Abstract
Plants sense gravity and regulate their morphology, known as gravitropism. In the process of gravitropism, changes in the gravity vector (gravistimulation) are transduced into certain intracellular signals. Previous studies showed that gravistimulation induced increases in the cytoplasmic calcium concentration ([Ca2+]c) consisting of an initial peak followed by a second one. However, it is obscure whether the dual peak is evoked by gravistimulation and/or rotation, since gravistimulation is generally accompanied by rotation under 1g condition. We analyzed [Ca2+]c in seedlings of Arabidopsis thaliana expressing aequorin under multiple gravitational acceleration conditions created by parabolic flights. The second [Ca2+]c peak was not induced when the seedlings were turned through 180o under microgravity condition, whereas the initial one was often observed. The second [Ca2+]c peak was strongly dependent on the gravitational acceleration. These results suggest that the second peak is induced by gravistimulation, whereas the initial one is induced by rotation.
