The CONSTANS (CO) gene plays a central role in the regulation of flowering time in Arabidopsis, and is the founding member of a family of 17 CO homologues. CO and CO homologues have been found in flowering plants, but not in yeast and animals. To address the question of the origin of CO, this gene family was analysed in the moss Physcomitrella patens, a phylogenetically distant organism. In Arabidopsis and rice, three classes of CO homologues exist. The same three classes were found in Physcomitrella, suggesting that this gene family has ancient origins in the plant kingdom. In Arabidopsis, CO and 5 other genes belong to Group 1. Since only three Group 1 genes were identified in Physcomitrella, the family of CO homologues appears to be smaller in Physcomitrella than in Arabidopsis, in agreement with observations made with other gene families. Further analysis demonstrated that the Physcomitrella Group 1 genes are most similar to Arabidopsis Group 1 genes COL3/COL4/COL5, which are closely related to, but distinct from CO. An essential feature of CO function in Arabidopsis is a circadian controlled rhythm of transcript abundance. The three closely related Physcomitrella Group 1 genes have diurnal expression patterns that are distinct from the pattern of CO expression, and that are mainly caused by direct light induction. Distinct diurnal expression patterns are also observed for CO homologues that are not involved in control of flowering time. Consistently, the Physcomitrella CO homologues are unable to promote flowering upon expression in Arabidopsis. Together, the findings indicate that the CO branch of Group 1 genes does not exist in Physcomitrella. The role of CO in flowering time control was possibly derived from an ancestral function of Group 1 genes in light signal transduction. The function of the three Physcomitrella CO homologues was studied by exploiting the feasibility of gene targeting. A disruptant was generated for each Group 1 CO homologue in Physcomitrella, whereas in Arabidopsis only CO has been inactivated to date. Phenotypical analysis of the disruptants revealed no developmental defects, nor an alteration of the phototropic growth response. The high degree of sequence conservation between the three genes and the similar expression patterns suggest redundancy. Therefore, simultaneous inactivation of all three genes may be necessary to elucidate their function
