Undergraduate Yale University Fort Collins, Colorado
Body of Abstract: The circadian clock controls rhythmic gene expression and downstream processes that give plants essential adaptations to fluctuating environments. However, many mechanisms of circadian regulation remain unclear. Here, we identified a pure clock-regulated F-box protein, CLOCK F-BOX CONTROL of HYPOCOTYL 1 (CFH1), which operates within an E3 ubiquitin ligase complex to guarantee a morning for plants essential for consistent regulation independent of light conditions. CFH1 expression is restricted to the early day, and both knockout and decoy exhibit long hypocotyl phenotypes, suggesting it is a morning-phased growth restrictor. The most dramatic hypocotyl defects are observed under red light, and CFH1 interacts with both Cullin-1, an essential E3 ubiquitin ligase complex component, and PIF3, a transcription factor involved in light-dependent growth and red light signaling. These data suggest that CFH1’s hypocotyl regulation results from targeting PIF3, representing novel circadian regulation of the red light signaling pathway. Double mutant screens further support that PIF3 is downstream of CFH1, and tobacco assays demonstrate that CFH1 and PIF3 can interact in vivo. In summary, CFH1 represents a pure clock output that imposes a morning state on plants by targeting PIF3 for degradation, illustrating a novel, post-translational integration of the circadian clock with downstream growth and light-sensing.