Induction of Actin-Based Cytoplasmic Contraction in the Siphonous Green Alga Acetabularia (Chlorophyceae) by Locally Restricted Calcium Influx

Perfused cell segments dissected from the stalk or from detached cap ray chambers of Acetabularia were used as an experimental system to study the induction of cytoplasmic contractions and concurrent cytoskeletal changes in plant cells. Immunofluorescence microscopy revealed that the actin cytoskeleton quickly rearranges upon induction of contraction by forming bundles oriented circumferentially around the affected area, whereas microtubules were not detected. Contraction is blocked by cytochalasin D or N-ethylmaleimide but is unaffected by microtubule specific inhibitors. Contraction requires external Ca²⁺ at concentrations of 1 μM or more, but fails to occur below 0.1 μM. Higher concentrations of Ca²⁺ up to 10 mM have no adverse effect. Contraction is prevented in the presence of micromolar Ca²⁺ by either 1 mM of the calcium channel blocker LaCl₃ or 10 μM of the calmodulin inhibitor fluphenazine. Calcium ionophore A 23187 (1 μM) does not perturb wound contraction per se but causes the entire cytoplasm of wounded or unwounded cells to contract slowly. These data suggest that a localized influx of calcium ions at the wound edge causes major rearrangements in the distribution of cytoskeletal actin prior to contraction in Acetabularia. An involvement of calmodulin in calcium signaling is proposed.