Anisotropic viscosity of the Chara (Characeae) rhizoid cytoplasm

To characterize cellular fluidity and mechanical processes, we determined the viscous properties of the cytoplasm of Chara contraria rhizoids in vivo by injecting and displacing superparamagnetic particles. After injection and a 24-h recovery period, the particles were moved to different positions within the rhizoid by an external magnet. The system was calibrated with solutions of known viscosities. The viscosity was determined based on the velocity at which individual beads moved toward the external magnet. The viscosity of the cytoplasm varied with direction of measurement (i.e., was highly anisotropic) and also varied between sites. The highest viscosity was observed near the endogenous statoliths (139 mP·s parallel and 78 mP·s perpendicular to the rhizoid axis). Depolymerization of actin filaments with latrunculin B reduced the viscosity significantly except around the nucleus but did not change the overall viscosity pattern. Microtubule depolymerization with oryzalin reduced viscosity especially between the nucleus and the statolith zone. The data indicate that F-actin but not microtubules affects statolith sedimentation and that cytoplasmic viscosity may be important for the gravisensing system.