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amyloplasts sediment due to the gravity force and trigger complex events that result in the reorientation of plant Coleoptiles of barley (Hordeum vulgare) were posi- organs relative to their normal orientation in the gravity tioned in a high gradient magnetic field (HGMF, field. This gravitropic curvature of all or part of the dynamic factor VH2/2 of 109-1010 Oe2 cm'1), generated organ(s) re-establishes the so-called gravitropic set point by a ferromagnetic wedge in a uniform magnetic field) angle (Digby and Firn, 1995). and rotated on a 1 rpm clinostat. After 4 h 90% of Previous work has shown that high gradient magnetic coleoptiles had curved toward the HGMF. The cells fields (HGMF ) exert ponderomotive forces on amylo- affected by HGMF showed clear intracellular displace- plasts that are proportional to the dynamic factor of the ment of amyloplasts. Coleoptiles in a magnetic field field VH2/2, and opposite to the field gradient. Since next to a non-ferromagnetic wedge showed no prefe- starch-filled amyloplasts are more diamagnetic than the rential curvature. The small size of the area of non- aqueous cytoplasm they move away from the denser uniformity of the HGMF allowed mapping of the sensit- magnetic field (Audus, 1960; Audus and Whish, 1964; ivity of the coleoptiles by varying the initial position of Kuznetsov and Hasenstein, 1996; Kuznetsov and the wedge relative to the coleoptile apex. When the Kuznetsov, 1989; Piruzyan et al., 1980). Therefore, ferromagnetic wedge was placed 1 mm below the cole- HGMF with a dynamic factor VH2/2 of c .1 09-1010 Oe2 optile tip only 58% of the coleoptiles curved toward cm'1 should be able to mimic the directional eVect of the wedge indicating that the cells most sensitive to gravity. Earlier work with roots of flax (Linum usitatissi- intracellular displacement of amyloplasts and thus mum) and Arabidopsis showed starch-dependent curvature gravity sensing are confined to the top 1 mm portion away from the stronger field areas, i.e. away from the of barley coleoptiles. Similar experiments with tomato edge of a magnetized ferromagnetic wedge or away from hypocotyls (Lycopersicum esculentum) also resulted in the depth of the gap between permanent magnets curvature toward the HGMF. The data strongly support ( Kuznetsov and Hasenstein, 1996). Roots of starchless the amyloplast-based gravity-sensing system in higher mutants of Arabidopsis did not respond to the HGMF, plants and the usefulness of HGMF to substitute grav- which implies that the relatively strong diamagnetic starch ity in shoots. is necessary for the magnetophoretic displacement of amyloplasts.

Magnetophoretic induction of curvature in coleoptiles and hypocotyls