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Pharmacological Alteration of the Actomyosin System and Subsequent Effects on Growth, Gravitropic Curvature, and Amyloplast Kinetics in Etiolated Arabidopsis Hypocotyls.   M. Palmieri, R. E. Edelmann and John Z. Kiss.  Dept. of Botany, Miami University, Oxford, OH, USA

   Gravitropism is the directed growth of a plant in response to gravity.  The process can be divided into the perception, signal transduction, signal transmission and response phases.  In flowering plants, perception occurs when amyloplasts sediment within shoot endodermal cells and root columella cells.  Signal transduction occurs when the mechanical settling of gravistimulated amyloplasts is converted into a biochemical signal, and actin microfilaments have been implicated in this process.  This study was undertaken to assess the relationship among the actomyosin system, gravicurvature, and amyloplast sedimentation in the statocytes of Arabidopsis hypocotyls.  Etiolated Arabidopsis seedlings with GFP-labeled amyloplasts in the endodermis were grown in Petri plates on media that either possessed or lacked actomyosin altering drugs.  The plates were then rotated 90° and photographed at intervals following reorientation.  In addition, we used a confocal laser scanning microscope with a horizontally-mounted stage to observe plastids in living cells following gravistimulation by reorientation.  Confocal images were captured digitally and plastid sedimentation kinetics were analyzed with image analysis software.  A comparison of plastid kinetics before and after actomyosin disruption will help to better define the role of statoliths and their interaction with the cytoskeleton as they traverse the statocyte during gravitropism.  [Supported by NASA: NCC2-1200 and NGT5-50480.]