Magnetophoretic Characterization of the Plant Gravity Receptor

Abstract

Physical characteristics (density p and magnetic susceptibility x) of amyloplasts were studied by magnetograviphoresis. The ratio of the velocities of individual amyloplasts in the presence and absence of a high gradient magnetic field (HGMF) permits the determination of the parameter Δ x/Δ p. The density of amyloplasts was measured by isopycnic centrifugation in metrizamide solutions and ranged from 1.36 to 1.38 g/cm3. The magnetic susceptibility x varied between -8.0 to -8.2x10−7emu and is close to that of starch. Magnetograviphoresis was sensitive enough to detect the reduction in the starch content of amyloplasts in light-deprived seedlings. Microscopy of flax roots showed intracellular displacement of amyloplasts in columella cells due to magnetophoresis. Continued application of HGMF resulted in curvature of roots away from HGMF, and coleoptiles toward the HGMF, similar to curvature after grayistimulation. Uniform magnetic fields caused neither curvature nor changes in growth rate. HGMF-induced curvature depended on the starch content of amyloplasts since wildtype of Arabidopsis curved but a starchless mutant did not. These data suggest that amyloplasts serve as the gravity receptor and that magnetic ponderomotive forces could substitute for gravity under is-gravity conditions.