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

Changes in the spatiotemporal concentration of free Ca2+ ([Ca2+]) in different organelles of the cell contribute to responses of plants to physiological and environmental stimuli. One example are [Ca2+] 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 [Ca2+] ([Ca2+](cyt)) can be detected. We have measured changes in [Ca2+](cyt) upon darkness in addition to the already known dark-induced increases in [Ca2+](stroma) in the aerial part of the Arabidopsis thaliana plant. These [Ca2+](cyt) transients depend on the photoperiod and time of day, peaking at anticipated dusk, and are superimposed on daily 24 h oscillations in [Ca2+](cyt). We also find that the magnitude of the dark-induced increases in Ca2+ in both the cytosol and chloroplasts are gated by the nuclear circadian oscillator. The modulation of the magnitude of dark-induced increases in [Ca2+](stroma) and [Ca2+](cyt) by transcriptional regulators in the nucleus that are part of the circadian oscillator demonstrates a new role for the circadian system in subcellular Ca2+ signalling, in addition to its role in driving circadian oscillations of [Ca2+] in the cytosol and chloroplasts.