Coordinated Transcriptional Regulation by the UVB Photoreceptor and Multiple Transcription Factors for Plant UV-B Responses

Non-damaging ultraviolet B(UV-B)light promotes photomorphogenic development and stress acclimation through UV-B-specific signal transduction in Arabidopsis.UV-B irradiation induces monomerization and nuclear translocation of the UV-B photoreceptor UV RESISTANCE LOCUS 8(UVR8).However,it is not clear how the nuclear localization of UVR8 leads to changes in global gene expression.Here,we reveal that nuclear UVR8 governs UV-B-responsive transcriptional networks in concert with several previously known transcription factors,including ELONGATED HYPOCOTYL 5(HY5)and PHYTOCHROME INTERACTING FACTOR 4(PIF4).Based on the transcriptomic analysis,we identify MYB13 as a novel positive regulator in UV-B-induced cotyledon expansion and stress acclimation.MYB13 is UV-B inducible and is predominantly expressed in the cotyledons.Our results demonstrate that MYB13 protein functions as a transcription factor to regulate the expression of genes involved in auxin response and flavonoid biosynthesis through direct binding with their promoters.In addition,photoactivated UVR8 interacts with MYB13 in a UV-B-dependent manner and differentially modulates the affinity of MYB13 with its targets.Taken together,our results elucidate the cooperative function of the UV-B photoreceptor UVR8 with various transcription factors in the nucleus to orchestrate the expression of specific sets of downstream genes and,ultimately,mediate plant responses to UV-B light.

Plant responses to UV‑B radiation:signaling,acclimation and stress tolerance

Ultraviolet-B(UV-B)light is an intrinsic part of sunlight that reaches the earth’s surface,and affects plant survival and adaptation.How plants respond to UV-B light is regulated by the wavelength,intensity and duration of UV-B radiation,and is also regulated by photosynthetically active radiation perceived by phytochrome and cryptochrome photore-ceptors.Non-damaging UV-B light promotes plant photomorphogenesis and UV-B acclimation which enhances plant tolerance against UV-B stress.However,high-level UV-B radiation induces DNA damage,generates reactive oxygen species(ROS)and impairs photosynthesis.Plants have evolved efficient mechanisms to utilize informational UV-B signal,and protect themselves from UV-B stress.UV RESISTANCE LOCUS8(UVR8)is a conserved plant-specific UV-B photoreceptor.It interacts with CONSTITUTIVELY PHOTOMORPHOGENIC1(COP1)to initiate UV-B-specific light signal-ing and regulate UV-B responsive gene expression.A set of transcription factors such as ELONGATED HYPOCOTYL5(HY5)function downstream of the UVR8-COP1 module to promote seedling de-etiolation for photomorphogenic development and biosynthesis of sunscreen flavonoids for UV-B stress tolerance.In addition to UVR8 signaling path-ways,plants subjected to damaging UV-B radiation initiate stress protection and repair mechanisms through UVR8-independent pathways.In this review,we summarize the emerging mechanisms underlying UV-B stress acclimation and protection in plants,primarily revealed in the model plant Arabidopsis thaliana.