Effects of exogenous γ-aminobutyric acid spray on sugar and acid metabolism in Cabernet Gernischet grape berries

Abstract:【Objective】Sugars and organic acids are metabolites that affect fruit quality, and together with volatile substances, determine the flavor quality of fruit. Appropriate sugar and acid content of wine grapes determine the flavor and quality of the wine. At present, the accumulation of sugar and organic acids in grapes is mainly improved through the regulation and cultivation measures of environmental factors such as temperature, light, water and soil. The main sugars are glucose and fructose, and the organic acids are mainly tartaric acid and malic acid, and also contain a small amount of succinic acid, oxalic acid and citric acid in grape berries. The synthesis and accumulation of sugars and organic acids are mainly regulated by sucrose metabolism-related enzymes and tricarboxylic acid cycle-related enzymes in berries. γ-aminobutyric acid (GABA) is a growth regulator widely present in plants, which can affect the accumulation of organic acids through the tricarboxylic acid cycle. In this study, the wine grape Cabernet Gernischet was used as the test material, and the grapevines were treated with different concentrations of GABA at each growth stage to determine the sugar and acid components, sugar and acid metabolism, and GABA synthesis-related enzyme activities. The purpose was to investigate the effect of GABA on the ripening process of grape berries, and provide a theoretical basis for improving quality and yield in agricultural production.【Methods】The foliar spray with 5 mmol · L^{- 1}(T1), 10 mmol·L^-1 (T2), 15 mmol·L^-1 (T3), and 20 mmol·L^-1 (T4) GABA solution were carried out at the flowering stage, berry setting stage, expansion stage and veraison stage, and the distilled water spray served as the control (CK). The contents of GABA, total soluble sugar, glucose, fructose, sucrose, oxalic acid, tartaric acid, shikimic acid, fumaropimaric acid, citrate acid, α-ketoglutaric acid, malate and glutamic acid as well as the activities of decarboxylase, sucrose metabolism-related enzymes and organic acid metabolism-related enzymes were determined.【Results】(1) The foliar spraying with GABA solution significantly increased the soluble sugar content in grape berries. The appropriate concentration of GABA treatment significantly increased the sucrose, glucose and fructose contents, and the activities of sucrose synthase synthesis (SuSy-s), sucrose phosphate synthase (SPS), acid invertase (AI), and neutral invertase (NI) significantly increased in grape berries, among which, the contents of total soluble sugar, sucrose, glucose and fructose with T2 treatment at the mature stage were 0.09, 0.63, 0.33 and 0.48 times more than those with CK, respectively. (2) The different concentrations of GABA significantly enhanced glutamate decarboxylase (GAD) activity and GABA content, among which, the GABA content with T3 treatment was 2.93 times more than that with CK at the maturity stage. (3) The appropriate concentration of GABA treatment can significantly increase the contents of oxalic acid, tartaric acid, fumaropimaric acid, citrate acid, α-ketoglutaric acid and malate in grape berries. Among them, oxalic acid, tartaric acid, fumaropimaric acid, citrate acid, α-ketoglutaric acid and malate contents with T2 treatment at the mature stage were 1.21, 1.08, 0.49, 2.26, 3.41 and 1.11 times more than that with CK, respectively. The activity of citrate acid synthase (CS), aconitic acid enzyme (ACO), isocitrate dehydrogenase (NAD-IDH), phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (NAD-MDH) and malate enzyme (NADP-ME) significantly increased with T2 treatment. (4) The correlation analysis indicated that GABA had an extremely significant positive correlation with NI activity and citric acid contents, and had a positive correlation with SPS, oxalic acid and Mit- ACO. The sucrose content had a very significant positive correlation with soluble sugar, glucose, fructose contents and SuSy-s activity, and an extremely significant negative correlation with SuSy-c and NI activities. In addition, the glucose and fructose contents were significantly and positively correlated with soluble sugar, sucrose contents and SuSy-s activity, negatively correlated with SuSy-c and NI activities, and significantly and negatively correlated with AI activity. The tartaric acid content was extremely positively correlated with oxalic acid, fumaropimaric acid contents and ME activity, and significantly and positively correlated with citrate acid content and MDH activity. The malate content was extremely positively correlated with GAD activity, shikimic acid content, MDH and PEPC activities, significantly and positively correlated with Mit-ACO activity, and significantly and negatively correlated with CS, IDH and ME activities.【Conclusion】Exogenous GABA promoted the sucrose accumulation by increasing the activities of SuSy-s and SPS and inhibiting the activity of SuSy-c, and promoted the accumulation of glucose and fructose by increasing the activities of AI and NI. Exogenous spraying with GABA increased GABA biosynthesis by increasing GAD activity, thereby increasing the rate of the TCA cycle, the activities of CS, ACO, NADIDH, PEPC and NAD-MDH, and NADP-ME activity were significantly improved, promoting the accumulation of oxalic acid, tartaric acid, shikimic acid, fumaropimaric acid, citrate acid, α-ketoglutaric acid and malate, and then improved the grape flavor quality, with the best efficacy of 10 mmol· L^{- 1} GABA treatment.