UV-B light (UV-B radiation) is known to inhibit plant growth, but the mechanism is not well understood. UVR8 (UV RESISTANCE LOCUS 8) is a UV-B light photoreceptor that mediates UV-B light responses in plants. We report here that UV-B inhibits plant growth by repressing plant steroid hormone brassinosteroid (BR)-promoted plant growth. UVR8 physically interacts with the functional dephosphorylated BES1 (BRI1-EMS-SUPPRESSOR1) and BIM1 (BES1-INTERACTING MYC-LIKE 1) transcription factors that mediate BR-regulated gene expression and plant growth to inhibit their activities. Genome-wide gene expression analysis defined a BES1-dependent UV-B-regulated transcriptome, which is enriched with genes involved in cell elongation and plant growth. We further showed that UV-B-activated and nucleus-localized UVR8 inhibited the DNA-binding activities of BES1/BIM1 to directly regulate transcription of growth-related genes. Our results therefore establish that UVR8-BES1/BIM1 interaction represents an early photoreceptor signaling mechanism in plants and serves as an important module integrating light and BR signaling.

No abstract

UV RESISTANCE LOCUS 8 (UVR8) is an ultraviolet-B (UVB) radiation photoreceptor that mediates light responses in plants. How plant UVR8 acts in response to UVB light is not well understood. Here, we report the identification and characterization of the Arabidopsis WRKY DNA-BINDING PROTEIN 36 (WRKY36) protein. WRKY36 interacts with UVR8 in yeast and Arabidopsis cells and it promotes hypocotyl elongation by inhibiting HY5 transcription. Inhibition of hypocotyl elongation under UVB requires the inhibition of WRKY36. WRKY36 binds to the W-box motif of the HY5 promoter to inhibit its transcription, while nuclear localized UVR8 directly interacts with WRKY36 to inhibit WRKY36-DNA binding both in vitro and in vivo, leading to the release of inhibition of HY5 transcription. These results indicate that WRKY36 is a negative regulator of HY5 and that UVB represses WRKY36 via UVR8 to promote the transcription of HY5 and photomorphogenesis. The UVR8-WRKY36 interaction in the nucleus represents a novel mechanism of early UVR8 signal transduction in Arabidopsis.

Light and temperature are two key environmental signals that profoundly affect plant growth and development, but underlying molecular mechanisms of how light and temperature signals affect the circadian clock are largely unknown. Here, we report that COR27 and COR28 are regulated not only by low temperatures but also by light signals. COR27 and COR28 are negative regulators of freezing tolerance but positive regulators of flowering, possibly representing a trade-off between freezing tolerance and flowering. Furthermore, loss-of-function mutations in COR27 and COR28 result in period lengthening of various circadian output rhythms and affect central clock gene expression. Also, the cor27 cor28 double mutation affects the pace of the circadian clock. Additionally, COR27 and COR28 are direct targets of CCA1, which represses their transcription via chromatin binding. Finally, we report that COR27 and COR28 bind to the chromatin of TOC1 and PRR5 to repress their transcription, suggesting that their effects on rhythms are in part due to their regulation of TOC1 and PRR5. These data demonstrate that blue light and low temperature-regulated COR27 and COR28 regulate the circadian clock as well as freezing tolerance and flowering time.

The UV-B photoreceptor UVR8 mediates multiple UV-B responses in plants, but the function of UVR8 in regulating root development has not previously been investigated. Here, we show that UV-B light inhibits Arabidopsis lateral root growth in a UVR8-dependent manner. Monomeric UVR8 inhibits auxin responses in a tissueautonomous manner and thereby regulates lateral root growth. Genome-wide gene expression analysis demonstrated that auxin and UV-B irradiation antagonistically regulate auxin-regulated gene expression. We further show that UVR8 physically interacts with MYB73/MYB77 (MYB DOMAIN PROTEIN 73/77) in a UV-Bdependent manner. UVR8 inhibits lateral root development via regulation of MYB73/MYB77. When activated by UV-B light, UVR8 localizes to the nucleus and inhibits the DNA-binding activities of MYB73/MYB77 and directly represses the transcription of their target auxin-responsive genes. Our results demonstrate that UV-B light and UVR8 are critical for both shoot morphogenesis and root development. The UV-B-dependent interaction of UVR8 and MYB73/MYB77 serves as an important module that integrates light and auxin signaling and represents a new UVR8 signaling mechanism in plants.

No abstract

Contents Summary1247I.Introduction1247II.The UVR8–COP1 pathway1248III.The UVR8–WRKY36 pathway1248IV.The UVR8–BES1/BIM1 pathway1249V.Other pathways1250VI.Conclusion and perspectives1250Acknowledgements1251References1251 Summary Ultraviolet‐B (UV‐B) light is an intrinsic part of sunlight that has significant effects on plant development and acclimation responses. UVR8 (UV Resistance Locus 8) is the long sought‐after UV‐B photoreceptor that mediates UV‐B light perception and signal transduction. UV‐B irradiation induces the monomerization and nuclear accumulation of UVR8 in plant cells to activate the UV‐B signaling pathway. The photoactivated UVR8 could transduce UV‐B signal via multiple mechanisms to regulate transcription and plant growth. Here, we summarize current understanding of UVR8‐mediated UV‐B signal transduction pathways, including UVR8–COP1 (CONSTITUTIVELY PHOTOMORPHOGENIC 1) and UVR8–WRKY36 (WRKY DNA‐BINDING PROTEIN 36), UVR8–BES1 (BRI1‐EMS‐SUPPRESSOR1) and BIM1 (BES1‐INTERACTING MYC‐LIKE 1).

No abstract