Our understanding of the specific low fluence UV-B responses in plants has taken a big step since the identification of the UV-B specific photoreceptor, UVR8 (Di et al. 2012; Hofmann 2012; Rizzini et al.
2011). To date, we have a good knowledge of UVR8 including the structural basis, the molecular mechanisms and the perception of UV-B radiation. However, most of these studies have been carried out in the model plant Arabidopsis thaliana. There remains a lack of understanding of UVR8 and its reaction partners in non-model plants. This research is the first study in relation to UVR8 and the UVR8- mediated low fluence UV-B signal transduction pathway in an important commercial crop, grapevine. It has been suggested that UVR8 in the model plant Arabidopsis thaliana is constitutively expressed in all plant tissues and developmental stages analysed to date (Heilmann & Jenkins 2013; Kaiserli & Jenkins 2007; Rizzini et al. 2011). Whereas in this study, VvUVR8 did exhibit a developmental pattern of regulation in Sauvignon blanc berries with a significantly higher expression pre-veraison compared with post-veraison. The studies in Arabidopsis thaliana also suggest UVR8 does not respond to different light quality (Heilmann & Jenkins 2013; Kaiserli & Jenkins 2007; Rizzini et al. 2011), which is consistent with the finding in this study that VvUVR8 gene expression did not show any UV-B responses in Sauvignon blanc berries and harvested skin. Similarly, its signalling partner VvCOP1 TF did not respond to UV-B radiation in berries or skin. However, the transcription factor VvHY5 did show a significant UV-B induction in both the berries at different development stages and berry skin at harvest. It is most likely that in grapevines VvHY5 plays a central role in the regulation of genes involved in photomorphogenic UV-B responses as suggested previously in Arabidopsis thaliana (Jenkins 2009), while the UV-B specific photoreceptor VvUVR8 and VvCOP1 TF are constitutive and respond to UV-B induced changes to VvHY5. As was discussed earlier in this thesis, of the transcription factors MYB- bHLH-WD40 that are known to be associated with VvFLS activity (Czemmel et al. 2009; Hichri et al.
2010; Matus et al. 2010), only VvMYB12 showed a significant UV-B response in grapevine. If the flavonol biosynthesis in grapevine is regulated by the UVR8-mediated low fluence UV-B response (discussed earlier in this thesis) and only VvHY5 of the signal transduction pathway shows a significant UV-B response, it is likely that VvHY5 participates in the regulation of VvMYB12 TF and therefore regulates VvFLS activity. It has been shown in Arabidopsis thaliana that HY5 regulates the expression of MYB12 in response to light and UV-B radiation (Stracke et al. 2010), but this still remains to be determined in grapevine. Future research should include confirming the signalling of the low fluence UV-B response pathway, from UV-B perception by the UV-B photoreceptor UVR8 and its reaction partners COP1 and HY5 to the regulation of the transcription factor complex MYB-bHLH-WD40 and therefore to the final UV-B induction of flavonol biosynthesis in non-model plants.
In addition to the biosynthetic genes and transcription factors that we have tested, there is strong evidence in Arabidopsis thaliana that repressor TFs are involved in UV-B regulated flavonoid biosynthesis (Albert et al. 2013; Wade et al. 2003), specifically in UVR8-mediated UV-B perception (Gruber et al. 2010; Heijde & Ulm 2013). In Arabidopsis thaliana, the repression of UVR8 by REPRESSOR OF PHOTOMORPHOGENESIS (RUP1 and RUP2) takes place by converting the biologically active monomer to the dimeric ground state (Heijde & Ulm 2013). Therefore, the UV-B effects on UVR8 is not due to variations in UVR8 levels but the efficiency of the monomer-dimerization process regulated by the RUP 1 and 2 levels. Certainly, it has been found that RUP1 and RUP2 levels are induced by UV-B exposure in Arabidopsis thaliana (Gruber et al. 2010). It is likely that RUP1 and RUP2 levels are induced by UV-B exposure to then provide negative feedback regulation of the UVR8 pathway involving direct RUP1/RUP2–UVR8 interaction. Therefore, it is important to understand the functions of the repressor TFs RUP1 and RUP2 as they play a critical role in regulating the UVR8-mediated low fluence UV-B response. In this study, the gene expression of VvUVR8 and VvCOP1 has been found to be constitutively expressed in grapevine, but the gene activity of repressors RUP1 and RUP2 remains to be determined. Whether these repressor TFs are induced by UV-B exposure and whether they regulate UVR8- mediated UV-B signal transduction to negatively regulate the UV-B induced flavonoid biosynthesis is important for the future understanding of UVR8 regulation. If the UVR8-mediated UV-B perception is regulated by the repressors RUP1 and RUP2 in grapevine, it is likely that RUP1 and RUP2 will show some induction after UV-B exposure in grapevine in a similar response to UV-B exposure in Arabidopsis
thaliana (Gruber et al. 2010). Future work should focus on the regulatory mechanism of these
repressor TFs in non-model species to improve our understanding.