Multiple circadian clock outputs regulate diel turnover of carbon and nitrogen reserves

Plants accumulate reserves in the daytime to support growth at night. Circadian regulation of diel reserve turnover was investigated by profiling starch, sugars, glucose 6-phosphate, organic acids and amino acids during a light-dark cycle and after transfer to continuous light in Arabidopsis wild-types and in mutants lacking dawn (lhy cca1), morning (prr7 prr9), dusk (toc1, gi) or evening (elf3) clock components. The metabolite time-series were integrated with published time-series for circadian clock transcripts to identify circadian outputs that regulate central metabolism. i) Starch accumulation was slower in elf3 and prr7 prr9. It is proposed that ELF3 positively regulates starch accumulation. ii) Reducing sugars were high early in the T-cycle in elf3, revealing that ELF3 negatively regulates sucrose recycling. iii) The pattern of starch mobilization was modified in all five mutants. A model is proposed in which dawn and dusk/evening components interact to pace degradation to anticipated dawn. iv) An endogenous oscillation of glucose 6-phosphate revealed that the clock buffers metabolism against the large influx of carbon from photosynthesis. v) Low levels of organic and amino acids in lhy cca1 and high levels in prr7 prr9 provide evidence that the dawn components positively regulate the accumulation of amino acid reserves.