Abstract

Circadian clocks allow optimization of growth and metabolism for organisms exposed to daily changes in environment, providing competitive advantage. Nuclear molecular oscillators form interlocking transcription-translation feedback loops, which are entrained by zeitgebers (time-givers), such as light and temperature, to provide reference to adjust the phase of the oscillator’s 24 h cycle. Here, we show that zeitgeber action of sugar into the Arabidopsis clock is distinct from light, acts early in the photoperiod and defines a ‘metabolic dawn’. By inhibiting photosynthesis in the presence of light, we reveal a role for photosynthesis in circadian entrainment acting through the morning-expressed PSEUDO RESPONSE REGULATOR 7 (PRR7) and show that prr7 mutants are insensitive to the effects of sugar on circadian period. Thus, photosynthesis has a profound effect to entrain and maintain robust circadian rhythms in Arabidopsis, demonstrating a critical role for metabolism in regulation of the circadian clock.