Comparisons of controlled environment and vineyard experiments in Sauvignon blanc grapes reveal similar UV-B signal transduction pathways for flavonol biosynthesis

UV-B Radiation Induced the Changes in the Amount of Amino Acids, Phenolics and Aroma Compounds in Vitis vinifera cv. Pinot Noir Berry under Field Conditions

High UV-B radiation can challenge Pinot noir growth in the wine-making region of the Southern Hemisphere. The aim of this work was to determine UV-B effects on amino acids, phenolic composition and aroma compounds of Pinot noir fruit. Sunlight exposure with or without UV-B did not affect fruit production capacity, °Brix and total amino acids in the vineyard over the two years. This research reported increased contents of skin anthocyanin and skin total phenolics in berry skins under UV-B. The research showed that there were no changes in C6 compounds. Some monoterpenes concentrations were decreased by UV-B. The information also indicated how important leaf canopy management was for vineyard management. Therefore, UV radiation potentially affected fruit ripeness and crop load, and even stimulated the accumulation of phenolic compounds that may affect Pinot noir quality. This research reported that canopy management (UV-B exposure) may be a good way for vineyard management to increase the accumulation of anthocyanins and tannins in berry skins.

Secondary metabolites and related genes in Vitis vinifera L. cv. Tempranillo grapes as influenced by ultraviolet radiation and berry development

The effects of UV radiation on Vitis vinifera cv Tempranillo grapes were studied under field conditions as influenced by ultraviolet (UV) band (UV-A and UV-B), UV-B level (ambient vs enhanced), grape phenological stage (pea-size, veraison, and harvest), grape component (skin, flesh, and seeds), and fraction from which phenolic UV-absorbing compounds (UVACs) were extracted (soluble vs insoluble). Ambient UV-B levels caused stronger effects than ambient UV-A. These effects included increases in flavonol contents (particularly quercetins and kaempferols), the expression of flavonol synthase and chalcone synthase genes (VvFLS4 and VvCHS1), and grape weight and size. In addition, the contents of flavanols and hydroxycinnamic acids increased under UV-B radiation at pea-size stage. All these compounds play physiological roles as antioxidants and UV screens. Synergic effects between UV-B and UV-A were observed. The responses of anthocyanins, stilbenes, and volatile compounds to UV were diffuse or nonexistent. Enhanced UV-B led to rather subtle changes in comparison with ambient UV-B, but differences between both treatments could be demonstrated by multivariate analysis. Pea-size and harvest were the phenological stages where the most significant responses to UV were found, while the skin was the most UV-responsive grape component. Soluble phenolic compounds were much more UV-responsive than insoluble compounds. In conclusion, UV radiation was essential for the induction of specific grape phenolic and volatile compounds. Given the physiological roles of these compounds, as well as their contribution to grape and wine quality, and their potential use as nutraceuticals, our results may have implications on the artificial manipulation of UV radiation.

To What Extent Are the Effects of UV Radiation on Grapes Conserved in the Resulting Wines?

Ultraviolet (UV) radiation strongly influences grape composition, but only a few studies have focused on how this influence is conserved in the resulting wines. Here we analyzed to what extent the changes induced by exposing Tempranillo grapes to UV radiation from budbreak to harvest were conserved in wine. By using different cut-off filters and lamps, we differentiated the effects of ambient levels of UV-A and UV-B wavelengths, as well as the effects of a realistic UV-B enhancement associated with climate change. Among phenolic compounds, the most consistent responses to UV were those of flavonols (particularly quercetin-, kaempferol-, isorhamnetin- and myricetin-glycosides), which significantly increased in wines whose grapes had been exposed to a synergic combination of UV-A and UV-B radiation. This confirms that flavonols are the phenolic compounds most reliably conserved from UV-exposed grapes to wine, despite the possible influence of the winemaking process. Flavonols are important compounds because they contribute to wine co-pigmentation by stabilizing anthocyanins, and they are interesting antioxidants and nutraceuticals. Hydroxycinnamic acids also increased under the same UV combination or under UV-A alone. Wine VOCs were much less reactive to the UV received by grapes than phenolic compounds, and only esters showed significantly higher values under (mainly) UV-A alone. This was surprising because (1) UV-A has been considered to be less important than UV-B to induce metabolic changes in plants, and (2) esters are produced during winemaking. Esters are relevant due to their contribution to the fruity aroma in wines. In general, the remaining phenolic compounds (stilbenes, flavanols, hydroxybenzoic acids, and anthocyanins) and VOCs (alcohols, hydrocarbons, and fatty acids), together with wine color and antioxidant capacity, showed inconsistent or non-significant responses to UV radiation. These results were summarized by a multivariate analysis. Our study opens up new possibilities to artificially manipulate UV radiation in grapevine cultivation to improve both grape and wine quality.

