A Self-Regulatory Circuit of CIRCADIAN CLOCK-ASSOCIATED1 Underlies the Circadian Clock Regulation of Temperature Responses in Arabidopsis

The circadian clock synchronizes biological processes to daily cycles of light and temperature. Clock components, including CIRCADIAN CLOCK-ASSOCIATED1 (CCA1), are also associated with cold acclimation. However, it is unknown how CCA1 activity is modulated in coordinating circadian rhythms and cold acclimation. Here, we report that self-regulation of Arabidopsis thaliana CCA1 activity by a splice variant, CCA1 beta, links the clock to cold acclimation. CCA1b interferes with the formation of CCA1 alpha-CCA1 alpha and LATE ELONGATED HYPOCOTYL (LHY)-LHY homodimers, as well as CCA1 alpha-LHY heterodimers, by forming nonfunctional heterodimers with reduced DNA binding affinity. Accordingly, the periods of circadian rhythms were shortened in CCA1 beta-overexpressing transgenic plants (35S:CCA1 beta), as observed in the cca1 lhy double mutant. In addition, the elongated hypocotyl and leaf petiole phenotypes of CCA1 alpha-overexpressing transgenic plants (35S:CCA1 alpha) were repressed by CCA1 beta coexpression. Notably, low temperatures suppressed CCA1 alternative splicing and thus reduced CCA1 beta production. Consequently, whereas the 35S:CCA1 alpha transgenic plants exhibited enhanced freezing tolerance, the 35S:CCA1 beta transgenic plants were sensitive to freezing, indicating that cold regulation of CCA1 alternative splicing contributes to freezing tolerance. On the basis of these findings, we propose that dynamic self-regulation of CCA1 underlies the clock regulation of temperature responses in Arabidopsis.