Seasonal Circuitry: transcriptional and post-translational regulation of photoperiodic flowering in Arabidopsis thaliana

Abstract

For plants, the transition towards reproductive development is timed so that it coincides with the most optimal external conditions. For many plant species, day length is an important cue in mediating this process. At the molecular level, the perception of day length for flowering is accomplished through the a variety of mechanisms, but the regulatory control of the genes encoding the CONSTANS (CO) transcription factor and the FLOWERING LOCUS T (FT) florigenic protein are most critical in Arabidopsis. Temporal control of the CO and FT daily expression pattern is mediated by the CYCLING DOF FACTOR (CDF) family of transcriptional repressors, and by the degradation of CDFs by the FLAVIN BINDING KELCH REPEAT F-BOX 1 (FKF1) photoreceptor in the evening of long days. This dissertation is an examination of the mechanisms by which CDFs perform their repressive function on their target genes CO and FT, and the further details of FKF1 function mediated through stability of the photoactive form of the light sensing LOV (Light Oxygen Voltage) domain. In this dissertation, first I review the framework of photoperiodic flowering through a presentation of the history of flowering time research and its context in circadian biology. Then, I move on to provide a review of the major factors and processes that have been found to play a role in the regulation of CO and FT in Arabidopsis. Later on, I describe our experimental findings about how the CDF transcription factors utilize the general co-repressor protein TOPLESS (TPL) to exert repression on CO and FT. I then discuss the implications of our findings on CO and FT regulation in Arabidopsis as well as other plant species. Next, I detail our characterization of a novel CDF transcription factor in Arabidopsis, CDF6, and its role in the repression of photoperiodic flowering. Finally, I present our preliminary findings that show the importance of photocycle rate kinetics of the FKF1 LOV domain on photoperiodic flowering regulation, demonstrating that the conversion of FKF1 into a shorter photocycle variant perturbs its function by reducing the light-memory into dusk and early night of long days.