Fig. 2: Time measurement mechanisms in the photoperiodic flowering pathway in Arabidopsis.

a The coincidence mechanisms control the formation of the FKF1-GI complex. The timing of FKF1 and GI protein expression coincides in long days but not in short days. With blue-light dependent FKF1 binding to GI, the FKF1-GI complex formation (shown in purple) is maximized in long-day afternoon. Thus, this regulation contains aspects that fit the internal and external coincidence models. In the long-day afternoon, the FKF1-GI complex degrades CDF and DELLA proteins and represses TOE1 function and simultaneously stabilizes CO protein. FKF1 can form a homodimer, but the one in the FKF1-GI complex is a monomer. FKF1 monomer also binds to COP1 monomer to prevent COP1 homo-dimerization, which requires its E3 ubiquitin ligase activity. COP1 degrades many transcription factors, including CO during the nighttime. b The external coincidence mechanism controls the stabilization of CO protein in long days. The diurnal oscillation patterns of CO are regulated by the circadian clock. CO expression occurs from the afternoon to evening in long days and at night in short days. Once CO protein is synthesized under light in long-day afternoons, CO protein stability is regulated by red, blue, and far-red light perceived by phyA, phyB, cry1, cry2, ZTL, and FKF1 proteins. The presence of each photoreceptor in long days is shown by different color bars corresponding to each absorbance. phyB and ZTL are negative regulators of CO, while the rest are positive regulators. FKF1 and PRR5 bind to CO in the long-day afternoon to stabilize CO, which in turn induces the expression of FT, leading to floral induction. The external coincidence between light signaling and CO protein only occurs in long days. Note that there is a short window at the beginning of the day in which CO is also stabilized. c The mechanism for generating bimodal expression of FT. In long days in which the red/far-red light ratio (R/FR) is adjusted to approximately 1 to mimic sunlight (denoted LD + FR), FT expression shows a bimodal pattern with morning and evening peaks. The evening peak is regulated by the external coincidence mechanism between CO and light signaling discussed in Fig.2b, and the morning peak is controlled by the phyA-mediated HIR (High Irradiance Response). In addition to CO, which is required for the morning FT induction, other factors (PIF7, ZTL, TOE1, and GI) are also involved in this regulation. CO protein is more stabilized in the morning in LD + FR than in LD.