Figure 6.

A Model for the Timekeeping Mechanism in Pharbitis Compared with Those for Arabidopsis and Rice.

A model for control of flowering under LD (left) and SD (right) in Arabidopsis (top), rice (middle), and Pharbitis (bottom). In Pharbitis, the activity of a clock-controlled gene (CCG) that regulates the transcription of Pn FT is plotted. The activity of this gene is regulated by a specific circadian clock whose phase is reset by light off. Under LDs, Pn FT mRNA does not accumulate because the rhythm in the activity of the CCG is terminated by light at dawn before it reaches its peak phase. By contrast, under SDs, Pn FT mRNA does accumulate because the rhythm in expression of the CCG reaches a peak phase during the longer night. In Arabidopsis, expression of CO mRNA while the plants are exposed to light under LDs stabilizes CO protein, enabling activation of FT transcription under these conditions. Under SDs, FT expression is not activated because CO mRNA is only expressed in the dark and the protein does not accumulate under these conditions. In rice, under LDs, expression of Hd1 mRNA while the plants are exposed to light causes Hd1 protein to act as a repressor of Hd3a transcription and this inhibits flowering. Under these conditions, the repressor form is also proposed to repress Hd3a transcription during the short night. By contrast, under SDs, the Hd1 repressor form is made early during the long night, but later the activator form is made and this activates Hd3a transcription.