The circadian clock is an endogenous timekeeping mechanism that coordinates diverse biological processes across diverse organisms. Emerging evidence recently underscores its critical role in modulating plant immune responses. Salicylic acid (SA) is a central phytohormone in plant immunity; however, the molecular mechanisms by which clock components influence SA signaling under biotic stress are poorly understood. Here, we report the pivotal role of the core clock components, PSEUDO-RESPONSE REGULATOR 5 (PRR5) and PRR7, in governing Arabidopsis immunity by direct suppression of SA signaling. We found that the transcriptional outputs of SA signaling showed rhythmic expression and were remarkably affected by PRR5 and PRR7. Genetic analyses revealed that PRR5 and PRR7 function genetically upstream of the SA receptor NONEXPRESSER OF PATHOGENESIS-RELATED GENES 1 (NPR1) to inhibit SA-mediated defenses. Biochemical assays confirmed physical interactions among PRR5/7, NPR1, and TGA3, highlighting a direct mechanism whereby PRR5/7 antagonize the transcriptional activity of the NPR1-TGA3 complex. The prr5 prr7 double mutant not only showed enhanced SA signaling but also boosted pathogen-associated molecular pattern-triggered immunity, highlighting their broad inhibitory function in plant immunity. These findings provide critical insights into the temporal dynamics of plant immunity and reveal key molecular targets for breeding crop varieties with an optimized balance between growth and immunity.
Li et al. (Wed,) studied this question.