Distinct light and clock modulation of cytosolic free Ca2+ oscillations and rhythmic CHLOROPHYLL A/B BINDING PROTEIN2 promoter activity in Arabidopsis.

Plants have circadian oscillations in the concentration of cytosolic free calcium (Ca(2+)). To dissect the circadian Ca(2+)-signaling network, we monitored circadian Ca(2+) oscillations under various light/dark conditions (including different spectra) in Arabidopsis thaliana wild type and photoreceptor and circadian clock mutants. Both red and blue light regulate circadian oscillations of Ca(2+). Red light signaling is mediated by PHYTOCHROME B (PHYB). Blue light signaling occurs through the redundant action of CRYPTOCHROME1 (CRY1) and CRY2. Blue light also increases the basal level of Ca(2+), and this response requires PHYB, CRY1, and CRY2. Light input into the oscillator controlling Ca(2+) rhythms is gated by EARLY FLOWERING3. Signals generated in the dark also regulate the circadian behavior of Ca(2+). Oscillations of Ca(2+) and CHLOROPHYLL A/B BINDING PROTEIN2 (CAB2) promoter activity are dependent on the rhythmic expression of LATE ELONGATED HYPOCOTYL and CIRCADIAN CLOCK-ASSOCIATED1, but Ca(2+) and CAB2 promoter activity are uncoupled in the timing of cab1 (toc1-1) mutant but not in toc1-2. We suggest that the circadian oscillations of Ca(2+) and CAB2 promoter activity are regulated by distinct oscillators with similar components that are used in a different manner and that these oscillators may be located in different cell types in Arabidopsis.