Intracellular Calcium Decreases Upon Hyper Gravity-Treatment of <Emphasis Type="Italic">Arabidopsis Thaliana</Emphasis> Cell Cultures

Cell cultures of Arabidopsis thaliana (A.t.) respond to changes in the gravitational field strength with fluctuations of the amount of cytosolic calcium (Ca²⁺). In parabolic flight experiments, where hyper- and μg phases follow each other, μg clearly increased Ca²⁺, while hyper-g caused a slight reduction. Since the latter observation had not been reported before, we studied this effect in more detail. Using a special centrifuge for heavy items (ZARM, Bremen, Germany), we determined the hyper-g-dependent intracellular Ca²⁺ level with transgenic cell lines expressing the Ca²⁺ sensor, cameleon. This sensor exhibits a shift in fluorescence from 480 to 530 nm in response to Ca²⁺ binding. The data show a drop in the intracellular Ca²⁺ concentration with a threshold gravity of around 3 g. This is above hypergravity levels achieved during parabolic flights (1.8 g). The use of mutants with different sub-cellular targets of cameleon expression (nucleus, tonoplast, plasma membrane) gave the same results, i.e. Ca²⁺ is obviously exported from several intracellular compartments.