Statoliths pull on microfilaments: experiments under microgravity

Previous videomicroscopy of Chara rhizoids during parabolic flights of rockets showed that the weightless statoliths moved basipetally. A hypothesis was offered that the removal of gravity force disturbed the initial balance between this force and the basipetally acting forces generated in a dynamic interaction of statoliths with microfilaments (MFs). The prediction of this hypothesis that the statoliths would not be displaced basipetally during the microgravity phase (MG-phase) after disorganizing the MFs was tested by videomicroscopy of a rhizoid treated with cytochalasin D (CD) immediately before the flight. The prediction was fully supported by the flight experiment. Additionally, by chemical fixation of many rhizoids at the end of the MG-phase it was shown that all rhizoids treated with CD before the flight had statoliths at the same location. i.e., sedimented an the apical cell wall, while all untreated rhizoids had statoliths considerably displaced basipetally from their normal position. Thus, a dynamical interaction involving shearing forces between MFs and statoliths appears highly probable.