Circadian clock and PIF4-mediated external coincidence mechanism coordinately integrates both of the cues from seasonal changes in photoperiod and temperature to regulate plant growth in Arabidopsis thaliana.

In Arabidopsis thaliana, the circadian clock regulates the photoperiodic plant growth including the elongation of hypocotyls in a short-days (SDs)-specific manner. The clock-controlled PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) gene encoding a basic helix-loop-helix (bHLH) transcription factor plays crucial roles in this regulation. The SDs-specific elongation of hypocotyls is best explained by accumulation of the active PIF4 proteins at the end of night specifically in SDs due to coincidence between internal (circadian clock) and external (photoperiod) cues. However, this external coincidence model was challenged with the recent finding that the elongation of hypocotyls is markedly promoted at high growth temperature (28˚C) even in long-days (LDs), implying that the model to explain the photoperiodic response of plant architecture appears to be conditional on ambient temperature. With regard to this problem, the results of this and previous studies showed that the model holds under a wide range of ambient temperature conditions (16˚C to 28˚C). We propose that the circadian clock and PIF4-mediated external coincidence mechanism coordinately integrates both of the cues from seasonal changes in photoperiod and temperature to regulate plant growth in natural habitats.