Light quality-dependent nuclear import of the plant photoreceptors phytochrome A and B

The phytochrome (phy) family of plant photoreceptors controls various aspec ts of photomorphogenesis. Overexpression of rice phyA-green fluorescent pro tein (GFP) and tobacco phyB-GFP fusion proteins in tobacco results in funct ional photoreceptors. phyA-GFP and phyB-GFP are localized in the cytosol of dark-adapted plants. In our experiments, red light treatment led to nuclea r translocation of phyA-GFP and phyB-GFP, albeit with different kinetics. R ed light-induced nuclear import of phyB-GFP, but not that of phyA-GFP, was inhibited by far-red light. Far-red light alone only induced nuclear transl ocation of phyA-GFP. These observations indicate that nuclear import of phy A-GFP is controlled by a very low fluence response, whereas translocation o f phyB-GFP is regulated by a low fluence response of phytochrome. Thus, lig ht-regulated nucleocytoplasmic partitioning of phyA and phyB is a major ste p in phytochrome signaling.