New fungus-resistant grapevine varieties display high and drought-independent thiol precursor levels

Abstract

The use of varieties tolerant to diseases is a long-term but promising option to reduce chemical input in viticulture. Several important breeding programs in Europe and abroad are starting to release a range of new hybrids performing well regarding fungi susceptibility and wine quality. Unfortunately, little attention is paid by the breeders to the adaptation of these varieties to climatic changes and to the aromatic potential such as thiol precursors. Indeed, varietal thiols (3-sulfanylhexan-ol (3SH) and its acetate or the 4-methyl-4-sulfanylpentan-2-one (4MSP)) are very powerful aromatic compounds in wines coming from odorless precursors in grapes and could contribute to the typicity of such varieties. This study aimed to characterize 6 new resistant varieties selected by INRAE (Floreal, G5 and 3159B for white grapes and Artaban, 3176N and G14 for red grapes) in comparison to Syrah to (i) quantify the thiol precursors in the fruits and to (ii) evaluate the influence of water deficit (WD) imposed on field-grown vines on these molecules. Grapes were picked-up at the arrest of phloem unloading to objectify the sampling at a precise physiological landmark and analyzed by LC-MS/MS. Six thiol precursors were quantified by isotopic dilution across all samples and only 3 were clearly identified and quantified: the glutathionylated (G3SH), cysteinylated (Cys3SH) and one dipeptidic precursors of 3SH (CysGly3SH). For all varieties, G3SH contents represented between 75 and 100% of the aromatic potential, followed by Cys3SH (0-16%) and finally the CysGly3SH (0-13%). The absolute concentrations of G3SH ranged from 31 to 132 µg/kg for white varieties and from 68 to 466 µg/kg for red ones. Surprisingly, 3176N had exceptional G3SH levels that can reach 466 µg/kg which corresponded to nearly 777 µg/L in volume concentration. The pedigree of this variety which includes Grenache as a progenitor could explain the high levels of thiol precursors as observed in the Rosé wines of Provence, a type of wines also characterized by high levels of varietal thiols. Whatever the variety, we did not find marked effects of WD on the contents in thiol precursors when expressed in µg/kg. When expressed in µg/berry to reflect the real impact of WD on rate of metabolite accumulation per organ, 3176N and Artaban showed significant differences between moderate and high WD treatments (p-value < 0.05, less amount of thiol precursors in WD grapes). Analyzing thiol precursors and more generally metabolites of interest in fruits requires to objectify the sampling date at a given physiological stage. This allows deciphering the effects of environmental factors on the accumulation of metabolites at organ or plant level and their consequences in the concentration of the fruit at harvest. In conclusion, resistant varieties seemed to be less impacted by WD than Vinifera ones, which is bode well for the development of these varieties in relation to climate change challenges.