Light-dependent destabilization of PHL in Arabidopsis.

Plants sense environmental stimuli such as light to regulate their flowering time. In Arabidopsis, phytochrome B (phyB) is the major photoreceptor that perceives red and far-red light, and destabilizes transcriptional regulator CONSTANS (CO) protein. However the mechanism that links photoreceptor and CO protein degradation is largely unknown. We recently showed that PHYTOCHROME-DEPENDENT LATE-FLOWERING (PHL) protein inhibits phyB signaling through direct protein-protein interaction. Here, we report that light exposure destabilizes PHL protein as is the case with CO. Fluorescence from PHL-YFP fusion protein expressed under the control of Cauliflower Mosaic Virus (CaMV) 35S promoter (35S::PHL-YFP) almost disappeared after four-hour treatment of white light. Furthermore, the similar results were also obtained from the analysis of PHL-GUS fusion protein expressed by PHL promoter (PHLpro::PHL-GUS phl-1). These results highlight the importance of post-transcriptional regulation in phyB-mediated flowering regulation and will give us hints how phyB regulates CO protein amount.