Are there multiple circadian clocks in plants?

We have reported that Arabidopsis might have genetically distinct circadian oscillators in multiple cell-types. 1 Rhythms of CHLOROPHYLL A/B BINDING PROTEIN2 (CAB2) promoter activity are 2.5 h longer in phytochromeB mutants in constant red light and in cryptocrome1 cry2 double mutant (hy4-1 fha-1) in constant blue light than the wild-type. 2 However, we found that cytosolic free Ca2+ ([Ca2+](cyt)) oscillations were undetectable in these mutants in the same light conditions. 1 Furthermore, mutants of CIRCADIAN CLOCK ASSOCIATED1 (CCA1) have short period rhythms of leaf movement but have arrhythmic [Ca2+](cyt) oscillations. More important, the timing of cab1-1 (toc1-1) mutant has short period rhythms of CAB2 promoter activity (similar to 21 h) but, surprisingly, has a wild-type period for circadian [Ca2+] cyt oscillations (similar to 24 h). In contrast, toc1-2, a TOC1 loss-of-function mutant, has a short period of both CAB2 and [Ca2+] cyt rhythms (similar to 21 h). Here we discuss the difference between the phenotypes of toc1-1 and toc1-2 and how rhythms of CAB2 promoter activity and circadian [Ca2+](cyt) oscillations might be regulated differently.