Cytoplasmic streaming and gravity sensing in Chara internodal cells.
Since the nineteenth century, the merits of two alternate models for explaining the mechanism of plant gravity perception have been discussed. The gravitational pressure model states that plant cells perceive gravity by sensing their relative buoyancy to that of the surrounding medium, whereas the more popular starch-statolith model states that intracellular sedimenting particles act as gravity sensors. Vertically-oriented Chara internodal cells exhibit a gravity dependent polarity of cytoplasmic streaming such that the downwardly-directed stream moves ca. 10% faster than the upwardly-directed stream. This polarity of cytoplasmic streaming is not simply a consequence of gravity acting directly on the cytoplasm but is rather under physiological control. When Chara internodal cells are placed in a medium more dense than themselves, the gravity-induced polarity of cytoplasmic streaming is reversed. This phenomenon cannot be explained by a model which relies on intracellular sedimenting particles as gravity sensors but is consistent with the gravitational pressure model for gravity sensing. We propose that gravity causes the internodal cells to settle within the confines of the extracellular matrix resulting in a tension between the plasma membrane and the extracellular matrix at the top of the cell and a compression between the plasma membrane and the extracellular matrix at the bottom of the cell. These stresses are proposed to act upon peptides which span the plasma membrane/extracellular matrix interface at the ends of the cells and which subsequently activate Ca2+ channels which in turn may induce a polarity of cytoplasmic streaming.