A transcription factor involved in controlling the timing of flowering in the model plant thale cress (Arabidopsis thaliana). It is produced in the leaves where it activates expression of the FT (FLOWERING LOCUS T) gene encoding the plant hormone florigen, responsible for initiating flowering. In A. thaliana, CO protein specifically works late in the afternoon on days when it is still light, thus ensuring flowering at the appropriate season for the plant. Only at such times is the CO protein stabilized through the action of light-sensitive components, including phytochrome and cryptochrome photoreceptors. A similar interplay between CO and florigen is thought to regulate daylength-sensitive onset of flowering in other species.