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UNRAVELLING TOUCH AND GRAVITY STIMULATION IN PLANT ROOTS.  G.D. Massa and S. Gilroy.  Dept of Biology, The Pennsylvania State University

   An initial characterization of primary and lateral root responses to obstacles led us to suspect that touching the obstacle modulates the graviresponse in these roots.  Since, under natural conditions, roots would rarely reorient in the absence of mechanical stimulation, we propose that both touch stimulation and gravistimulation are integrated simultaneously in cells of the root cap, leading to the observed biphasic bending response.  Experiments with roots that were touched and then gravistimulated and placed on a clinostat show that touching the root cap, but not elsewhere in the root, leads to an inhibition of subsequent gravicurvature.

   To identify possible signaling mechanisms, we have analyzed ionic signaling using Arabidopsis expressing GFP’s sensitive to Ca²⁺ and pH. In addition we have analyzed the touch response in mutants such as the Arabidopsis potassium channel mutant akt1-1, and the response of these roots to a barrier appears similar to that of wild-type seedlings.  We have also applied a pharmacological approach, for example wild-type roots were treated with Gd³⁺, proposed to inhibit apoplastic calcium influx through mechanically gated channels at the plasma membrane.  While this chemical did transiently reduce the growth rate of treated roots, the biphasic bending response to an obstacle appears unchanged.  In conjunction with the results of Ca²⁺ imaging these results suggest sustained Ca²⁺ increase at the plasma membrane may not mediate the long-term response to an obstacle.  The effects of touch on the cytoskeleton are being analyzed using plants transformed with GFP fusion protein complexes.  (Supported by NSF: IBN9874445, NASA: NAG2-1366)

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