title: Reorientation of seedlings in the earth's gravitational field induces cytosolic calcium transients.
abstract: 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(2+)](c)) as an important factor, but direct evidence for changes in [Ca(2+)](c) is currently lacking. We now have made measurements of [Ca(2+)](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 [Ca(2+)](c) signaling occurs in response to gravistimulation with kinetics very different from [Ca(2+)](c) transients evoked by other mechanical stimuli (e.g. movement and wind). [Ca(2+)](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-(1-naphthyl) phthalamic acid interfere with gravi-induced [Ca(2+)](c) responses and addition of methyl indole-3-acetic acid to whole seedlings induces long-lived [Ca(2+)](c) transients, suggesting that changes in auxin transport may interact with [Ca(2+)](c). Permanent nonaxial rotation of seedlings on a two-dimensional clinostat, however, produced a sustained elevation of the [Ca(2+)](c) level. This probably reflects permanent displacement of gravity-sensing cellular components and/or disturbance of cytoskeletal tension. It is concluded that [Ca(2+)](c) is part of the gravity transduction mechanism in young Arabidopsis seedlings.
authors.affiliation: Zentrum für Biochemie und Molekularbiologie, Christian-Albrechts-Universität, Am Botanischen Garten 1-9 (Room 517), D-24118 Kiel, Germany. cplieth@zbm.uni-kiel.de
authors.forename: Christoph
authors.initials: C
authors.lastname: Plieth
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Trewavas
mesh_terms: D000222:Adaptation, Physiological; D000331:Aequorin / Q000235:genetics / Q000378:metabolism; D017360:Arabidopsis / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D003593:Cytoplasm / Q000502:physiology; D004867:Equipment Design; D018522:Gravitropism / Q000502:physiology*; D018466:Gravity Sensing / Q000502:physiology*; D007093:Imidazoles; D007210:Indoleacetic Acids / Q000494:pharmacology; D010797:Phthalimides / Q000494:pharmacology; D011719:Pyrazines / Q000378:metabolism; D013314:Stress, Mechanical; D014283:Triiodobenzoic Acids / Q000494:pharmacology
chemical_list: D007093:Imidazoles; D007210:Indoleacetic Acids; D010797:Phthalimides; D011719:Pyrazines; D014283:Triiodobenzoic Acids; C002687:alpha-naphthylphthalamic acid; C017144:coelenterazine; D000331:Aequorin; D002118:Calcium