Activation and negative feedback regulation of SlHY5 transcription by the SlBBX20/21-SlHY5 transcription factor module in UV-B signaling.

In tomato (Solanum lycopersicum) and other plants, the photoreceptor UV-RESISTANCE LOCUS 8 regulates plant UV-B photomorphogenesis by modulating the transcription of many genes, the majority of which depends on the transcription factor ELONGATED HYPOCOTYL 5 (HY5). HY5 transcription is induced and then rapidly attenuated by UV-B. However, neither the transcription factors that activate HY5 transcription nor the mechanism for its attenuation during UV-B signaling is known. Here, we report that the tomato B-BOX (BBX) transcription factors SlBBX20 and SlBBX21 interact with SlHY5 and bind to the SlHY5 promoter to activate its transcription. UV-B-induced SlHY5 expression and SlHY5-controlled UV-B responses are normal in slbbx20 and slbbx21 single mutants, but strongly compromised in the slbbx20 slbbx21 double mutant. Surprisingly, UV-B responses are also compromised in lines overexpressing SlBBX20 or SlBBX21. Both SlHY5 and SlBBX20 bind to G-box1 in the SlHY5 promoter. SlHY5 outcompetes SlBBX20 for binding to the SlHY5 promoter in vitro, and inhibits the association of SlBBX20 with the SlHY5 promoter in vivo. Overexpressing 35S:SlHY5-FLAG in the WT background inhibits UV-B-induced endogenous SlHY5 expression. Together, our results reveal the critical role of the SlBBX20/21-SlHY5 module in activating the expression of SlHY5, the gene product of which inhibits its own gene transcription under UV-B, forming an autoregulatory negative feedback loop that balances SlHY5 transcription in plants.

Expression of Tomato UVR8 in Arabidopsis reveals conserved photoreceptor function.

UV RESISTANCE LOCUS 8 (UVR8) is a photoreceptor that regulates UV-B photomorphogenesis in plants. UV-B photon perception promotes UVR8 homodimer dissociation into monomer, which is reverted to homodimer post UV-B, forming a complete photocycle. UVR8 monomer interacts with CONSTITUTIVELY PHOTOMORPHOGENEIC 1 (COP1) to initiate UV-B signaling. The function and mechanism of Arabidopsis UVR8 (AtUVR8) are extensively investigated, however, little is known about UVR8 and its signaling mechanisms in other plant species. Tomato is a widely used model plant for horticulture research. In this report we tested whether an ortholog of AtUVR8 in Tomato (SIUVR8) can complement Arabidopsis uvr8 mutant and whether the above-mentioned key signaling mechanisms of UVR8 are conserved. Heterologous expressed SIUVR8 in an Arabidopsis uvr8 null mutant rescued the uvr8 mutant in the tested UV-B responses including hypocotyl elongation, UV-B target gene expression and anthocyanin accumulation, demonstrating that the SIUVR8 is a putative UV-B photoreceptor. Moreover, in response to UV-B, SIUVR8 forms a protein complex with Arabidopsis COP1 in plants, suggesting conserved signaling mechanism. SIUVR8 exhibits similar photocycle as AtUVR8 in plants, which highlights conserved photoreceptor activation and inactivation mechanisms.

The C-terminal 17 amino acids of the photoreceptor UVR8 is involved in the fine-tuning of UV-B signaling.

Plant UV-B responses are mediated by the photoreceptor UV RESISTANCE LOCUS 8 (UVR8). In response to UV-B irradiation, UVR8 homodimers dissociate into monomers that bind to the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1). The interaction of the C27 domain in the C-terminal tail of UVR8 with the WD40 domain of COP1 is critical for UV-B signaling. However, the function of the last 17 amino acids (C17) of the C-terminus of UVR8, which are adjacent to C27, is unknown, although they are largely conserved in land plants. In this study, we established that Arabidopsis thaliana UVR8 C17 binds to full-length UVR8, but not to COP1, and reduces COP1 binding to the remaining portion of UVR8, including C27. We hypothesized that overexpression of C17 in a wild-type background would have a dominant negative effect on UVR8 activity; however, C17 overexpression caused strong silencing of endogenous UVR8, precluding a detailed analysis. We therefore generated YFP-UVR8N423 transgenic lines, in which C17 was deleted, to examine C17 function indirectly. YFP-UVR8N423 was more active than YFP-UVR8, suggesting that C17 inhibits UV-B signaling by attenuating binding between C27 and COP1. Our study reveals an inhibitory role for UVR8 C17 in fine-tuning UVR8-COP1 interactions during UV-B signaling.

Pivotal roles of Tomato photoreceptor SlUVR8 in seedling development and UV-B stress tolerance.

UV RESISTANCE LOCUS 8 (UVR8) is a UV-B photoreceptor that regulates various aspects of plant photomorphogenesis. Physiological functions of UVR8 have been extensively investigated in Arabidopsis. However, functions of Tomato UVR8 (SlUVR8) are largely unknown. To analyze physiological functions of SlUVR8, we generated sluvr8 knock-out mutant lines with CRISPR-CAS9 gene editing approach. At seedling stage, SlUVR8 regulates hypocotyl elongation and anthocyanin accumulation under UV-B. Moreover, SlUVR8 regulates acclimation to low dose UV-B and promotes tolerance to elevated UV-B stress. These results revealed pivotal roles of SlUVR8 in the regulation of Tomato seedling development and UV-B stress tolerance. The manipulation of photoreceptor SlUVR8 may represent a powerful tool to improve Tomato plant performance in nature where high dose UV-B is present.