Circadian gating of dark‐induced increases in chloroplast‐ and cytosolic‐free calcium in Arabidopsis

Summary Changes in the spatiotemporal concentration of free Ca 2+ ([Ca 2+ ]) in different organelles of the cell contribute to responses of plants to physiological and environmental stimuli. One example are [Ca 2+ ] increases in the stroma of chloroplasts during light‐to‐dark transitions; however, the function and mechanisms responsible are unknown, in part because there is a disagreement in the literature concerning whether corresponding dark‐induced changes in cytosolic [Ca 2+ ] ([Ca 2+ ] cyt ) can be detected. We have measured changes in [Ca 2+ ] cyt upon darkness in addition to the already known dark‐induced increases in [Ca 2+ ] stroma in the aerial part of the Arabidopsis thaliana plant. These [Ca 2+ ] cyt transients depend on the photoperiod and time of day, peaking at anticipated dusk, and are superimposed on daily 24 h oscillations in [Ca 2+ ] cyt . We also find that the magnitude of the dark‐induced increases in Ca 2+ in both the cytosol and chloroplasts are gated by the nuclear circadian oscillator. The modulation of the magnitude of dark‐induced increases in [Ca 2+ ] stroma and [Ca 2+ ] cyt by transcriptional regulators in the nucleus that are part of the circadian oscillator demonstrates a new role for the circadian system in subcellular Ca 2+ signalling, in addition to its role in driving circadian oscillations of [Ca 2+ ] in the cytosol and chloroplasts.