FKF1 Conveys Timing Information for CONSTANS Stabilization in Photoperiodic Flowering

Spring Into Flower In spring, plants respond to increasing day length and shifts in the spectrum of solar irradiance by releasing the flowering induction pathway, which includes expression of the FLOWERING LOCUS T (FT) protein. Song et al. (p. 1045) have now identified a trio of controls brought to bear on FT gene expression by the FKF1 (FLAVIN-BINDING, KELCH REPEAT, F-BOX 1) protein. FKF1 removes a repressor and also stabilizes the activating CONSTANS (CO) protein in the afternoons through a binding interaction enhanced by blue light—an increasing component of solar irradiation during spring. Then FKF1 itself helps to activate transcription of the CO gene. Thus, by removing the repressor and shoring up the activator, FKF1 sets flowering on its way. A plant protein sensitive to blue light links longer afternoons to more flowering. Plants use day-length information to coordinate flowering time with the appropriate season to maximize reproduction. In Arabidopsis, the long day–specific expression of CONSTANS (CO) protein is crucial for flowering induction. Although light signaling regulates CO protein stability, the mechanism by which CO is stabilized in the long-day afternoon has remained elusive. Here, we demonstrate that FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1) protein stabilizes CO protein in the afternoon in long days. FKF1 interacts with CO through its LOV domain, and blue light enhances this interaction. In addition, FKF1 simultaneously removes CYCLING DOF FACTOR 1 (CDF1), which represses CO and FLOWERING LOCUS T (FT) transcription. Together with CO transcriptional regulation, FKF1 protein controls robust FT mRNA induction through multiple feedforward mechanisms that accurately control flowering timing.