Response of Zebrina pendula leaves to enhanced UV-B radiation

Plants inevitably receive harmful UV-B radiation when exposed to solar energy, so they have developed a variety of strategies to protect against UV-B radiation damage during long-term evolution. In this study, Zebrina pendulaSchnizl. was used to investigate the plant defence against UV-B radiation because of its strong adaptability to sunlight changes, and the colour of its leaves changes significantly under different sunlight intensities. The experiment was carried out to study the changes of Z. pendula leaves under three light conditions: artificial daylight (control check); shading 50%; and artificial daylight + UV-B, aiming to explore the mechanism of defence against UV-B radiation by observing changes in leaf morphological structure, anthocyanin content and distribution. Results showed that the single leaf area increased but leaves became thinner, and the anthocyanin content in the epidermal cells decreased under 50% shading. In contrast, under daylight + UV-B, the single leaf area decreased but thickness increased (mainly due to the increase of the thickness of the upper epidermis and the palisade tissue), the trichomes increased. In addition, the anthocyanin content in the epidermal cells and phenylalanine ammonia-lyase (PAL) activity increased, and the leaf colour became redder, also, the photosynthetic pigment content in mesophyll cells and the biomass per unit volume increased significantly under daylight + UV-B. Thus, when UV-B radiation was enhanced, Z. pendula leaves reduced the exposure to UV-B radiation by reducing the area, and reflect some UV-B radiation by growing trichomes. The UV-B transmittance was effectively reduced by increasing the single leaf thickness and anthocyanin content to block or absorb partial UV-B. Through the above comprehensive defence strategies, Z. pendula effectively avoided the damage of UV-B radiation to mesophyll tissue.

Ambient Ultraviolet B Signal Modulates Tea Flavor Characteristics via Shifting a Metabolic Flux in Flavonoid Biosynthesis

Tea leaves contain an extraordinarily high level of flavonoids that contribute to tea health benefits and flavor characteristics, but the regulatory mechanism of ambient ultraviolet B (UV-B) on tea flavonoid enrichment remains unclear. Here, we report that ambient UV-B modulates tea quality by inducing a metabolic flux in flavonoid biosynthesis. UV-B absence decreased bitter- and astringent-tasting flavonol glycosides (kaempferol-7-O-glucoside, myricetin-3-O-glucoside, and quercetin-7-O-glucoside) but increased non-galloylated catechins. Conversely, supplementary UV-B increased flavonols and decreased catechins in tea leaves. These responses were achieved via CsHY5, which mediates the UV-B-induced MYB12 activation and binds to the promoters of flavonoid biosynthetic genes (CsFLS, CsLARa, and CsDFRa), leading to flavonoid changes. Transcriptomic data indicated that UV-B-induced tea flavonoid regulation is responsive to multiple biotic and abiotic environmental stresses. These findings improve our understanding of light-regulated tea astringency and bitterness underlying shading effects and seasonal light changes and provide novel insights into tea cultivation management and processing.

Phenolic characteristics acquired by berry skins of Vitis vinifera cv. Tempranillo in response to close-to-ambient solar ultraviolet radiation are mostly reflected in the resulting wines

BACKGROUND

It is widely recognized that ambient levels of solar ultraviolet (UV) radiation strongly influence the phenolic composition of grape skins. However, it is unknown to what extent this influence is reflected in the resulting wines.

RESULTS

Tempranillo grapevines were exposed or non-exposed to close-to-ambient solar UV levels using appropriate filters, and the phenolic profiles and antioxidant capacity of both grape skins and the resulting wines were analyzed. In total, 47 phenolic compounds were identified in skins and wines, including flavonols, anthocyanins, flavanols, stilbenes, and hydroxycinnamic and hydroxybenzoic acids. In UV-exposed grape skins, flavonols and anthocyanins increased, whereas flavanols and hydroxybenzoic acids showed no significant change. These characteristics were conserved in the resulting wines. However, for stilbenes, hydroxycinnamic acids and antioxidant capacity, the effect of UV on grape skins was not conserved in wines, probably as a result of changes during winemaking. In addition, color intensity, total phenols and total polyphenol index of wines elaborated from UV-exposed grapes increased (although non-significantly) compared to those made from non-UV-exposed grapes.

