RESUMO
The ultraviolet-B (UV-B) portion of the solar radiation functions as an environmental signal for which plants have evolved specific and sensitive UV-B perception systems. The UV-B-specific UV RESPONSE LOCUS 8 (UVR8) and the multifunctional E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) are key regulators of the UV-B response. We show here that uvr8-null mutants are deficient in UV-B-induced photomorphogenesis and hypersensitive to UV-B stress, whereas overexpression of UVR8 results in enhanced UV-B photomorphogenesis, acclimation and tolerance to UV-B stress. By using sun simulators, we provide evidence at the physiological level that UV-B acclimation mediated by the UV-B-specific photoregulatory pathway is indeed required for survival in sunlight. At the molecular level, we demonstrate that the wild type but not the mutant UVR8 and COP1 proteins directly interact in a UV-B-dependent, rapid manner in planta. These data collectively suggest that UV-B-specific interaction of COP1 and UVR8 in the nucleus is a very early step in signalling and responsible for the plant's coordinated response to UV-B ensuring UV-B acclimation and protection in the natural environment.
Assuntos
Proteínas de Arabidopsis/fisiologia , Arabidopsis/fisiologia , Proteínas Cromossômicas não Histona/fisiologia , Raios Ultravioleta , Aclimatação , Proteínas de Arabidopsis/genética , Proteínas Cromossômicas não Histona/genética , Regulação da Expressão Gênica de Plantas , Hipocótilo/crescimento & desenvolvimento , Hipocótilo/fisiologia , Mutação , Ligação Proteica , Transdução de Sinais/fisiologia , Estresse Fisiológico , Luz Solar , Ubiquitina-Proteína LigasesRESUMO
Plants respond to low levels of UV-B radiation with a coordinated photomorphogenic response that allows acclimation to this environmental stress factor. The key players in this UV-B response are COP1 (an E3 ubiquitin ligase), UVR8 (a ß-propeller protein), and HY5 (a bZIP transcription factor). We have shown previously that an elevated UV-B-specific response is associated with dwarf growth, indicating the importance of balancing UV-B-specific signaling. Negative regulators of this pathway are not known, however. Here, we describe two highly related WD40-repeat proteins, REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2, that interact directly with UVR8 as potent repressors of UV-B signaling. Both genes were transcriptionally activated by UV-B in a COP1-, UVR8-, and HY5-dependent manner. rup1 rup2 double mutants showed an enhanced response to UV-B and elevated UV-B tolerance after acclimation. Overexpression of RUP2 resulted in reduced UV-B-induced photomorphogenesis and impaired acclimation, leading to hypersensitivity to UV-B stress. These results are consistent with an important regulatory role for RUP1 and RUP2, which act downstream of UVR8-COP1 in a negative feedback loop impinging on UVR8 function, balancing UV-B defense measures and plant growth.
Assuntos
Aclimatação/efeitos da radiação , Arabidopsis/crescimento & desenvolvimento , Arabidopsis/efeitos da radiação , Retroalimentação Fisiológica/efeitos da radiação , Morfogênese/efeitos da radiação , Estresse Fisiológico/efeitos da radiação , Raios Ultravioleta , Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Regulação da Expressão Gênica de Plantas/efeitos da radiação , Mutação/genética , Ligação Proteica/efeitos da radiação , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Transdução de Sinais/efeitos da radiaçãoRESUMO
Plants fend off potentially damaging ultraviolet (UV)-B radiation by synthesizing and accumulating UV-B-absorbing flavonols that function as sunscreens. Regulation of this biosynthetic pathway is largely transcriptional and controlled by a network of transcription factors, among which the PRODUCTION OF FLAVONOL GLYCOSIDES (PFG) family of R2R3-MYB transcription factors was recently identified with a pivotal function. Here, we describe the response of Arabidopsis seedlings to narrow-band UV-B radiation at the level of phenylpropanoid pathway genes using whole-genome transcriptional profiling and identify the corresponding flavonol glycosides accumulating under UV-B. We further show that the bZIP transcriptional regulator ELONGATED HYPOCOTYL5 (HY5) is required for the transcriptional activation of the PFG1/MYB12 and PFG3/MYB111 genes under UV-B and visible light. A synthetic protein composed of HY5 with the VP16 activation domain is sufficient to activate PFG1/MYB12 expression in planta. However, even though myb11 myb12 myb111 triple mutants have strongly reduced CHS levels in darkness as well as in constant light, neither light- nor UV-B-inducibility seems impaired. Notwithstanding this, absence of the three PFG family transcription factors results in reduced UV-B tolerance, whereas PFG1/MYB12 overexpression leads to an increased tolerance. Thus, our data suggest that HY5-dependent regulation of PFG gene expression contributes to the establishment of UV-B tolerance.