Transcriptional activation and repression in the plant circadian clock: Revisiting core oscillator feedback loops and output pathways.

The plant circadian clock operates through interconnected transcriptional-translational feedback loops, comprising circadian-regulated transcriptional activators and repressors at the core oscillator. These transcription factors (TFs) not only regulate the timing of gene expression within the oscillator but also orchestrate downstream transcriptomic regulation in output pathways by directly binding to promoters of key genes. To date, most plant TFs identified in these loops are transcriptional repressors, supporting a clock model dominated by repressive interactions. However, a number of studies have reported the presence of transcriptional activators in newly identified feedback loops. Interestingly, some TFs can function as both activators and repressors of circadian-regulated genes, offering new insights into clock regulatory mechanisms. This review revisits transcriptional activation and repression in core oscillator genes, offering a conceptual framework to advance the understanding of their roles, particularly in the regulation of clock output pathways.