Circadian Control of Chloroplast Transcription by a Nuclear-Encoded Timing Signal

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Science  15 Mar 2013:
Vol. 339, Issue 6125, pp. 1316-1319
DOI: 10.1126/science.1230397

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Synchronizing Photosynthetic Capacity

Coordination of photosynthetic activity with sunlight benefits plant productivity. Noordally et al. (p. 1316) analyzed how the Arabidopsis circadian clock keeps the chloroplasts working in tune with the Sun. SIGMA FACTOR5 (SIG5) is encoded in the cell nucleus and reflects circadian cycles with changes in its own transcript abundance. SIG5 acts, however, in the chloroplast, where it supports photosystem II production.


Circadian timekeeping in plants increases photosynthesis and productivity. There are circadian oscillations in the abundance of many chloroplast-encoded transcripts, but it is not known how the circadian clock regulates chloroplast transcription or the photosynthetic apparatus. We show that, in Arabidopsis, nuclear-encoded SIGMA FACTOR5 (SIG5) controls circadian rhythms of transcription of several chloroplast genes, revealing one pathway by which the nuclear-encoded circadian oscillator controls rhythms of chloroplast gene expression. We also show that SIG5 mediates the circadian gating of light input to a chloroplast-encoded gene. We have identified an evolutionarily conserved mechanism that communicates circadian timing information between organelles with distinct genetic systems and have established a new level of integration between eukaryotic circadian clocks and organelles of endosymbiotic origin.

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