The effect of the external medium on the gravitropic curvature of rice (Oryza sativa, Poaceae) roots

Abstract

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.

Curvature Induced by Amyloplast Magnetophoresis in Protonemata of the Moss Ceratodon purpureus1

“…An alternate hypothesis of gravitropic sensing is based on pressure exerted by the mass of the entire protoplast on tension-or compression-sensitive components of the plasma membrane and cell wall (Wayne et al, 1992;Staves et al, 1997). If this hypothesis were true for moss protonemata, then an HGMF should produce curvature in the direction opposite to that observed.…”

Touch Sensing and Thigmotropism

“…In higher plants, statoliths are dense, starch-filled amyloplasts inside specialized cells (Sack 1997;Kiss 2000). In contrast, the gravitational pressure theory states that the entire protoplast acts as the gravity sensor and the tension and compression by the protoplast against the extracellular matrix initiates the graviresponse (Wayne and Staves 1996;Staves et al 1997aStaves et al , 1997b.…”

Plant Gravitropism. Unraveling the Ups and Downs of a Complex Process

“…Despite growing experimental support for plastidbased gravity susception, the alternative gravitational pressure model, which proposes that it is the pressure exerted by the entire cell that mediates gravity perception, has not been discounted (Staves et al, 1997). This model was tested recently in single-cell protonemata of the moss Ceratodon purpureus, which exhibit upward bending when reoriented horizontally.…”