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Studies of Plastid Sedimentation Kinetics in Arabidopsis Hypocotyls Using a Cryofixation Method.  M. Palmieri and J.Z. Kiss  Botany Department, Miami University, Oxford, OH.  

   Gravitropism, the directed growth of a plant in response to gravity, comprises three temporal phases:  perception, signal transduction, and response.  In higher plants, perception occurs in specialized cells (statocytes) that contain dense, starch-filled, sedimentable amyloplasts.  Signal transduction occurs when the mechanical settling of gravistimulated amyloplasts is converted into a biochemical signal.  The F-actin cytoskeleton has been implicated in the signal transduction phase of gravitropism.  Studies utilizing pharmacological depolymerization of F-actin with latrunculin B indicate a promotion of gravitropic curvature in roots, hypocotyls and inflorescence stems.  Why gravitropic curvature is promoted when the F-actin cytoskeleton is disrupted remains unclear.  One hypothesis is that settling amyloplasts may terminate the curvature response, and disruption of the cytoskeleton potentially interferes with this process. 

   This study was undertaken to assess the relationship between amyloplast sedimentation and the F-actin cytoskeleton in the statocytes of Arabidopsis hypocotyls during gravitropism.  Etiolated Arabidopsis seedlings were placed on media that either possessed or lacked latrunculin B, and then they were reoriented and cryofixed at intervals following reorientation.  The specimens were further processed via freeze substitution and visualized via light microscopy.  Digital images were captured and plastid position was analyzed with image analysis software.  Results indicate that cytoskeletal disruption causes a dramatic change in plastid sedimentation kinetics.  Whereas amyloplasts settle to the new cell bottom within 5 minutes after reorientation in control specimens, they cluster and are not settled after 10 minutes in specimens exposed to latrunculin B.  Reduced amyloplast mobility after cytoskeletal disruption is consistent with an active mechanism of statolith transport within the statocyte.  [Supported by NASA: NCC2-1200 and NGT5-50480].