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.

The internodal cells of the characean alga Nitellopsis 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 downward-streaming 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 < 1) by treatment with neutral red (NR). The NR effect may be the result of membrane hyperpolarization, caused by the opening of K+ channels. The K+ channel blocker TEA Cl- inhibits the NR effect. External Ca2+ is required for normal graviresponsiveness. The [Ca2+] of the medium determines the polarity of cytoplasmic streaming. Less than 1 micromole Ca2+ resulted in a PR < 1 while greater than 1 micromole Ca2+ resulted in the normal gravity response. The voltage-dependent Ca(2+)-channel blocker, nifedipine, inhibited the gravity response in a reversible manner, while treatment with LaCl3 resulted in a PR < 1, indicating the presence of two types of Ca2+ 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 of Nitellopsis. Calculations are presented that indicate that the weight of the protoplasm may provide enough potential energy to open ion channels.

Biological systems synthesize proteins with an almost exclusive use of L-amino acids and virtually none of the D isomer. There has been no satisfactory explanation for the origin of this use of the L isomer. Research presented here shows that at pH 5, transfer of phenylalanine from the adenylate anhydride to ester occurs and is 95-97% efficient for the L isomer and only about 50% efficient for the D isomer. The origin of the use of the L isomer, given D-ribose nucleotides, may be based in part on this stereoselectivity.

The roots of rice seedlings, growing in artificial pond water, exhibit robust gravitropic curvature when placed perpendicular to the vector of gravity. To determine whether the statolith theory (in which intracellular sedimenting particles are responsible for gravity sensing) or the gravitational pressure theory (in which the entire protoplast acts as the gravity sensor) best accounts for gravity sensing in rice roots, we changed the physical properties of the external medium with impermeant solutes and examined the effect on gravitropism. As the density of the external medium is increased, the rate of gravitropic curvature decreases. The decrease in the rate of gravicurvature cannot be attributed to an inhibition of growth, since rice roots grown in 100 Osm/m3 (0.248 MPa) solutions of different densities all support the same root growth rate but inhibit gravicurvature increasingly with increasing density. By contrast, the sedimentation rate of amyloplasts in the columella cells is unaffected by the external density. These results are consistent with the gravitational pressure theory of gravity sensing, but cannot be explained by the statolith theory.

It is generally thought that sedimenting plastids are responsible for gravity sensing in higher plants. We directly tested the model generated by the current statolith hypothesis that the gravity sensing that leads to gravitropism results from an interaction between the plastids and actin microfilaments. We find that the primary roots of rice, corn, and cress undergo normal gravitropism and growth even when exposed to cytochalasin D, a disruptor of actin microfilaments. These results indicate that an interaction between amyloplasts and the actin cytoskeleton is not critical for gravity sensing in higher plants and weaken the current statolith hypothesis.