CONCLUSION

The phenolic composition of grape skins exposed to close-to-ambient solar UV could predict, to some extent, the phenolic composition of the resulting wines, particularly regarding higher contents of flavonols and anthocyanins. Thus, manipulating the UV radiation received by grape skins could improve wine quality by positively influencing color stability and healthy properties. To our knowledge, this is the first study in which the effects of solar UV radiation on phenolic composition have been assessed from grape skins to wine. © 2019 Society of Chemical Industry.

Lower levels of UV-B light trigger the adaptive responses by inducing plant antioxidant metabolism and flavonoid biosynthesis in Medicago sativa seedlings

Ultraviolet-B (UV-B) light, as an intrinsic part of sunlight, has more significant effects on plant growth and photomorphogenesis than other organisms due to plant's sessile growth pattern. In our studies, we have observed that alfalfa (Medicago sativa L.) seedlings are very sensitive to UV-B performance. Seedlings have grown better at lower levels of UV-B light (UV-B irradiation dosage <17.35 μW cm-2 day-1), and have higher UV-resistance. However, the higher levels of UV-B light (UV-B irradiation dosage >17.35 μW cm-2 day-1) has caused severe stress injuries to alfalfa seedlings, and seriously inhibited its growth and development. Chlorophyll biosynthesis and chlorophyll fluorescence have been suppressed under all different dosage of UV-B light conditions. Plant antioxidant enzymes were induced by lower levels of UV-B, but greatly inhibited under higher levels of UV-B light. The contents of flavonoid compounds significantly increased under UV-B light compared with controls, and that was more significant under lower levels of UV-B than higher levels of UV-B. Therefore, we have assumed that the significant induction of plant antioxidant capacity and flavonoid excessive accumulation play a central role in alfalfa UV-B tolerance to lower levels of UV-B irradiation.

Metabolite profiling and transcriptomic analyses reveal an essential role of UVR8-mediated signal transduction pathway in regulating flavonoid biosynthesis in tea plants (Camellia sinensis) in response to shading

BACKGROUND

Tea is the most popular nonalcoholic beverage worldwide for its pleasant characteristics and healthful properties. Catechins, theanine and caffeine are the major natural products in tea buds and leaves that determine tea qualities such as infusion colors, tastes and fragrances, as well as their health benefits. Shading is a traditional and effective practice to modify natural product accumulation and to enhance the tea quality in tea plantation. However, the mechanism underlying the shading effects is not fully understood. This study aims to explore the regulation of flavonoid biosynthesis in Camellia sinensis under shading by using both metabolomic and transcriptional analyses.

RESULTS

While shading enhanced chlorophyll accumulation, major catechins, including C, EC, GC and EGC, decreased significantly in tea buds throughout the whole shading period. The reduction of catechins and flavonols were consistent with the simultaneous down-regulation of biosynthetic genes and TFs associated with flavonoid biosynthesis. Of 16 genes involved in the flavonoid biosynthetic pathway, F3'H and FLS significantly decreased throughout shading while the others (PAL, CHSs, DFR, ANS, ANR and LAR, etc.) temporally decreased in early or late shading stages. Gene co-expression cluster analysis suggested that a number of photoreceptors and potential genes involved in UV-B signal transductions (UVR8_L, HY5, COP1 and RUP1/2) showed decreasing expression patterns consistent with structural genes (F3'H, FLS, ANS, ANR, LAR, DFR and CHSs) and potential TFs (MYB4, MYB12, MYB14 and MYB111) involved in flavonoid biosynthesis, when compared with genes in the UV-A/blue and red/far-red light signal transductions. The KEGG enrichment and matrix correlation analyses also attributed the regulation of catechin biosynthesis to the UVR8-mediated signal transduction pathway. Further UV-B treatment in the controlled environment confirmed UV-B induction on flavonols and EGCG accumulation in tea leaves.

CONCLUSIONS

We proposed that catechin biosynthesis in C. sinensis leaves is predominantly regulated by UV through the UVR8-mediated signal transduction pathway to MYB12/MYB4 downstream effectors, to modulate flavonoid accumulation. Our study provides new insights into our understanding of regulatory mechanisms for shading-enhanced tea quality.