EXPERIMENTAL STUDIES ON THE STABILITY OF THE CORTICAL MICROTUBULE CYTOSKELETON IN RELATION TO POLARITY AND CELL ELONGATION IN THE COENOCYTIC GREEN-ALGA, CHAETOMORPHA-MONILIGERA

The stability and ordered assembly of cytoskeletal microtubules (MTs) and the relationship between cell growth and MT cytoskeleton in the coenocytic green alga, Chaetomorpha moniligera Kjellman were examined. The cytoplasm of cylindrical growing cells of Chaetomorpha is covered with dense arrays of longitudinally arranged cortical MTs which constitute the MT cytoskeleton. Seventy-five percent of MTs of the cytoskeleton disappeared within 4 h, with 25% remaining after 20 h following cold treatment. On terminating MT assembly with amiprophos-methyl (APM), the number of MTs decreased by 75% within 4 h. The remaining MTs disappeared gradually within 24 h. The MT cytoskeleton of Chaetomorpha would thus appear to be composed of at least two kinds of MTs differing in stability. The MT cytoskeleton returned to normal after treatment with APM for less than 48 h. However, this did not occur after treatment with APM for more than 48 h, and the MT arrays became random. Cell elongation ceased completely within 24 h after treatment with APM for less than 48 h but was restored within 24 h after removing APM. The restoration of cell elongation was no longer evident after removaI of APM for more than 48 h. The results indicate that assembly of MTs into ordered arrays depends on cell polarity and that in turn cell elongation is dependent on the polar-dependent arrays of MTs.