An evolutionarily conserved DOF-CONSTANS module controls photoperiodic signaling in plants

The response of plants to day length constitutes a crucial regulatory process that evolved very early in the plant evolutionary lineage. In close association with the circadian clock, photoperiod signalling entitled the evolving green eukaryote to predict seasonal variability and attune its physiological responses to the environment. This is reflected in the complex signalling pathways that govern modern plant developmental switches, such as the floral transition. A Chlamydomonas single copy gene CrDOF controls the transcription of downstream genes depending on the photoperiod. In Chlamydomonas, CrDOF enhances the expression of the plant CONSTANS (CO) homolog (CrCO) in Short Days by direct binding to its promoter, while it represses cell division gene expression in Long Days independently of CrCO. Overexpression or down regulation of CrDOF has a pronounced influence in algal growth and viability. Furthermore, expression of CrDOF in Arabidopsis delays flowering by promoting the stability of CYCLING DOF FACTORS (CDFs) proteins responsible for the reduced expression of the floral activator genes CO and FLOWERING LOCUS T (FT). Chlamydomonas DOF constitutes an evolutionarily conserved step of the photoperiod response whose function is conserved in angiosperms despite of the wide expansion and diversification of the DOF gene family in modern plants. The conservation of the DOF-CO module in the green lineage could be an important indication of how higher plants have evolved from simple unicellular algae by the inheritance of evolutionarily conserved gene toolkits for key developmental programs.