Epidermal CCA1 and PMR5 contribute to nonhost resistance in Arabidopsis.

Nonhost resistance (NHR) is the most robust and durable resistance in plants, but its spatiotemporal regulation is poorly understood. The circadian clock functions in a tissue-specific manner and regulates individual physiological processes in plants. Using mutant and RNA-seq analyses, we revealed a role of CIRCADIAN CLOCK ASSOCIATED1 (CCA1) in tissue-specific and time-of-day-specific regulation of NHR to Pyricularia oryzae (syn. Magnaporthe oryzae) in Arabidopsis thaliana (Arabidopsis). Targeted perturbation of CCA1 function in epidermis compromised time-of-day-specific regulation of NHR to P. oryzae in Arabidopsis. RNA-seq analysis showed that P. oryzae inoculation alters the transcriptome in penetration 2 (pen2) plants and identified POWDERY MILDEW RESISTANCE 5 (PMR5) as a candidate gene of direct targets of CCA1. Time-of-day-specific penetration resistance to P. oryzae was reduced in Arabidopsis pen2 pmr5 mutant plants. These findings suggest that epidermal CCA1 and PMR5 contribute to the establishment of time-of-day-specific NHR to P. oryzae in Arabidopsis.