrastructural level in freeze-substituted hyphal tip cells of Fusarium acuminatum. Nontreated and control cells had numerous microtubules throughout. After just 10 min of exposure to MBC, almost no cytoplasmic microtubules were present, except near spindle pole bodies. After 45 min of exposure to MBC, no microtubules were present in hyphal tip cells, but they were present in the relatively quiescent subapical cells. These observations suggested that there are different rates of turnover for cytoplasmic microtubules in apical and subapical cells and for microtubules near spindle pole bodies and that MBC acts by inhibiting microtubule assembly. A statistical analysis of the distribution of intracytoplasmic vesicles in thick sections of cells treated with MBC, D20 or MBC + D20 was obtained by use of a high-voltage electron microscope. More than 50% of the vesicles in the apical 30 Am of control cells were found to lie within 2 pin of the tip cell apex. MBC treatment caused this vesicle distribution to become uniform, resulting in a substantial increase in the number of vesicles in subapical regions. The reduction in the number of cytoplasmic microtubules, induced by MBC, apparently inhibited intracellular transport of these vesicles and rendered random the longitudinal orientation of mitochondria. In most cases, D20 appeared capable of preventing these MBC-effects through stabilization of microtubules. These observations support the "vesicle hypothesis" of tip growth and establish a transport role for cytoplasmic microtubules in fungal morphogenesis. © 1980, Rockefeller University Press., All rights reserved.