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

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 degrees, 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) ethaneN, 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.