Are there multiple circadian clocks in plants?

Abstract

We have reported that Arabidopsis might have genetically distinct circadian oscillators in multiple cell-types. 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. However, we found that cytosolic free Ca²⁺ ([Ca²⁺]_cyt) oscillations were undetectable in these mutants in the same light conditions1. 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 (~21 h) but, surprisingly, has a wild-type period for circadian [Ca²⁺]_cyt oscillations (~24 h). In contrast, toc1-2, a TOC1 loss-of-function mutant, has a short period of both CAB2 and [Ca²⁺]_cyt rhythms (~21h). 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.

Acknowledgements

This research was funded by the USA National Institute of Mental Health (R01 MH43836) to C.H.J. and the BBSRC UK to A.A.R.W., who is also grateful to the Royal Society of London for the award of a University Research Fellowship. CTH is supported by a CAPES, Brazil Scholarship.

Figures and Tables

Figure 1 Models of how the toc1-1 mutation might differently affect cell-type specific circadian oscillators. The single mutant toc1-1 have 21 h rhythms of CAB2 promoter activity and 24 h-rhythms of [Ca²⁺]_cyt oscillations. The toc1-1 mutation is a single amino acid change in the CCT domain. The CCT domain is involved in protein-protein interaction and/or nuclear localization. We have proposed that circadian oscillators with different periods are present in different cell-types. The luminescence generated by CAB2 promoter-drived luciferase (from the CAB2:luc) is probably originated in the epidermis and mesophyll cells. In this model, we propose that the mutation on the CCT domain impairs the mutated TOC1 interaction with the hypothetical protein Z in these cells-types. In contrast, in other cell-types, the mutated TOC1 still interacts with other hypothetical proteins (W), despite the mutation in the CCT domain. In those cell-types, the circadian oscillator could still run with a 24 h period for [Ca²⁺]_cyt rhythms (from the 35S:AEQ construct). One possible identity for Z and W are the members of the PHYTOCHROME INTERACTING FACTOR (PIF) related PIF3-LIKE (PIL) family.

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