Molecules and Cells

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Fig. 1. Photoperiodic flowering regulation by the induction of FT expression under long-day conditions. In Arabidopsis, a high abundance of CDF proteins accumulates and represses the expression CO and FT genes simultaneously by binding to promoter regions of these genes in the morning. FKF1 protein, the expression of which coincides with that of GI in the afternoon, absorbs blue light and forms a protein complex with GI. The protein complex stimulates the degradation of CDF proteins on CO and FT promoters. Removal of CDF repression enables FBH transcription factors to gain access to the CO promoter. FBH proteins activate CO transcription throughout the rest of day. CO protein is post-translationally modulated by light quality. The protein is degraded by the COP1?SPAs complex in the dark, but is sequestered from the complex by the inhibitory function of blue light-activated CRY2. In the morning, CO is stabilized by the far-red light photoreceptor PHYA and immediately destabilized by the blue light photoreceptor ZTL and the formation of a protein complex between HOS1 and red light-absorbed PHYB. In the afternoon, CO accumulates as a consequence of the activities of FKF1, GI, and PHYA. Together with GI, blue light activation of FKF1 increases CO stability. Thus, GI indirectly regulates CO stability via the inhibition of ZTL function. In turn, FT expression is induced by CO and CIBs. CO binds to the FT promoter directly as well as indirectly with other transcription factors. In addition, CIB proteins interact with blue light-activated CRY2 and directly bind to the FT promoter. These CO and CIBs activate FT transcription at dusk, leading to the promotion of reproductive transition in long days.
Mol. Cells 2016;39:715~721
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