Time for a change

Abstract

The circadian clock of Arabidopsis, a popular model organism for plants, is more complex than expected, with negative feedback loops based on the repression of gene expression having a less exclusive role than previously thought.

Keywords: Arabidopsis; circadian rhythm; circadian rhythms; evening element; transcription factors.

Figure 1.. The circadian clock of Arabidopsis.

(A) By demonstrating that a transcription factor called RVE8 increases the expression of hundreds of evening phased genes including the clock gene called TOC1 in Arabidopsis, Hsu, Devisetty and Harmer have shown that the circadian clocks of plants are more complex (see part B) than previously thought (Hsu et al., 2013). Early in the morning (left; light background), the proteins CCA1 and LHY repress the expression of TOC1 by binding to the evening element (EE) in the promoter region of this gene: the suppression is represented by the violet line with the flat end. As the day progresses, however, other proteins down regulate the genes that code for CCA1 and LHY, and this allows the expression of TOC1 to increase at dusk: the solid blue line shows the level of TOC1 mRNA. RVE8 contributes to this increase by binding to the evening element of the TOC1 gene. Levels of TOC1 mRNA decrease during the evening as RVE8 levels (orange dotted line) fall off and CCA1 protein levels (yellow dotted line) start to rise again. (B) Simplified representation of the circadian clock of Arabidopsis: again, the violet lines represent a protein repressing the expression of a gene, the green arrow shows that CCA1/LHY increase the production of certain PRR proteins during the day, and the red arrows show the roles played by RVE8 and two similar proteins (RVE4 and RVE6) during the day and in the evening (dark background), as revealed by Hsu et al. The dashed red arrow indicates an interaction that only occurs in specific conditions. The three PRR proteins shown here are similar to TOC1: they are produced throughout the afternoon and early evening and they act to reduce the production of CCA1 and LHY. Finally ELF4, LUX and ELF3 are components of an evening protein complex that represses the expression of morning genes such as PRR9. The waves represent evening output genes induced by RVE8.