Immunolocalization of myosin in rhizoids of Chara globularis Thuill.

Myosin-related proteins have been localized immunocytochemically in gravity-sensing rhizoids of the green alga Chara globularis using a monoclonal antibody against the heavy chain of myosin from mouse 3T3 cells and a polyclonal antibody to bovine skeletal and smooth muscle myosin. In the basal zone of the rhizoids which contain a large vacuole, streaming endoplasm and stationary cortical cytoplasm, the monoclonal antibody stained myosin-related proteins as diffusely fluorescing endoplasmic strands. This pattern is similar to the arrangement of subcortical actin filament bundles. In the apical zone which contains an aggregation of ER membranes and secretory vesicles for tip growth, diffuse immunofluorescence was detected; the intensity of the signal increasing towards the apical cell wall. The most prominent myosin-staining was associated with the surface of statoliths in the apical zone. The polyclonal antibody produced a punctate staining pattern in the basal zone, caused by myosin-related proteins associated with the surface of organelles in the streaming endoplasm and the periphery of the nucleus. In the apical zone, this antibody revealed myosin-immunofluorescence on the surface of statoliths in methacrylate-embedded rhizoids. Neither antibody revealed myosin-immunofluorescence on the surface of organelles and vesicles in the relatively stationary cytoplasm of the subapical zone. These results indicate (i) that different classes of myosin are involved in the various transport processes in Chara rhizoids; (ii) that cytoplasmic streaming in rhizoids is driven by actomyosin, corresponding to the findings on Chara internodal cells; (iii) that actin dependent control of statolith position and active movement is mediated by myosin-related proteins associated with the statolith surfaces; and (iv) that myosin-related proteins an involved in the process of tip growth.