Mapping the Core of the <i>Arabidopsis</i> Circadian Clock Defines the Network Structure of the Oscillator

TLDR

The circadian clock in many organisms consists of coupled morning and evening oscillators, and in Arabidopsis this coupling was previously thought to involve TOC1 activating morning‑expressed genes. TOC1 represses oscillator genes by rhythmically associating with their promoters, and hormone‑dependent induction together with RNAi studies show that TOC1 blocks activation of morning genes at night. These results overturn the prevailing model, demonstrating that TOC1 acts as a general repressor linking the morning and evening loops of the Arabidopsis circadian oscillator.

Abstract

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 TIMING OF CAB EXPRESSION 1 (TOC1) was proposed to activate a subset of morning-expressed oscillator genes. Here, we show that TOC1 does not function as an activator but rather 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 RNA interference 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.