Abstract
Plant circadian clock controls many physiological events under diurnal and seasonal changes in natural light/dark cycles. The best characterized is the photoperiodic control of flowering time in A. thaliana. Recently, it was reported that the clock regulates daily timing of rhythmic hypocotyl elongation.
Here, we found that this regulation of hypocotyl elongation is dependent on the photoperiod in a non-linear fashion. Genetic evidence will be presented for that the clock regulates the photoperiodic control of hypocotyl elongation by modulating the expression of PIF4/PIF5 (bHLH factors), which act positively hypocotyl elongation, particularly, in a manner that certain lengths of dark-period are necessary to enhance their transcription during late nighttime. Taken together, we propose the model that the photoperiodic control of hypocotyl elongation is best explained by the accumulation of PIF4/PIF5 during nighttime of short-days, due to coincidence between the internal (circadian rhythm) and external (photoperiod) time cues.
