Plant Physiology Preview
Published on December 29, 2005; 10.1104/pp.105.075119
Received December 5, 2005
Returned for revision December 13, 2005
Accepted December 13, 2005
A universal role for inositol 1,4,5-trisphosphate-mediated signaling in plant gravitropism
Imara Y. Perera *, Chiu-Yueh Hung , Shari Brady , Gloria K. Muday , and Wendy F. Boss
Department of Botany, North Carolina State University, Raleigh, North Carolina 27695-7612
Department of Biology, Wake Forest University, Winston-Salem, North Carolina, 27109-7325
* Corresponding author; email: imara_perera{at}ncsu.edu.
Inositol 1,4,5-trisphosphate (InsP3) has been implicated in the early signaling events of plants linking gravity sensing to the initiation of the gravitropic response. However at present, the contribution of the phosphoinositide signaling pathway in plant gravitropism is not well understood. In order to delineate the role of InsP3 in plant gravitropism, we generated Arabidopsis thaliana plants constitutively expressing the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase), an enzyme that specifically hydrolyzes InsP3. The transgenic plants show no significant differences in growth and life cycle compared to wild type plants, although basal InsP3 levels are reduced by greater than 90% compared to wild type plants. With gravistimulation, InsP3 levels in inflorescence stems of transgenic plants show no detectable change, whereas in wild type plant inflorescences, InsP3 levels increase  3 fold within the first 5-15 min of gravistimulation, preceding visible bending. Furthermore, gravitropic bending of the roots, hypocotyls, and inflorescence stems of the InsP 5-ptase transgenic plants is reduced by 30% compared with the wild type. Additionally, the cold "memory" response of the transgenic plants is attenuated, indicating that InsP3 contributes to gravisignaling in the cold. The transgenic roots were shown to have altered calcium sensitivity in controlling gravitropic response, a reduction in basipetal IAA transport, and a delay in the asymmetric auxin-induced GUS expression with gravistimulation as compared to the controls. The compromised gravitropic response in all the major axes of growth in the transgenic Arabidopsis plants reveals a universal role for InsP3 in the gravity signal transduction cascade of plants.
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