BIANNUALREPORT2012-2013

The beat of time: molecular mechanisms of the plant circadian clock

Plants, as many other organisms, synchronize the timing of their physiology and development by using an endogenous mechanism called circadian clock. Perception of environmental changes during the day-night cycle is crucial for circadian function, which relies on transcriptional feedback loops at the core of a central oscillator.

However, deciphering the oscillator transcriptional regulatory code is a major challenge due to the interplay among clock activators and repressors, which are responsible for the generation of the loops. The team led by CSIC Investigator Paloma Mas has shown that the morning and evening oscillators in the small plant Arabidopsis thaliana are connected through the repressing activity of the key clock component known as TOC1. These results overturn the canonical, long-standing model of the plant circadian clock, in which TOC1 was presumed to be an activator of a reduced number of oscillator genes, not a general repressor of oscillator gene transcription, as has now been demonstrated. The studies were pioneer in using the ChIP-Seq technique to identify the target genes of TOC1. Biochemical, molecular and genetic analysis also allowed to define the regulatory network at the core of the clock in plants.

The team has also demonstrated that chromatin remodeling is a prevalent regulatory mechanism at the core of the clock. The functional properties of chromatin are modulated by various mechanisms, including, among others, posttranslational modifications of histones, incorporation of histone variants and DNA methylation. It has now been shown that the peak-to-trough circadian oscillation is paralleled by the sequential accumulation of different histone marks at the promoters of core clock genes. Mechanistically, histone methylation functions as a transition mark, modulating the progression from circadian activation to repression. Despite divergences in oscillator components, a chromatin-dependent mechanism of clock gene activation appears to be common to both plant and mammal circadian systems.