Description:
<p>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 ([Ca<sup>2+</sup>]<sub>c</sub>) as an important factor, but direct evidence for changes in [Ca<sup>2+</sup>]<sub>c</sub> is currently lacking. We now have made measurements of [Ca<sup>2+</sup>]<sub>c</sub> 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 [Ca<sup>2+</sup>]<sub>c</sub> signaling occurs in response to gravistimulation with kinetics very different from [Ca<sup>2+</sup>]<sub>c</sub> transients evoked by other mechanical stimuli (e.g. movement and wind). [Ca<sup>2+</sup>]<sub>c</sub> 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-(1-naphthyl) phthalamic acid interfere with gravi-induced [Ca<sup>2+</sup>]<sub>c</sub> responses and addition of methyl indole-3-acetic acid to whole seedlings induces long-lived [Ca<sup>2+</sup>]<sub>c</sub> transients, suggesting that changes in auxin transport may interact with [Ca<sup>2+</sup>]<sub>c</sub>. Permanent nonaxial rotation of seedlings on a two-dimensional clinostat, however, produced a sustained elevation of the [Ca<sup>2+</sup>]<sub>c</sub> level. This probably reflects permanent displacement of gravity-sensing cellular components and/or disturbance of cytoskeletal tension. It is concluded that [Ca<sup>2+</sup>]<sub>c</sub> is part of the gravity transduction mechanism in young Arabidopsis seedlings.</p>