Ongoing climate change causes extreme weather events to increase both in frequency and intensity, significantly affecting vine physiology and grape berry composition at harvest. Elevated temperatures during the growing ...
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Ongoing climate change causes extreme weather events to increase both in frequency and intensity, significantly affecting vine physiology and grape berry composition at harvest. Elevated temperatures during the growing season increase must sugar content, while reducing organic acids and anthocyanins levels, modifying wine quality and typicality. Viticultural practices such as leaf-to-fruit ratio manipulations combined with applications of plant hormones can potentially be used to adapt to climate change and maintain the sustainability of wine production. To optimize adaptation strategies, we need to gain better insights into the links between sugars and anthocyanins accumulation during berry development and ripening.Fruiting-cuttings made up of one vertical shoot with one grape cluster of cv. Cabernet Sauvignon were grown between 2013 and 2018 in a naturally lighted and semi-controlled greenhouse. Various leaf-to-fruit ratios (2 to12 leaves per cluster) were applied, and combined with spraying abscisic acid (ABA, 400 mg.L-1) on berries at the pre-veraison stage. To explore the biological mechanisms underlying the effect of leaf-to-fruit ratio manipulation and application of exogenous ABA on berry composition, transcript abundance of genes related to sugar, anthocyanins and ABA metabolism were studied in this material. To further analyze the potential interplay between sugar and ABA signaling on anthocyanins biosynthesis, in vitro berry culture experiments were also conducted. Berries were cultured 2-3 weeks on solid medium supplemented with different sugar levels (0%, 2%, 8%) combined with ABA (200 µM), ABA biosynthesis inhibitor (NDGA), or hexokinase inhibitors (NAG, GDH).The results confirmed that reducing the leaf-to-fruit ratio had a significant effect on berry composition: reduction of sugar and anthocyanin content, slight increase in total organic acids and modification of the free amino acids composition. Exogenous ABA application increased sugars and anthocyanins concentrations, and partially restored the coupling between sugar and anthocyanins accumulation under low leaf-to-fruit ratios, without affecting the amino acids/sugar/organic acids ratios. Transcript abundance analysis revealed that several anthocyanin biosynthetic genes (CHS2, CHS3, CHI, F3H, DFR, LODX, UFGT, MybA1 and MybA2) were decreased under low leaf-to-fruit ratio, whereas some genes (CHI, F3H, F3’5’H, LODX, UFGT, MybA1 and MybA2) were up-regulated after exogenous ABA treatment. Exogenous ABA had little effect on the transcript abundance of sugar accumulation related genes, and leaf-to-fruit ratio also had little effect on ABA biosynthetic genes. Berry in vitro culture experiments showed that ABA and elevated concentrations of glucose synergistically induced anthocyanins biosynthesis during ripening. Both ABA and glucose decreased Phenylalanine concentration in ripening berries, most probably due to their role in promoting anthocyanins synthesis, and two-way crossed ANOVA analysis indicate a significant interaction between sugar and ABA levels and anthocyanin accumulation in berries.In conclusion, our results showed that ABA and sugar signaling synergistically interact to regulate the expression of anthocyanin biosynthetic genes and increase anthocyanins accumulation during berry ripening. Thus, exogenous ABA application was able to increase the ratio of anthocyanins to sugar under low leaf-to-fruit ratio at harvest, and combining leaf-to-fruit ratio manipulation with exogenous ABA applications may offer a fine-tuned way to reduce sugar concentration, while maintaining anthocyanin concentrations in grape berry, potentially offering a way to partially alleviate climate change related high temperature effects.
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