Model of photoperiod flowering-time regulation in sorghum. Phytochrome B mediates light signaling, providing information about day length required for photoperiod detection. Light and the circadian clock regulate expression of PRR37 resulting in higher expression in LD vs. SD. PRR37, a floral repressor, inhibits the activity of SbCO, a floral activator resulting in delayed flowering in LD. In inductive SD, SbCO activates expression of SbEHD1, SbCN8 and SbCN12 genes thereby promoting flowering. Thin lines represent possible mechanism of flowering time regulation.