The effects of UV radiation on flavonoid accumulation and gene expression in Sauvignon blanc grape berries

Leaf removal is a normal commercial practice in viticulture to lower disease pressure through reduction in humidity and increased light exposure. However, New Zealand has 30-40% higher levels of UV radiation compared to similar latitudes in the Northern hemisphere, an increase of light exposure may therefore have a profound influence on berry composition. The aim of this research was to determine the effects of leaf removal and UV-B on flavonoid biosynthesis and regulation of gene activity in Vitis vinifera L. var. Sauvignon blanc grapes, particularly those genes associated with the low and high fluence UV-B responses. During the 2010 and 2011 seasons, three north-south oriented rows of Sauvignon blanc grapevines in the Lincoln University research vineyard were chosen to study the specific effects of UV-B on flavonoid accumulation and gene expression. This was achieved by using a combination of leaf removal and plastic screening techniques, which alter the light environment that the fruits are exposed to. UV-B exposure had a dramatic effect on the physical appearance of the berries causing specific pigmentation post-veraison. However, the accumulation of the total soluble solids and berry tissue development were not affected either by leaf removal around the fruiting zone or increased exposure to UV-B. In contrast, flavonols, particularly quercetin and kaempferol glycosides, increased substantially on fruit exposure due to UV-B. Spatial analysis of these flavonols located the changes to the berry skin. To understand the role of UV-B in regulating flavonoid biosynthesis, the transcript abundance for genes and transcription factors that are involved in the flavonoid biosynthetic pathway were analysed by qRT-PCR. These genes include five flavonol synthase genes (VvFLS1-5), three chalcone synthase genes (VvCHS1-3), and genes for the MYB-bHLH-WD40 transcription factor complex (VvMYB12, VvMYCA1 and VvWDRs). Of the five identified VvFLS, only two (VvFLS4 and VvFLS5) were found to be transcriptionally active in Sauvignon blanc berries and responsive to UV-B exposure. Of three VvCHS, only VvCHS1 and VvCHS2 showed a significant UV-B induction. Of the transcription factor genes, only VvMYB12 was found to be responsive to UV-B radiation and its expression pattern was consistent with VvFLS. The relative involvement of genes in the high and low fluence UV-B transduction pathways (VvUVR8, VvHY5 and VvCOP1; PR genes and VvMAPK3; respectively) were also studied. VvUVR8 did not response to UV-B but showed a developmental regulation. A similar lack of a UV-B response was found for VvCOP1. In contrast, VvHY5 showed a UV-B induction and higher levels of transcript abundance took place at harvest. All of PR genes showed a significant developmental regulation, with very low or no detectable transcript abundance pre-veraison and high transcript abundance post-veraison. These PR genes were not responsive to the natural fluence UV-B in the vineyard environment, but some showed a significant UV-B induction to a relatively high fluence UV-B under the controlled environment. Overall, this study clearly demonstrates the effect of UV-B exposure on the flavonoid biosynthesis in Sauvignon blanc grapes is achieved through a complex regulation of genes and transcription factors involved in the flavonoid biosynthetic pathway. This research makes a substantial contribution, not only in improvement of our scientific understanding of UV-B responses in important commercial species, but also provides valuable information for vineyard management to ultimately improve wine quality.