The principal immune mechanism against biotrophic pathogens in plants is the resistance (R)-gene-mediated defence1. It was proposed to share components with the broad-spectrum basal defence machinery2. However, the underlying molecular mechanism is largely unknown. Here we report the identification of novel genes involved in R-gene-mediated resistance against downy mildew in Arabidopsis and their regulatory control by the circadian regulator, CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1). Numerical clustering based on phenotypes of these gene mutants revealed that programmed cell death (PCD) is the major contributor to resistance. Mutants compromised in the R-gene-mediated PCD were also defective in basal resistance, establishing an interconnection between these two distinct defence mechanisms. Surprisingly, we found that these new defence genes are under circadian control by CCA1, allowing plants to ‘anticipate’ infection at dawn when the pathogen normally disperses the spores and time immune responses according to the perception of different pathogenic signals upon infection. Temporal control of the defence genes by CCA1 differentiates their involvement in basal and R-gene-mediated defence. Our study has revealed a key functional link between the circadian clock and plant immunity.
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We thank S. Brady for performing mixed model ANOVA of the data and advice on data analyses; J. Li for sharing the protocol for elf18 treatment, E. Tobin for providing the CCA1OE transgenic line, R. McClung for CCA1:LUC, LHY:LUC, ztl-4 lines. H. Lu for helpful discussion of the work, F. Ausubel, P. Benfey, S. Brady, J. Siedow and R. Mohan for critiquing the manuscript. This work was supported by a grant from NSF (MCB-0519898) to X.D. and a grant (HG004659) to X.-D.F.
The authors declare no competing financial interests.
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Wang, W., Barnaby, J., Tada, Y. et al. Timing of plant immune responses by a central circadian regulator. Nature 470, 110–114 (2011). https://doi.org/10.1038/nature09766
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