Tree Genetics and Molecular Breeding 2025, Vol.15, No.2, 62-69 http://genbreedpublisher.com/index.php/tgmb 64 Enzymes such as hexokinase (VvHXK) and fructokinase (VvFK) are crucial in the process of sugar synthesis. They add phosphoric acid to glucose and fructose. This step is a prerequisite for the utilization of sugar by plants. The expressions of VvHXK and VvFK are controlled by factors such as sugar content and hormone levels in plants, and these regulations affect the accumulation of sugar in fruits (Lecourieux et al., 2009; Xu et al., 2021). 3.2 Metabolic pathways contributing to sugar content Grapevines produce carbohydrates through photosynthesis. These sugars are eventually transported to the fruits for storage, and the distribution of sugars among various organs is very important. Ren et al. (2020) found that the expression levels of sucrose synthase VvSS3 and some sugar transport proteins were related to the amount of sugar in the fruit. Carbohydrates can be effectively transported from the plant to the fruit, and the sugar content of the fruit can be significantly increased. Environmental factors can also affect the sugar accumulation of fruits. When water is scarce, sugar transport genes such as VvHT1 and VvSUC11 will have expression changes to varying degrees, affecting the transport and distribution of sugar (Medici et al., 2014). Hormones such as abscisic acid (ABA) induced by stress can also regulate the expression of genes related to sugar and affect the sweetness of fruits (Lecourieux et al., 2009). 3.3 Transcription factors regulating sugar content Transcription factors such as VvMYB and VvNAC are crucial in regulating sugar metabolism. They can activate or inhibit certain genes related to sugar transport and metabolism, affecting the sugar content in fruits. Météier et al. (2019) and Huang et al. (2021) found that VvMYB can regulate the expression of genes related to sugar transport, while VvNAC is more involved in plants’ responses to stress and affects sugar accumulation. There is also a synergistic effect among different sugar-related genes and transcription factors. Ren et al. (2020) demonstrated that when both VvSWEET protein and sucrose synthase VvSS3 were highly expressed simultaneously, the hexose content in the fruit would increase significantly. Huang et al. (2021) hold that the interaction between VvWRKY22 and VvSnRK1 demonstrates the complex regulatory network behind sugar metabolism. 3.4 Functional analysis of a gene regulating sugar content in grapevine The VvGH9 gene (glycoside hydrolase) is activated under the induction of sugars such as sucrose, fructose and glucose. Najafi et al.’s research in 2022 found that when VvGH9 was overexpressed in grape callus, the accumulation of sugar would significantly increase (Figure 1). The function of this gene is to play a role in the process of breaking down polysaccharides into monosaccharides. Its research enables relevant researchers to better understand the genetic regulatory mechanism behind sugar accumulation in grape fruits. Figure 1 Grapevine embryogenic callus system (cv. Thompson Seedless) and stable transformation with the GFP gene (Adopted from Najafi et al., 2022) Image caption: (a) Unopened leaves cultured on initiation medium (bar=6 mm) produced (b) sectors of embryogenic and non-embryogenic callus (bar=2 mm). (c) Embryogenic callus proliferated and (d) produced somatic embryos ready for transformation (bars=1 mm). (e) Somatic embryos after 72 h of co-cultivation with Agrobacterium tumefaciens under white light and (f) UV light (bars=0.5 mm). (g) Embryogenic callus induced from somatic embryos under white light and (h) UV light (bars=0.75 mm) (Adopted from Najafi et al., 2022)
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