Abstract
The plant circadian clock generates daily rhythms in the activity of many biological processes and serves their photoperiodic responses. In A. thaliana, the clock regulates the long-day-specific promotion of flowering time and short-day-specific elongation of hypocotyl length. For the photoperiodic control of flowering, it was known B-box containing transcriptional factor CO is activated at the end of day time specifically under long day conditions to induce the FT gene expression. On the other hand, it is speculated bHLH transcriptional factors PIF4/5 are activated at the end of nigh time to enhance hypocotyl elongation. To make sure the assumption, here, we screened PIF4/5-dependent genes whose expressions are induced at the end of night specifically in short days. We identified the ATHB2 gene encoding a HD-Zip protein as a candidate. Examination of ATHB2 transcriptional profiles under various photoperiodic conditions in photoreceptor and clock mutant backgrouds makes it possible to suggest that both the circadian clock and photoreceptor phyB coordinately control the PIF4-ATHB2 output pathway responsible for the short-day-specific photoperiodic control of hypocotyl elongation.
