Light regulation of circadian clock-controlled gene expression in rice

Using transgenic rice seedlings expressing a firefly luciferase (luc) gene under the control of a rice CAB (chlorophyll a/b binding protein) promoter, we demonstrated how light affects circadian clocks in the transcription of the CAB gene. Rhythmic luc expression was observed for more than 5 d under constant light and dark (DD) conditions after light/dark entrainment. Afte r a light pulse was applied at different time points in DD various temporal patterns of CAB gene expression were individually observed. We first exami ned two distinct properties related to the entrainment mechanism of the cir cadian clock: fluence-rate dependence of free-running periods (FRPs) and ph ase resetting by alight pulse. Although fluence-rate dependent shortening o f FRP was demonstrated, the FRP in DD was almost equal to that in constant light of a middle fluence-rate, indicating that this fluence-rate dependenc e may not fully describe the entrainment of the circadian clock in rice. Ty pical phase responses of the circadian clock by a single light pulse were a lso observed at the transcriptional level in rice seedlings. Thus, the phas e resettings upon the light/dark transitions of daily cycles may be suffici ent to explain the entrainment mechanisms of rice. We have further demonstr ated that, in addition to having a gating effect to acute response, a light pulse can activate the circadian clock-controlled CAB1R gene expression at the first circadian peak in a phase-dependent manner. This suggests that l ight activates circadian clock activity in the diurnal CAB gene expression under daily light/dark cycles.