Reorientation of seedlings in the earth's gravitational field induces cytosolic calcium transients

The gravitational field controls plant growth, morphology, and development. However, the underlying transduction mechanisms are not well understood. Much indirect evidence has implicated the cytoplasmic free calcium concentration ([Ca2+](c)) as an important factor, but direct evidence for changes in [Ca2+](c) is currently lacking. We now have made measurements of [Ca2+](c) in groups of young seedlings of Arabidopsis expressing aequorin in the cytoplasm and reconstituted in vivo with cp-coelenterazine, a synthetic high-affinity luminophore. Distinct [Ca2+](c) signaling occurs in response to gravistimulation with kinetics very different from [Ca2+](c) transients evoked by other mechanical stimuli (e.g. movement and wind). [Ca2+](c) changes produced in response to gravistimulation are transient but with a duration of many minutes and dependent on stimulus strength (i.e. the angle of displacement). The auxin transport blockers 2,3,5-tri-iodo benzoic acid and N-(I-naphthyl) phthalamic acid interfere with gravi-induced [Ca2+](c) responses and addition of methyl indole-3-acetic acid to whole seedlings induces long-lived [Ca2+](c) transients, suggesting that changes in auxin transport may interact with [Ca2+](c). Permanent nonaxial rotation of seedlings on a two-dimensional clinostat, however, produced a sustained elevation of the [Ca2+](c) level. This probably reflects permanent displacement of gravity-sensing cellular components and/or disturbance of cytoskeletal tension. It is concluded that [Ca2+](c) is part of the gravity transduction mechanism in young Arabidopsis seedlings.