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Published on November 30, 2007; 10.1104/pp.107.106450

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Received July 31, 2007
Accepted November 16, 2007

Cytoplasmic calcium increases in response to changes in the gravity vector in hypocotyls and petioles of Arabidopsis seedlings

Masatsugu Toyota , Takuya Furuichi , Hitoshi Tatsumi , and Masahiro Sokabe *

Department of Physiology, Nagoya University Graduate School of Medicine, 65 Tsurumai, Showa-ku, Nagoya, Aichi 466-8550, Japan; Molecular Plant Physiology, University of Erlangen, Staudtstrasse 5, Erlangen D-91058, Germany; ICORP/SORST, Cell Mechanosensing Project, Japan Science and Technology Agency, 65 Tsurumai, Showa-ku, Nagoya, Aichi 466-8550, Japan; Department of Molecular Physiology, National Institute for Physiological Sciences, Okazaki, Aichi 444-8585, Japan

* Corresponding author; email: msokabe{at}med.nagoya-u.ac.jp.

Plants respond to a large variety of environmental signals including changes in the gravity vector (gravistimulation). In 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 ultra-sensitive 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 through 180o, 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 suggest 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 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.






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