Molecular Mechanism Of The Subcellular Localization And Activity Of Arabidopsis UV-B Photoreceptor

Light is one of the most important environmental cue for plant growth and development.Ultraviolet(UV)-B light of low fluence directs photomorphogenic development and UV-B acclimation in Arabidopsis.UV RESISTANCE LOCUS 8(UVR8),a plant-specific chromoprotein,is responsible for UV-B light perception.Upon the exposure to UV-B light,homodimeric UVR8 performs rapid monomerization and associates with CONSTITUTIVELY PHOTOMORPHOGENIC 1(COP1)to mediate light signaling cascades in the photomorphogenic developmental process.UVR8 localizes in both the cytoplasm and the nucleus.However,it is not clear how UV-B light regulates the subcellular localization and physiological activity of UVR8.Firstly,to study the effect of UVR8 conformation on its localization,we used yellow fluorescence protein(YFP)-UVR8WT/W285F/R338A/uvr8-6 transgenic plants.We show that UV-B light promotes the accumulation of UVR8 monomers in the nucleus.The nuclear translocation assays reveal that with short-term UV-B illumination,nuclear UVR8 dimers rapidly switch to monomers.As the exposure to UV-B prolongs,cytoplasmic UVR8 dimers are monomerized and then translocated into the nucleus.To study the biological activity of subcellular UVR8,we used YFP-glucocorticoid receptor(GR)-UVR8WT/W285F/R338A/uvr8-6 transgenic plants.The results reveal that UVR8 monomers in the nucleus promote UV-B-induced photomorphogenesis.In addition,two types of UVR8-interacting proteins,COP1 and REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1(RUP1)/RUP2,play positive and negative roles respectively in the maintenance of nuclear UVR8 monomers.Secondly,to study the mechanism of UVR8 subcellular localization regulating downstream gene expression,we used YFP-GR-UVR8/uvr8-6 transgenic plants in+dexamethasone(DEX)and-DEX condition for high-throughput transcriptome sequencing.Based on our analysis,nuclear UVR8 mainly regulates UV-B-mediated transcriptome changes,partially through some transcription factors.In response to UV-B light,nuclear UVR8 regulates multiple hormone pathways through the known ELONGATED HYPOCOTYL(HY5)and PHYTOCHROME INTERACTING FACTOR 4(PIF4)transcription factors,and regulates the gene expression of auxin pathways mainly through PIF4.Thirdly,nuclear UVR8 may regulate UVR8-mediated gene expression via the unknown transcription factor.We reveal that UV-B light induces MYB13 expression and MYB13 enrichment in cotyledons.The genetic data support MYB13 is positive regulator of UV-B-induced cotyledon expansion and stress acclimation.The biochemical data show that MYB13 binds to the promoters of SMALL AUXIN UP-REGULATED RNA 27(SAUR27),SAUR28 and SAUR66 genes and the key genes of the flavonoid biosynthesis,including CHALCONE SYNTHASE(CHS),CHALCONE ISOMERASE(CHI)and FLAVONOL SYNTHASE(FLS).Both in vitro and in vivo results show that UVR8 monomers interact with MYB13.Moreover,UVR8 monomers repress MYB13 binding to these SAUR promoters.When MYB13 protein levels increase,UVR8 monomers is not able to completely inhibit MYB13's DNA-binding activity,so the transcripts of SAUR27,SAUR28 and SAUR66 genes accumulate and promote cotyledon expansion.However,UVR8 monomers promote MYB13 binding the promoters of CHS and CHI genes,and activating their expression and flavonoid accumulation,thus enhancing the adaptability of plants to UV-B stress.In this study,we propose that dual-source nuclear UVR8 monomers direct UV-B-induced photo morphogenesis and reveal the mechanism of UVR8 subcellular localization regulating downstream gene expression.Nuclear UVR8 mainly regulates UV-B-mediated transcriptome changes,partially through the known transcription factor.In addition,we identify a new functional factor MYB13 in UV-B signaling pathway.Photoactivated UVR8 interacts with MYB13 to regulate UV-B-induced cotyledon expansion and stress acclimation.Together,we reveal the molecular mechanism by which UV-B light regulates the subcellular localization and activity of UVR8.