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PLASTID SEDIMENTATION KINETICS IN ENDODERMAL CELLS OF ARABIDOPSIS.  M. Palmieri and J. Z. Kiss.  Department of Botany, Miami University, Oxford, Ohio.

   The endodermal cells of hypocotyls in flowering plants possess starch-filled organelles called amyloplasts, which appear to function in gravity perception.  Following reorientation of seedlings, the amyloplasts settle to the new bottom of the cell.   The seedlings then exhibit a differential growth response in which the roots grow towards the gravity vector, and the hypocotyls grow away from gravity.  The magnitude of this graviresponse has been correlated to plastid sedimentation in the columella cells of roots.  Research in our laboratory has shown that a decrease in the amyloplast starch content causes less plastid sedimentation, resulting in reduced gravitropic curvature of starch-deficient Arabidopsis seedlings.  This study aims to determine the sedimentation kinetics of amyloplasts in the hypocotyl endodermal cells of Arabidopsis thaliana.  Wild-type Arabidopsis seedlings were dark-grown and fixed at regular intervals following reorientation with respect to gravity.  One micrometer median longitudinal sections were stained with Toluidine Blue and amyloplast position was visualized using light microscopy.  Brightfield images were captured digitally, and image analysis was performed using Image Pro Plus software (Version 3.0; Media Cybernetics, Silver Spring, MD).  The plastid positions and sedimentation rates in wild-type Arabidopsis provide important baseline data for future experiments involving disruption of the F-actin cytoskeleton and analysis of the effect of this disruption on plastid sedimentation kinetics and the graviresponse. 

(Supported by NASA: NCC2-1200, and NIH: 1R15GM 57806-01.)