Changes of the actin filament system in the green alga Micrasterias denticulata induced by different cytoskeleton inhibitors

Two different techniques have been adapted for Micrasterias denticulata to depict the actin cytoskeleton of both untreated and inhibitor-treated devel oping cells: the "quickstaining method", where the cells are fixed in a mix ture of glutaraldehyde and formaldehyde followed by staining with phalloidi n without embedding, and the "methacrylate method", where the cells are als o fixed by aldehydes and where the embedding medium is removed prior to inc ubation with an actin antibody Both methods produce sufficient preservation and visualization of actin microfilaments (MFs) and confirm earlier observ ations on the presence of a cortical actin MF network in both the growing a nd the nongrowing semicell as well as of a basketlike MF arrangement around the migrating nucleus. The results show that a network of actin MFs is ess ential for the proper development of the young lobes of M. denticulata. Ear ly developmental stages expanding uniformly at the beginning of growth lack any netlike actin MF arrangement. The actin cytoskeleton in developing cel ls treated with the actin-targeting agents cytochalasin D and latrunculin B is markedly influenced. Cytochalasin D, which produces the most pronounced effects, causes a breakdown of the network of actin MFs, resulting in brig ht actin clusters as well as in short and abnormally thick actin fragments particularly in cortical cell regions. In latrunculin B-treated cells remna nts of the former actin MF network are still visible, yet most of the actin cytoskeleton appears collapsed and is reduced to short filament pieces. Th e disturbance of the actin MF system visualized in the present study correl ates with the severe morphological and ultrastructural changes occurring in desmid cells as a consequence of both drugs. The dinitroanilin herbicide o ryzalin, known to deploymerize cytoplasmic microtubules, causes also an imp airment of the actin cytoskeleton in M. denticulata though not sufficient t o influence normal cell growth and differentiation.