FKF1 and GIGANTEA Complex Formation Is Required for Day-Length Measurement in Arabidopsis

Precise timing ofCONSTANS (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 regulateCO transcription inArabidopsis . We demonstrate that FKF1 and GI proteins form a complex in a blue-light–dependent manner. The timing of this interaction regulates the timing of daytimeCO expression. FKF1 function is dependent on GI, which interacts with aCO repressor, CYCLING DOF FACTOR 1 (CDF1), and controls CDF1 stability. GI, FKF1, and CDF1 proteins associate withCO chromatin. Thus, the FKF1-GI complex forms on theCO promoter in late afternoon to regulateCO expression, providing a mechanistic view of how the coincidence of light with circadian timing regulates photoperiodic flowering.