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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Cytoplasmic Calcium Increases in Response to Changes in the Gravity Vector in Hypocotyls and Petioles of Arabidopsis Seedlings¹

Department of Physiology, Nagoya University Graduate School of Medicine, Showa-ku, Nagoya, Aichi 466–8550, Japan (M.T., T.F., H.T., M.S.); Molecular Plant Physiology, University of Erlangen, Erlangen D–91058, Germany (T.F.); International Cooperative Research Project/Solution Oriented Research for Science and Technology, Cell Mechanosensing Project, Japan Science and Technology Agency, Showa-ku, Nagoya, Aichi 466–8550, Japan (M.S.); and Department of Molecular Physiology, National Institute for Physiological Sciences, Okazaki, Aichi 444–8585, Japan (M.S.)

Plants respond to a large variety of environmental signals, including changes in the gravity vector (gravistimulation). In Arabidopsis (Arabidopsis thaliana) seedlings, gravistimulation is known to increase the cytoplasmic free calcium concentration ([Ca²⁺]_c). However, organs responsible for the [Ca²⁺]_c increase and the underlying cellular/molecular mechanisms remain to be solved. In this study, using Arabidopsis seedlings expressing apoaequorin, a Ca²⁺-sensitive luminescent protein in combination with an ultrasensitive photon counting camera, we clarified the organs where [Ca²⁺]_c increases in response to gravistimulation and characterized the physiological and pharmacological properties of the [Ca²⁺]_c increase. When the seedlings were gravistimulated by turning 180°, they showed a transient biphasic [Ca²⁺]_c increase in their hypocotyls and petioles. The second peak of the [Ca²⁺]_c increase depended on the angle but not the speed of rotation, whereas the initial peak showed diametrically opposite characters. This suggests that the second [Ca²⁺]_c increase is specific for changes in the gravity vector. The potential mechanosensitive Ca²⁺-permeable channel (MSCC) inhibitors Gd³⁺ and La³⁺, the Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), and the endomembrane Ca²⁺-permeable channel inhibitor ruthenium red suppressed the second [Ca²⁺]_c increase, suggesting that it arises from Ca²⁺ influx via putative MSCCs in the plasma membrane and Ca²⁺ release from intracellular Ca²⁺ stores. Moreover, the second [Ca²⁺]_c increase was attenuated by actin-disrupting drugs cytochalasin B and latrunculin B but not by microtubule-disrupting drugs oryzalin and nocodazole, implying that actin filaments are partially involved in the hypothetical activation of Ca²⁺-permeable channels. These results suggest that the second [Ca²⁺]_c increase via MSCCs is a gravity response in the hypocotyl and petiole of Arabidopsis seedlings.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Masahiro Sokabe (msokabe{at}med.nagoya-u.ac.jp).

www.plantphysiol.org/cgi/doi/10.1104/pp.107.106450

^* Corresponding author; e-mail msokabe{at}med.nagoya-u.ac.jp.

Received July 31, 2007; accepted November 16, 2007; published November 30, 2007.