Tree Genetics and Molecular Breeding 2025, Vol.15, No.2, 62-69 http://genbreedpublisher.com/index.php/tgmb 65 4 Mechanistic Insights into the Co-Regulation of Fruit Size and Sugar Content 4.1 Cross-talk between pathways controlling size and sugar content Whether the nutrition is sufficient and the changes in hormone signals will both affect the development of the grape from the time it blooms until the fruit begins to form. The signals of sugar and hormones are interrelated and work together at the early stage of fruit development. Domingos et al. (2016) found that SWEET14 sugar transporters and some hormone-related transcription factors such as MADS-box and AP2 type ethylene response factors were crucial in this process, indicating that sugar transport and hormone metabolism occur synergistically and jointly contribute to the normal development of fruits. The earlier research by Lecourieux et al. (2009) demonstrated that sugar-inducible protein kinases such as VvSK1 regulate the transport and accumulation of hexose, indicating that the sugar content and fruit size are balanced through a complex network. 4.2 Role of epigenetic modifications and small RNAs DNA methylation, histone modification and chromatin remodeling can all affect gene expression and the development process of fruits. The research conducted by Tang et al. in 2020 indicated that when there was a problem with DNA demethylase, the fruit ripening would slow down in tomatoes, suggesting that this mechanism might be equally important in grapes. Small RNAs such as microRNAs and long non-coding RNAs also participate in regulating many metabolic processes. Wong and Matus (2017) discovered in grapes by using the method of network analysis that microRNA might regulate the metabolic pathway of phenylalanine, which is related to some important secondary metabolites and these substances have an impact on the overall quality of grapes. 4.3 Integrative omics approaches in grapevine breeding for optimized traits The approach of integrative omics can provide a more comprehensive view of which genes, metabolites and regulatory factors are interrelated. Studies have shown that the integrated network established in this way has discovered clear synergy relationships among different types of data. Savoi et al. (2017) found that some genes and transcription factors involved in flavonoid synthesis would be jointly regulated under conditions of insufficient water, affecting the sugar accumulation and size of fruits. Wong and Matus (2017) demonstrated that the combined analysis of gene co-expression networks and promoter regulatory elements identified new transcription factors and microRNAs involved in the regulation of key metabolic pathways. 5 Case Study 5.1 Selection criteria for case study The genes in the case study must have been confirmed to have specific functions in the physiological processes of grapes, especially in controlling fruit size and sugar accumulation. These genes need to be verified through multiple experimental methods to ensure a comprehensive and reliable understanding of their functions. Only when they have a significant impact on both fruit size and sugar content do they have practical value as key candidate genes in breeding projects. 5.2 Detailed analysis of a key gene (VvGH9) VvGH9 belongs to the glycosyl hydrolase family. These enzymes can break down polysaccharides into monosaccharides and are key participants in carbohydrate metabolism. Xu et al. (2021) found that the expression level of VvGH9 was very high at the initial stage of grape fruit development, while the sugar content of the fruit at this stage was still relatively low (Figure 2). The results of RNA sequencing and real-time quantitative PCR indicated that the expression of VvGH9 was activated by exogenous sugars such as sucrose, fructose, and glucose, suggesting that it might be crucial in the sensing and metabolic regulation of sugars. Najafi et al. (2022) demonstrated that when VvGH9 was overexpressed in grape callus and tomato models, it could significantly increase sugar accumulation even in a low-glucose environment, indicating its potential to enhance the sweetness and quality of the fruit. The expression characteristics of VvGH9 and its response to sugar stimulation also suggest that it may be useful in the process of regulating sugar input and metabolic balance during berry development. Lecourieux et al.’s research in 2014 suggested that this balance is crucial to the final size and sugar content of the fruit, which in turn are the determinants of grapevines quality and market price.
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