Precise timing of CONSTANS (CO) gene expression is necessary for day-length discrimination for photoperiodic flowering. The FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1) and GIGANTEA (GI) proteins regulate CO transcription in Arabidopsis. We demonstrate that FKF1and GI proteins form a complex in a blue-light dependent manner. The timing of this interaction regulates the timing of daytime CO expression. FKF1 function is dependent on GI, which interacts with a CO repressor, CYCLING DOF FACTOR 1 (CDF1), and controls CDF1 stability. GI, FKF1, and CDF1 proteins associate with CO chromatin. Thus, the FKF1-GI complex forms on the CO promoter in late afternoon to regulate CO expression, providing a mechanistic view of how the coincidence of light with circadian timing regulates photoperiodic flowering.Many plants monitor seasonal changes in day-length to regulate flowering time for successful reproduction (1). In Arabidopsis, regulation of daytime CO expression is the primary process of time measurement in the photoperiodic flowering pathway (2, 3). FKF1 and GI proteins positively regulate CO transcription (4, 5). FKF1 and GI gene expression has similar diurnal patterns (5, 6), implying that these proteins may interact to regulate CO.We tested their direct interaction in yeast and found that FKF1 interacts with GI (Fig. 1A). Our results obtained using truncated FKF1 proteins suggests that this interaction occurs through the FKF1 LOV (Light, Oxygen, or Voltage) domain (Fig. 1A). In addition, the GI N-terminus was sufficient to interact with FKF1 ( fig. S1).To assess whether this interaction occurs in vivo, and whether it is modulated by photoperiod or light conditions, we generated transgenic plants constitutively expressing both haemagglutinin (HA)-tagged FKF1 (HA-FKF1) and tandem affinity purification (TAP)-tagged GI (GI-TAP) proteins [35S∷HA-FKF1 35S∷GI-TAP lines (7)] for coimmunoprecipitation experiments. In the 35S∷HA-FKF1 35S∷GI-TAP #18 / fkf1 line, a similar amount of GI-TAP protein was precipitated at every time point in both long-day (16 hours light / 8 hours dark) and short-day (8 hours light / 16 hours dark) conditions (Fig. 1, B and C). HA-FKF1 protein was coimmunoprecipitated with GI-TAP protein (Fig. 1, B and C), demonstrating that GI-TAP and HA-FKF1 proteins form a complex in vivo. In both daylength conditions, the amount of coimmunoprecipitated HA-FKF1 protein increased until 4 hours after light onset, remained constant for the rest of day, and declined in the dark (Fig. 1, B and C), suggesting that light or the circadian clock modulate the FKF1 and GI interaction.We therefore analyzed the interaction in dark-grown samples. A minimal amount of HA-FKF1 was coimmunoprecipitated with GI-TAP protein in the dark (Fig. 1D) Next we analyzed how light quality (wavelength) affects this interaction. Similar amounts of FKF1 and GI interacted in blue-light irradiated samples (Fig. 1E) compared to white-light grown samples, but little interaction was observed in red-light irradiated samples ( Fig. 1E), indicati...

The circadian clock is required for adaptive responses to daily and seasonal changes in environmental conditions1-3. Light and the circadian clock interact to consolidate the phase of hypocotyl cell elongation to dawn under diurnal cycles in Arabidopsis thaliana4-7. Here we identify a protein complex (Evening Complex) composed of EARLY FLOWERING 3 (ELF3), EARLY FLOWERING 4 (ELF4) and the transcription factor LUX ARRHYTHMO (LUX) that directly regulates plant growth8-12. ELF3 is both necessary and sufficient to form a complex between ELF4 and LUX, and the complex is diurnally regulated, peaking at dusk. ELF3, ELF4 and LUX are required for the proper expression of the growth-promoting transcription factors PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) and PIF5 under diurnal conditions4,6,13. LUX targets the complex to the promoters of PIF4 and PIF5 in vivo. Mutations in PIF4 and/or PIF5 are epistatic to the loss of the ELF4-ELF3-LUX complex, suggesting that regulation of PIF4 and PIF5 is a critical function of the complex. Therefore, the Evening Complex underlies the molecular basis for circadian gating of hypocotyl growth in the early evening.

The temporal control of CONSTANS (CO) expression and activity is a key mechanism in photoperiodic flowering in Arabidopsis. FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1) protein regulates CO transcription, although the molecular mechanism is unknown. We demonstrate here that FKF1 controls the stability of a Dof transcription factor, CYCLING DOF FACTOR 1 (CDF1). FKF1 physically interacts with CDF1, and CDF1 protein is more stable in fkf1 mutants. Plants with elevated levels of CDF1 flower late and have reduced expression of CO. CDF1 and CO are expressed in the same tissues, and CDF1 binds to the CO promoter. Thus, FKF1 controls daily CO expression in part by degrading CDF1, a repressor of CO transcription.

Many plants use information about changing day length (photoperiod) to align their flowering time with seasonal changes to increase reproductive success. A mechanism for photoperiodic time measurement is present in leaves, and the day-length-specific induction of the FLOWERING LOCUS T (FT) gene, which encodes florigen, is a major final output of the pathway. Here, we summarize the current understanding of the molecular mechanisms by which photoperiodic information is perceived in order to trigger FT expression in Arabidopsis as well as in the primary cereals wheat, barley, and rice. In these plants, the differences in photoperiod are measured by interactions between circadian-clock-regulated components, such as CONSTANS (CO), and light signaling. The interactions happen under certain day length conditions, as previously predicted by the external coincidence model. In these plants, the coincidence mechanisms are governed by multilayered regulation with numerous conserved as well as unique regulatory components, highlighting the breadth of photoperiodic regulation across plant species.