The core clock transcription factor TOC1 is a direct regulator of 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 conditions. In addition, TOC1-ox plants were more susceptible than Col-0 following inoculation at dawn, while inoculation at night lead to enhanced resistance in toc1-2 plants versus C24 plants, suggesting 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.