Gravity-dependent polarity of cytoplasmic streaming inNitellopsis

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Abstract
The internodal cells of the characean algaNitellopsis obtusa were chosen to investigate the effect of gravity on cytoplasmic streaming. Horizontal cells exhibit streaming with equal velocities in both directions, whereas in vertically oriented cells, the downwardstreaming cytoplasm flows ca. 10% faster than the upward-streaming cytoplasm. These results are independent of the orientation of the morphological top and bottom of the cell. We define the ratio of the velocity of the downward- to the upward-streaming cytoplasm as the polar ratio (PR). The normal polarity of a cell can be reversed (PR+ channels. The K+ channel blocker TEA Cl− inhibits the NR effect. External Ca2+ is required for normal graviresponsivness. The [Ca2+] of the medium determines the polarity of cytoplasmic streaming. Less than 1 μM Ca2+ resulted in a PR2+ resulted in the normal gravity response. The voltage-dependent Ca2+ -channel blocker, nifedipine, inhibited the gravity response in a reversible manner, while treatment with LaCl3 resulted in a PR2+ channels. A new model for graviperception is presented in which the whole cell acts as the gravity sensor, and the plasma membrane acts as the gravireceptor. This is supported by ligation and UV irradiation experiments which indicate that the membranes at both ends of the cell are required for graviperception. The density of the external medium also affects the PR ofNitellopsis. Calculations are presented that indicate that the weight of the protoplasm may provide enough potential energy to open ion channels.