International Journal of Horticulture, 2025, Vol.15, No.1, 8-20 http://hortherbpublisher.com/index.php/ijh 15 making it a popular choice in both fresh and processed forms (Constantino et al., 2021). The ability to maintain quality attributes through treatments like methyl jasmonate further enhances the marketability of pitaya by preserving its taste and nutritional benefits during storage and processing (Li et al., 2018). 7 Case Studies 7.1 Molecular regulatory mechanisms of sugar accumulation in pitaya The sweetness of pitaya fruit is primarily determined by the content of soluble sugars, including glucose, fructose, and sucrose. The metabolism and accumulation of sugars are regulated by various enzymes and their corresponding genes, such as sucrose synthase (SuSy) and invertase (INV). Studies have shown that transcription factors play a critical role in regulating the expression of sugar metabolism-related genes (Wei et al., 2019; Mou et al., 2022). Mou et al. (2022), through qPCR, dual-luciferase reporter assays, and gel mobility shift assays, revealed the functions of two Dof transcription factors in pitaya, HpDof1.7 and HpDof5.4, providing important insights into the regulatory mechanisms of sugar metabolism. These transcription factors bind to the promoters of sugar metabolism genes (HpSuSy1 and HpINV2) and sugar transporter genes (HpTMT2 and HpSWEET14), activating their expression and promoting the synthesis and accumulation of glucose and fructose (Figure 3). The results demonstrate a significant positive correlation between the expression of HpDof1.7 and HpDof5.4 and sugar accumulation during pitaya maturation, providing a theoretical foundation for enhancing fruit sweetness through molecular breeding. 7.2 Molecular mechanism of citramalic acid metabolism in pitaya Organic acids in pitaya fruit not only influence flavor but also play a critical role in its nutritional value and market acceptance. As an unconventional organic acid, citramalic acid dominates during the early stages of fruit development. A study focusing on four pitaya cultivars—'GHB,' 'GHH,' 'WCHL,' and 'YCHL'—explored the metabolic mechanism of citramalic acid and the roles of key associated genes through metabolomics, gene expression analysis, and subcellular localization experiments (Chen et al., 2022). The results showed that citramalic acid content peaked during the early fruit development stages (S3–S4) and gradually decreased thereafter. The expression of the HuIPMS2 gene was highly correlated with citramalic acid levels, and its encoded protein was localized to the chloroplast. It is hypothesized that HuIPMS2 regulates the citramalic acid biosynthesis pathway, thereby influencing the early fruit flavor. Pitaya cultivars 'WCHL' and 'GHB' exhibited higher citramalic acid accumulation, consistent with the expression trend of HuIPMS2, suggesting that this gene may impact fruit flavor by modulating citramalic acid synthesis. 7.3 Research on enhancing pitaya flavor by optimizing sugar-acid balance Yellow-peel pitaya is highly favored by consumers due to their unique appearance, sweet flavor, and rich nutritional content. A study focusing on two yellow-peel pitaya species, ‘WCHL’ (H. undatus) and ‘YCHL’ (H. megalanthus), utilized metabolite profiling and transcriptomic analyses to uncover the changes in sugars and organic acids during fruit development and their molecular regulatory mechanisms (Xie et al., 2022). The findings revealed that the dominant flavor components in ‘WCHL’ pitaya are glucose and malic acid, whereas sucrose, fructose, and citric acid predominate in ‘YCHL’. Starch accumulation was prominent during the early developmental stages, transitioning into soluble sugars during maturation. Total phenols and flavonoids were abundant in the early stages, while ascorbic acid levels were notably higher in ‘WCHL’ (Figure 4). Transcriptomic analyses identified 27 key genes, such as PMI1 and PMI3, associated with ascorbic acid biosynthesis, and 18 critical genes, including SuSy and FRK, involved in sugar metabolism. Additionally, the expression of key genes related to the TCA cycle, such as PEPC3 and CMS1, significantly impacted organic acid metabolism.
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