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Analysis of the Triphasic Motor Response in Maize Root Gravitropism.  E. Natori, M.L. Evans, and H. Ishikawa  Dept. of Plant Cellular and Molecular Biology, The Ohio State University, Columbus, OH.

   Maize (Zea mays, L.) root gravitropism displays several phases of curvature.  There is an early phase (Phase 1) of downward curvature followed by a period of curvature reversal or straightening (S) and then a second phase of downward curvature (Phase 2).   When the root apex is maintained at a constant angle, the same phases appear, indicating that they are not induced by change in angle as the response proceeds.  Under conditions where the root apex is held at a constant angle of gravistimulation, Phase 2 curvature continues indefinitely.  The straightening phase of the gravitropic response is initiated in the distal elongation zone (DEZ).  A comparison of the dependence of the rate of straightening on fixed angle of orientation of the DEZ with the dependence of Phase 1 and Phase 2 curvature on this angle showed the following: 1. Both Phase 1 and S are induced even at very low angles of stimulation, while the threshold for Phase 2 is high (30^o).  2. The rate of straightening (loss of curvature, deg/min) declines at higher angles of stimulation while the rates (curvature development, deg/min) of Phases 1 and 2 increase.  3. Straightening is not observed at angles of 70^o or higher.  Instead the roots progress from Phase 1 to Phase 2 without an intervening S phase.  The prolonged steady curvature of Phase 2 under constant stimulus input is especially intriguing since it indicates that there is steady signal output to the curving region long after the initial dynamic events of reorientation.  These observations have implications for the statolith model of gravisensing.

(supported by NASA: NNA04CC65G)