The core clock transcription factor TOC1 binds directly to defence gene promoters regulating immunity in Arabidopsis

The plant circadian clock drives temporal differences in susceptibility to pathogens. We investigated the role of TIMING OF CAB EXPRESSION 1 (TOC1) in the regulation of defence against Botrytis cinerea in Arabidopsis. The temporal variation in susceptibility to B. cinerea observed in wild-type Arabidopsis was abolished in TOC1-ox and toc1-2 plants under both diurnal and constant light conditions. In addition, TOC1-ox plants were more susceptible than Col-0 following inoculation at dawn, while inoculation at night led to enhanced resistance in toc1-2 plants versus C24 plants, suggesting that TOC1 is a negative regulator of immunity. RNA-seq analysis showed that the genes mis-regulated in toc1-2 plants had significant enrichment for terms related to biotic stress, an overrepresentation of G-box elements in their promoters and included genes encoding key transcription factors (TFs) involved in defence against necrotrophic pathogens. Chromatin immunoprecipitation-qPCR showed that TOC1 occupies G-box containing regions of the defence TFs ERF4, ORA47, ORA59 and WRKY33 in a pathogen-responsive and MYC2-dependent manner. We suggest that the phased TOC1 occupancy of defence gene promoters contributes to the gating of plant immunity against necrotrophic pathogens, while the MYC2-dependent release of TOC1 in response to pathogen detection allows plants to mount an acute immune response.