Statoliths motions in gravity-perceiving plant cells: does actomyosin counteract gravity?

Statocytes from plant root caps are characterized by a polar arrangement of cell organelles and sedimented statoliths. Cortical microtubules and actin microfilaments contribute to development and maintenance of this polarity, whereas the lack of endoplasmic microtubules and prominent bundles of actin microfilaments probably facilitates sedimentation of statoliths. High-resolution video microscopy shows permanent motion of statoliths even when sedimented. After immunofluorescence microscopy using antibodies against actin and myosin II the most prominent labeling was observed at and around sedimented statoliths. Experiments under microgravity have demonstrated that the positioning of statoliths depends on the external gravitational force and on internal forces, probably exerted by the actomyosin complex, and that transformation of the gravistimulus evidently occurs in close vicinity to the statoliths. These results suggest that graviperception occurs dynamically within the cytoplasm via small-distance sedimentation rather than statically at the lowermost site of sedimentation. It is hypothesized that root cap cells are comparing randomized motions with oriented motions of statoliths and thereby perceiving gravity.