Studies are increasingly showing that metabolic and circadian (~24 h) pathways are strongly interconnected, with the circadian system regulating the metabolic state of the cell, and metabolic products feeding back to entrain the oscillator. In plants, probably the most significant impact of the circadian system on metabolism is in its reciprocal regulation of photosynthesis; however, the pathways by which this occurs are still poorly understood. We have previously shown that members of the basic helix-loop-helix (bHLH) transcription factor PHYTOCHROME INTERACTING FACTOR (PIF) family are involved in the photosynthate entrainment of the circadian oscillator. In this paper, using Arabidopsis
mutants and overexpression lines, we examine how temperature and light quality affect PIF-mediated sucrose signaling to the oscillator and examine the contributions of individual PIF members. Our results also show that the quality of light is important for PIF signaling, with red and blue lights having the opposite effects, and that temperature affects PIF-mediated sucrose signaling. We propose the light sensitivity of PIF-mediated sucrose entrainment of the oscillator may be important in enabling plants to distinguish between sucrose produced de novo from photosynthesis during the day and the sucrose products of starch degradation at the end of the night.
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