Molecular basis of seasonal time measurement in Arabidopsis

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Several organisms have evolved the ability to measure daylength, or photoperiod, allowing them to adjust their development in anticipation of annual seasonal changes. Daylength measurement requires the integration of temporal information, provided by the circadian system, with light/dark discrimination, initiated by specific photoreceptors. Here we demonstrate that in Arabidopsis this integration takes place at the level of CONSTANS (CO)1 function. CO is a transcriptional activator that accelerates flowering time in long days, at least in part by inducing the expression of FLOWERING LOCUS T (FT)2,3,4,5. First, we show that precise clock control of the timing of CO expression, such that it is high during daytime only in long days, is critical for daylength discrimination. We then provide evidence that CO activation of FT expression requires the presence of light perceived through cryptochrome 2 (cry2) or phytochrome A (phyA). We conclude that an external coincidence mechanism, based on the endogenous circadian control of CO messenger RNA levels, and the modulation of CO function by light, constitutes the molecular basis for the regulation of flowering time by daylength in Arabidopsis.

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Figure 1: CO expression in the toc1-1 mutant and in wild-type plants.
Figure 2: FT expression in the toc1-1 mutant and in wild-type plants.
Figure 3: CO and FT expression in wild-type plants.
Figure 4: FT and CO expression in CO-overexpressing plants, and in wild-type, cry2 and phyA mutants.
Figure 5: Acute induction of FT expression by light.


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We thank J. J. Casal, S. Harmer, P. Mas and F. Harmon for critical reading of the manuscript. This work was supported by an NIH grant to S.A.K. The work of M.J.Y. was initially supported by Conicet, Antorchas and the University of Buenos Aires and, more recently, by the Pew Foundation.

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Correspondence to Steve A. Kay.

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Yanovsky, M., Kay, S. Molecular basis of seasonal time measurement in Arabidopsis. Nature 419, 308–312 (2002) doi:10.1038/nature00996

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