Day-length and the circadian clock control critical aspects of plant development such as the onset of reproduction by the photoperiodic pathway.(1) CONSTANS (CO) regulates the expression of a florigenic mobile signal from leaves to the apical meristem and thus is central to the regulation of photoperiodic flowering.(2) This regulatory control is present in all higher plants,(3) but the time in evolution when it arose was unknown. We have shown that the genomes of green microalgae encode members of the CONSTANS-like (COL) protein family. One of these genes, the Chlamydomonas reinhardtii CO homolog (CrCO), can complement the co mutation in Arabidopsis.(4) CrCO expression is controlled by the clock and photoperiod in Chlamydomonas and at the same time is involved in the correct timing of several circadian output processes such as the accumulation of starch or the coordination of cell growth and division. We have proposed that, since very early in the evolutionary lineage that gave rise to higher plants, CO homologs have been involved in the photoperiod control of important developmental processes, and that the recruitment of COL proteins in other roles may have been crucial for their evolutionary success.
