PSEUDO RESPONSE REGULATORs stabilize CONSTANS protein to promote flowering in response to day length.

Seasonal reproduction in many organisms requires detection of day length. This is achieved by integrating information on the light environment with an internal photoperiodic time-keeping mechanism. promotes flowering in response to long days (LDs), and CONSTANS (CO) transcription factor represents a photoperiodic timer whose stability is higher when plants are exposed to light under LDs. Here, we show that PSEUDO RESPONSE REGULATOR (PRR) proteins directly mediate this stabilization. PRRs interact with and stabilize CO at specific times during the day, thereby mediating its accumulation under LDs. PRR-mediated stabilization increases binding of CO to the promoter of (), leading to enhanced transcription and early flowering under these conditions. PRRs were previously reported to contribute to timekeeping by regulating transcription through their roles in the circadian clock. We propose an additional role for PRRs in which they act upon CO protein to promote flowering, directly coupling information on light exposure to the timekeeper and allowing recognition of LDs.