Flowering is often triggered by exposing plants to appropriate day lengths. This response requires an endogenous timer called the circadian clock to measure the duration of the day or night. This timer also controls daily rhythms in gene expression and behavioural patterns such as leaf movements. Several Arabidopsis mutations affect both circadian processes and flowering time; but how the effect of these mutations on the circadian clock is related to their influence on flowering remains unknown. Here we show that expression of CONSTANS (CO), a gene that accelerates flowering in response to long days, is modulated by the circadian clock and day length. Expression of a CO target gene, called FLOWERING LOCUS T (FT), is restricted to a similar time of day as expression of CO. Three mutations that affect circadian rhythms and flowering time alter CO and FT expression in ways that are consistent with their effects on flowering. In addition, the late flowering phenotype of such mutants is corrected by overexpressing CO. Thus, CO acts between the circadian clock and the control of flowering, suggesting mechanisms by which day length regulates flowering time.

Several genes are known to regulate circadian rhythms in Arabidopsis, but the identity of the central oscillator has not been established. LHY and CCA1 are related MYB-like transcription factors proposed to be closely involved. Here we demonstrate that, as shown previously for CCA1, inactivation of LHY shortens the period of circadian rhythms in gene expression and leaf movements. By constructing lhy cca1-1 double mutants, we show that LHY and CCA1 are partially redundant and essential for the maintenance of circadian rhythms in constant light. Under light/dark cycles the lhy cca1-1 plants show dramatically earlier phases of expression of GI and TOC1, genes associated with the generation of circadian rhythms and the promotion of LHY and CCA1 expression. We conclude that LHY and CCA1 appear to be negative regulatory elements required for central oscillator function.