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

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.