In Arabidopsis thaliana, the rhythm of sugar production by photosynthesis sets the timing of the circadian clock, by regulating the expression of circadian clock genes. In plants, the production of sugar by photosynthesis is a key metabolic output of the circadian clock. This study demonstrates that the rhythmic endogenous sugar signals can set the timing of the circadian clock in Arabidopsis by regulating the expression of circadian clock genes. The authors propose the concept of a 'metabolic dawn' that describes the resetting of the circadian clock in response to a peak in endogenous sugars produced by photosynthesis. Circadian clocks provide a competitive advantage in an environment that is heavily influenced by the rotation of the Earth1,2, by driving daily rhythms in behaviour, physiology and metabolism in bacteria, fungi, plants and animals3,4. Circadian clocks comprise transcription–translation feedback loops, which are entrained by environmental signals such as light and temperature to adjust the phase of rhythms to match the local environment3. The production of sugars by photosynthesis is a key metabolic output of the circadian clock in plants2,5. Here we show that these rhythmic, endogenous sugar signals can entrain circadian rhythms in Arabidopsis thaliana by regulating the gene expression of circadian clock components early in the photoperiod, thus defining a ‘metabolic dawn’. By inhibiting photosynthesis, we demonstrate that endogenous oscillations in sugar levels provide metabolic feedback to the circadian oscillator through the morning-expressed gene PSEUDO-RESPONSE REGULATOR 7 (PRR7), and we identify that prr7 mutants are insensitive to the effects of sucrose on the circadian period. Thus, photosynthesis has a marked effect on the entrainment and maintenance of robust circadian rhythms in A. thaliana, demonstrating that metabolism has a crucial role in regulation of the circadian clock.

Photosynthetic entrainment of the Arabidopsis thaliana circadian clock