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Effect of arg1 Mutation on Plastid Sedimentation in Arabidopsis Hypocotyls.  N.S. Kumar and J.Z. Kiss, Department of Botany, Miami University, Oxford, OH.

   The ARG1 (altered response to gravity) protein is hypothesized to be involved in signal transduction phase of gravitropism, and ARG1 is essential for normal gravitropism in roots and hypocotyls of Arabidopsis. Previous studies have characterized gravitropic curvatures in roots and hypocotyls of dark-grown seedlings and found that they are impaired in gravitropism. The arg1 mutants demonstrated lower curvature angles compared to the wild-type. We wanted to investigate whether arg1 mutation affects plastid movement in the gravity-perceiving endodermal cells of hypocotyls. For this, we conducted light microscopic studies to analyze plastid sedimentation rates in hypocotyls of light-grown WS and arg1 seedlings. Movement of plastids was studied in vertical and 90° reoriented seedlings that were processed by cryofixation and freeze substitution techniques. The hypocotyls were sectioned, stained and observed using light microscopy. Results show that arg1 mutation affects plastid movement in hypocotyls. Plastid movement is reduced in the endodermal cells of arg1 compared to those of the wild-type. Our data also indicate that arg1 mutation reduces gravitropism in hypocotyls by modulating plastid sedimentation and that arg1 mutants are likely to be altered in the actin cytoskeleton. Therefore, these studies support the hypothesis that ARG1 is involved in the early events of gravitropic signal transduction in shoots.

(Supported by NASA grant: NCC2-1200).