Punctual coordination: switching on and off for flowering during a day.

The photoperiodic flowering of Arabidopsis is shown to be explained in part by the Bünning's external coincidence model in which clock-controlled expression of CO and stabilization of CO protein by light have important roles. The floral activators, GI and CO, together with ZTL, FKF and CDF1 have been shown to be central for the induction of FT expression during evening to promote the photoperiodic flowering of Arabidopsis. Here we discuss a role of diurnal accumulation of a floral repressor SVP protein in the repression of the FT and SOC1 expression during daytime. A punctual coordination of the diurnal regulation of both positive and negative regulators by circadian clock appears to be important for the photoperiodic flowering in Arabidopsis.