Mapping the Core of the Arabidopsis Circadian Clock Defines the Network Structure of the Oscillator

In many organisms, the circadian clock is composed of functionally coupled morning and evening oscillators. In Arabidopsis, oscillator coupling relies on a core loop in which the evening oscillator component TOC1 (TIMING OF CAB EXPRESSION1) was proposed to activate a subset of morning-expressed oscillator genes. Here, we show that TOC1 does not function as an activator but as a general repressor of oscillator gene expression. Repression occurs through TOC1 rhythmic association to the promoters of the oscillator genes. Hormone-dependent induction of TOC1 and analysis of RNAi plants show that TOC1 prevents the activation of morning-expressed genes at night. Our study overturns the prevailing model of the Arabidopsis circadian clock showing that the morning and evening oscillator loops are connected through the repressing activity of TOC1.