Motility in the siphonous green alga Bryopsis. II. Chloroplast movement requires organized arrays of both microtubules and actin filaments.

The cortical cytoplasm of the giant cells of Bryopsis contains hundreds of interconnected microtubule (MT) bundles aligned along the cell's long axis. Actin fibers show an extensive but not exclusive superposition with these MT bundles. Chloroplasts move parallel to the bundles. Colchicine (0.5 mM), vinblastine (0.1 mM), and the herbicide ami-prophosmethyl (APM, 1-5 microM) strongly inhibit chloroplast movement and severely disrupt both the MT and the actin network. Additionally, APM leads to the appearance of large actin bundles up to 5 microns in diameter and several tens of micron in length. Erythro-9-[3-(2-hydroxynonyl)]adenine (EHNA, 1 mM) does not block chloroplast movement, but affects chloroplast behavior by causing transient aggregations. The MT network is not significantly changed by EHNA, but actin fibers converge in large, radially symmetric complexes in regions of chloroplast aggregates. Cytochalasin D (CD, 1-10 micrograms/ml) leads to a significant but transient reduction of chloroplast speed within the first 60 min, as the actin network breaks down into small foci. Within the next 1 to 3 h of treatment, these foci segregate into massive clusters where chloroplasts remain immobilized. At the same time, chloroplast movement recovers in other areas of the cell. This recovery coincides with the reappearance of actin filament bundles in these cell regions. The MT cytoskeleton is not significantly affected by CD. These data are inconsistent with a mechanism of chloroplast movement in Bryopsis based solely on either MTs or actin, but instead they suggest an intimate interaction of both cytoskeletal networks in maintaining the spatial organization of the cytoplasm and in supporting chloroplast movement.