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

Changes in the spatiotemporal concentration of free Ca ([Ca ]) in different organelles of the cell contribute to responses of plants to physiological and environmental stimuli. One example are [Ca ] 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 ] ([Ca ] ) can be detected. We have measured changes in [Ca ] upon darkness in addition to the already known dark-induced increases in [Ca ] in the aerial part of the Arabidopsis thaliana plant. These [Ca ] transients depend on the photoperiod and time of day, peaking at anticipated dusk, and are superimposed on daily 24 h oscillations in [Ca ] . We also find that the magnitude of the dark-induced increases in Ca in both the cytosol and chloroplasts are gated by the nuclear circadian oscillator. The modulation of the magnitude of dark-induced increases in [Ca ] and [Ca ] by transcriptional regulators in the nucleus that are part of the circadian oscillator demonstrates a new role for the circadian system in subcellular Ca signalling, in addition to its role in driving circadian oscillations of [Ca ] in the cytosol and chloroplasts.