TOC1 phosphorylation disproportionally enhances chromatin binding at rhythmic gene promoters

Protein phosphorylation is a key regulatory mechanism in circadian systems. TIMING OF CAB EXPRESSION 1(TOC1) is a core transcriptional repressor in the plant circadian system that is phosphorylated near its N-terminus. Phenotype testing of single and multi-phosphosite mutants shows incomplete rescue of the short period toc1 mutant. We establish that TOC1 phosphorylation (particularly at S175) is necessary for optimal interaction with FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and PHYTOCHROME INTERACTING FACTOR 5 (PIF5) at the CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) promoter to downregulate CCA1 expression. At the same time, expression of the closely related LATE ELONGATED HYPOCOTYL (LHY) also requires TOC1, but is independent of the TOC1 phosphorylation state, suggesting different TOC1-dependent mechanisms in the repression of these two genes. We additionally show how phosphorylation-dependent interactions of TOC1 at specific clock gene promoters selectively regulate these circadian system components more acutely than non-rhythmic genes. Our genome-wide analysis shows that the phosphostate of TOC1 is important for optimal chromatin presence and robust rhythmic gene expression.