The ultraviolet (UV)-B wavelength (280–315 nm) is an indispensable component of the sunlight that reaches the earth’s surface. Longwavelength and low-fluence UV-B specifically induces plant photomorphogenesis, which is characterized by inhibition of hypocotyl elongation, promotion of anthocyanin accumulation, and eventually tolerance against UV-B stress (Jenkins, 2009). In Arabidopsis thaliana, UV-B light is perceived by UV RESISTANCE LOCUS 8 (UVR8), a plant-specific chromoprotein. UVR8 forms a symmetric homodimer in the absence of UV-B light. The tryptophan residues on the surface of the UVR8 dimer serve as the intrinsic chromophore to sense UV-B light. Upon UV-B irradiation, the homodimeric interface, which is stabilized by arginine residues, is disrupted, leading to monomerization of UVR8 (Rizzini et al., 2011; Christie et al., 2012; Wu et al., 2012). UVR8 is present in both the cytoplasm and the nucleus, and UV-B treatment stimulates the accumulation of UVR8 in the nucleus (Kaiserli and Jenkins, 2007). However, it is not clear how UV-B regulates the subcellular localization and physiological activity of UVR8. In this study, we explore the mechanism of the buildup of the nuclear UVR8 pool, dissect physiological roles of subcellular UVR8, and reveal positive and negative regulators of UVR8 localization in the photomorphogenic UV-B signaling process.

We first performed a nuclear fractionation assay to examine UVR8 conformation states in distinct subcellular fractions, which was subsequently analyzed in SDS–PAGE without sample boiling, a well-accepted way to study UVR8 conformation (Rizzini et al., 2011; Christie et al., 2012; O’Hara and Jenkins, 2012; Wu et al., 2012; Huang et al., 2014). In ÀUV-B-grown seedlings, most of the cytosolic YELLOW FLUORESCENT PROTEIN (YFP)-UVR8WT was dimeric, while both its dimeric and monomeric forms were present in the nucleus. In+ UV-B-grown seedlings, however, most of the cytosolic YFP-UVR8WT became monomeric, while only the monomeric form was present in the nucleus (Figure 1 A). We proposed two possible sources for the nuclear accumulation of UVR8 monomers under UV-B light:(1) through the translocation of the dissociated cytosolic monomers from the cytosol to the nucleus, and/or (2) from the dissociated monomers within the nucleus. As UV-B treatment resulted in rapid UVR8 monomerization in the cytosolic and nuclear fractions, respectively (Supplemental Figure 1), we conclude that the nuclear UVR8 monomers can be derived from the dimer-to-monomer switch within the nucleus.