Restoration of gravitropic sensitivity in starch-deficient mutants of Arabidopsis by hypergravity

Despite the extensive study of plant gravitropism, there have been few expe riments which have utilized hypergravity as a tool to investigate gravisens itivity in flowering plants. Previous studies have shown that starch-defici ent mutants of Arabidopsis are less sensitive to gravity compared to the wi ld-type (WT), In this report, the question addressed was whether hypergravi ty could restore the sensitivity of starch-deficient mutants of Arabidopsis , The strains examined include a WT, a starchless mutant and a reduced-star ch mutant. Vertical orientation studies with dark-grown seedlings indicate that increased centrifugal acceleration improves orientation relative to th e acceleration vector for all strains, even the WT. For starchless roots, g rowth of seedlings under constant 5 g acceleration was required to restore orientation to the level of the WT at 1 g, In contrast, approximately 10 g was required to restore the orientation of the starchless mutant hypocotyls to a WT level at 1 g, Examination of plastid position in root cap columell a cells of the starchless mutant revealed tt-tat the restoration of gravitr opic sensitivity was correlated with the sedimentation of plastids toward t he distal cell wall. Even in WT plants, hypergravity caused greater sedimen tation of plastids and improved gravitropic capability. Collectively, these experiments support the hypothesis of a statolith-based system of gravity perception in plants. As far as is known, this is the first report to use h ypergravity to study the mechanisms of gravitropism in Arabidopsis.