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 (Ca(2+)(c)). However, organs responsible for the Ca(2+)(c) increase and the underlying cellular/molecular mechanisms remain to be solved. In this study, using Arabidopsis seedlings expressing apoaequorin, a Ca(2+)-sensitive luminescent protein in combination with an ultrasensitive photon counting camera, we clarified the organs where Ca(2+)(c) increases in response to gravistimulation and characterized the physiological and pharmacological properties of the Ca(2+)(c) increase. When the seedlings were gravistimulated by turning 180 degrees, they showed a transient biphasic Ca(2+)(c) increase in their hypocotyls and petioles. The second peak of the Ca(2+)(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 Ca(2+)(c) increase is specific for changes in the gravity vector. The potential mechanosensitive Ca(2+)-permeable channel (MSCC) inhibitors Gd(3+) and La(3+), the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), and the endomembrane Ca(2+)-permeable channel inhibitor ruthenium red suppressed the second Ca(2+)(c) increase, suggesting that it arises from Ca(2+) influx via putative MSCCs in the plasma membrane and Ca(2+) release from intracellular Ca(2+) stores. Moreover, the second Ca(2+)(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 Ca(2+)-permeable channels. These results suggest that the second Ca(2+)(c) increase via MSCCs is a gravity response in the hypocotyl and petiole of Arabidopsis seedlings.