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The Role of the F-Actin Cytoskeleton in Signal Transduction Mechanisms of Gravitropism in Arabidopsis Hypocotyls.  Maria Palmieri and John Z. Kiss, Miami University, Oxford, OH 45056.

   Gravitropism, the directed growth of a plant in response to gravity, occurs in several temporal stages:  perception, signal transduction, and response.  Plants possess specialized cells termed statocytes, which function in graviperception.  These cells contain statoliths (in higher plants, amyloplasts), which are dense structures that move relative to gravity.  In terms of gravitropic signal transduction, the F-actin cytoskeleton has been implicated in this process of gravitropism in both roots and stem-like organs.  In a recent study, Yamamoto and Kiss (2002; Plant Physiol 128: 669 – 681) reported a promotion of gravitropic curvature concomitant with application of the actin-disrupting drug latrunculin B in inflorescence stems and hypocotyls of Arabidopsis.  Why gravitropic curvature is promoted in shoots and hypocotyls when the MF network is disrupted remains unclear. 
   This study was undertaken to assess the relationship between amyloplast sedimentation and the actin cytoskeleton in the endodermal cells (= 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 90° with respect to the gravity vector 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.  A comparison of plastid sedimentation kinetics before and after cystoskeletal disruption with latrunculin B should better define the role of statoliths and their interaction with the cytoskeleton as they traverse the statocyte during gravitropism. 

[Supported by the National Aeronautics and Space Administration (grant no. NCC2–1200) and the Academic Challenge program through the Department of Botany at Miami University.]