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The Rate of Plastid Sedimentation in the Hypocotyls of the arg1 (altered response to gravity) Mutant of Arabidopsis.  N.S. Kumar, M. Palmieri and J.Z. Kiss. Department of Botany, Miami University, Oxford, OH 45056.

   Gravity perception in plants occurs by the sedimentation of starch-filled amyloplasts called statoliths. These statoliths are located in the endodermal cells of stem-like organs and columella cells of root cap. Gravity perception leads to signal transduction that eventually results in tropic curvature in plants. In these experiments, we studied arg1 mutants (kindly provided by Dr. P. Masson, University of Wisconsin), which are impaired in gravitropism. The ARG1 gene encodes a DnaJ-like protein whose coiled coil region is homologous to the coiled coil of cytoskeleton-associated proteins. Studies have shown that arg1 is impaired in the early stages of gravitropism, but it is not known if plastid movement is affected by the arg1 mutation. This study was conducted to examine whether the arg1 mutation affects gravitropism by modulating plastid sedimentation in gravity perceiving endodermal cells of hypocotyls. Light microscopic studies were conducted to analyze plastid sedimentation in hypocotyls of light-grown seedlings. Sedimentation of plastids was studied in vertical and 90° reoriented arg1 and wild-type (Wassewskija-WS) seedlings using cryofixation/freeze substitution, since these methods provide a reliable indication of rapid cellular events. Results on plastid movement in arg1 versus wild type will be presented. This research will provide insight in the role of ARG1 in the gravitropism pathway in plants.

(Supported by NASA grant: NCC2-1200).