Editors' ChoiceCircadian Rhythms

Leaves Have More Than One Clock

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Science's STKE  24 Sep 2002:
Vol. 2002, Issue 151, pp. tw347
DOI: 10.1126/stke.2002.151.tw347

Animals and plants exhibit rhythmic behaviors and regulation of gene expression that are entrained by the light-dark cycle but that can persist even in the absence of light-dark signaling. These are called circadian rhythms, and the molecular mechanism for them is the circadian clock. There are many questions regarding the roles for a central clock versus peripheral clocks. Thain et al. discovered another gene with rhythmic expression in roots and leaves--the gene for chalcone synthase (CHS). CHS expression did not follow the same period as a well-known rhythmically expressed gene for chlorophyll a/b-binding protein (CAB), which suggests different autonomous clocks controlling the expression of these two genes. The promoter of the mustard plant or Arabidopsis CHS gene was fused to a luciferase reporter gene, and the expression of the reporter was monitored after a transition to continuous light in Arabidopsis. The reporter was expressed in roots and in the epidermal layer of leaves. The period of expression for CHS was longer than that for CAB, which is expressed in the mesophyll layer of leaves, and was also phase-shifted, with the first peak occurring just before predicted dawn. Analysis of the expression in two clock mutants, det1 and toc1 (the presumptive light input sensor and the oscillator, respectively), decreased the period of both CAB and CHS expression, but their expression was still such that the CHS periodicity was longer than that for CAB. These results suggest that the leaf clocks both use the DET1 and TOC1 components. CAB is part of the light-harvesting complex, whereas CHS is involved in flavonoid biosythesis. The authors propose that the mesophyll and epidermal layers of the leaf have independent clocks that have some conserved components, but that there may be tissue-specific regulators that alter the periodicity of the clocks in the same organ.

S. C. Thain, G. Murtas, J. R. Lynn, R. B. McGrath, A. J. Millar, The circadian clock that controls gene expression in Arabidopsis is tissue specific. Plant Physiol. 130, 102-110 (2002). [Abstract] [Full Text]

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