The article discusses the regulation of the CONSTANS (CO) protein by photoreceptors in Arabidopsis, which is crucial for photoperiodic flowering. CO promotes flowering in long days by stabilizing its nuclear protein during the evening, while it is degraded by the proteasome in the morning or darkness. Photoreceptors regulate CO stability, generating daily rhythms in CO abundance. This regulation refines the circadian rhythm of CO mRNA and is central to how day length controls flowering. The study shows that light stabilizes CO protein, and photoreceptors like phytochrome (phy) and cryptochrome (cry) regulate CO stability. Under long days (LDs), CO protein accumulates in the evening, activating the FT gene, which promotes flowering. In contrast, under short days (SDs), CO protein levels are lower, reducing FT expression. The proteasome degrades CO in the dark, and ubiquitination plays a role in this process. The research also highlights the role of photoreceptor mutations in CO activity. For example, the phyB mutation increases CO protein abundance under red light, while cry1 and cry2 mutations reduce CO levels under blue light. These findings suggest that cryptochromes stabilize CO during the photoperiod, counteracting the degradation mediated by phyB. The balance between phyB and cryptochromes determines CO abundance, which in turn affects FT expression and flowering time. The study provides a mechanism by which light influences CO stability and, consequently, flowering. It demonstrates that CO protein is stabilized at the end of a long day, allowing the activation of FT gene expression. This regulation ensures that plants respond appropriately to seasonal changes in day length, enabling them to flower at the right time. The findings contribute to understanding the molecular mechanisms underlying photoperiodic flowering in plants.The article discusses the regulation of the CONSTANS (CO) protein by photoreceptors in Arabidopsis, which is crucial for photoperiodic flowering. CO promotes flowering in long days by stabilizing its nuclear protein during the evening, while it is degraded by the proteasome in the morning or darkness. Photoreceptors regulate CO stability, generating daily rhythms in CO abundance. This regulation refines the circadian rhythm of CO mRNA and is central to how day length controls flowering. The study shows that light stabilizes CO protein, and photoreceptors like phytochrome (phy) and cryptochrome (cry) regulate CO stability. Under long days (LDs), CO protein accumulates in the evening, activating the FT gene, which promotes flowering. In contrast, under short days (SDs), CO protein levels are lower, reducing FT expression. The proteasome degrades CO in the dark, and ubiquitination plays a role in this process. The research also highlights the role of photoreceptor mutations in CO activity. For example, the phyB mutation increases CO protein abundance under red light, while cry1 and cry2 mutations reduce CO levels under blue light. These findings suggest that cryptochromes stabilize CO during the photoperiod, counteracting the degradation mediated by phyB. The balance between phyB and cryptochromes determines CO abundance, which in turn affects FT expression and flowering time. The study provides a mechanism by which light influences CO stability and, consequently, flowering. It demonstrates that CO protein is stabilized at the end of a long day, allowing the activation of FT gene expression. This regulation ensures that plants respond appropriately to seasonal changes in day length, enabling them to flower at the right time. The findings contribute to understanding the molecular mechanisms underlying photoperiodic flowering in plants.