Slave to the Rhythm

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Science  19 Dec 2008:
Vol. 322, Issue 5909, pp. 1753
DOI: 10.1126/science.322.5909.1753g

Most organisms, from bacteria to humans, harbor endogenous clocks that cycle with a period of about 24 hours. These clocks function within individual cells and comprise regulatory feedback loops of transcriptional and post-translational processes. Plants are thought to use a circadian clock consisting of three light-sensitive, interlocked transcription-translation feedback loops. Because experiments have generally used plants grown on agar plates—where the roots are exposed to light—the fact that a different circadian clock operates in plant roots has been obscured. By growing plants hydroponically with the roots in darkness, James et al. (p. 1832) discovered that the root circadian clock is a stripped down version of the clock that operates in the shoots, operating on only one of the feedback loops and regulating only a small number of genes. In roots, two of the feedback loops are inactivated in that two clock components (CCA1 and LHY) do not regulate gene expression like they do in shoots. However, the shoot and root clocks are synchronized under normal day/night conditions, possibly by circulating metabolic signals, making the root clock essentially a “slave” to the shoot clock.

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