International Journal of Horticulture, 2025, Vol.15, No.1, 8-20 http://hortherbpublisher.com/index.php/ijh 10 2.2 Sugar metabolism pathways and key enzymes The metabolism of sugars in pitaya involves several biochemical pathways and key enzymes that regulate the synthesis, breakdown, and transport of sugars. Key enzymes such as sucrose synthase (HpSuSy1) and invertase (HpINV2) play pivotal roles in the hydrolysis of sucrose into glucose and fructose, which are the predominant sugars in pitaya (Wei et al., 2019; Luo et al., 2021). The expression of these enzymes is regulated by transcription factors like HpWRKY3, which activates the transcription of HpSuSy1 and HpINV2, thereby enhancing sugar accumulation during fruit maturation. The SWEET family of sugar transporters, particularly HuSWEET12a and HuSWEET13d, are crucial for sugar transport and accumulation in pitaya fruits. These transporters facilitate the movement of sugars across cell membranes, contributing to the overall sugar content in the fruit (Jiang et al., 2023). The regulatory mechanisms involving Dof transcription factors (HpDof1.7 and HpDof5.4) further modulate the expression of sugar metabolism-related genes, enhancing the activities of HpSuSy1, HpINV2, and sugar transporter genes like HpTMT2 andHpSWEET14 (Mou et al., 2022; Zou and Sun, 2023). 2.3 Influence of sugar metabolism on flavor and sweetness The metabolism of sugars in pitaya significantly impacts its flavor and sweetness, which are critical determinants of fruit quality and consumer preference (Gou et al., 2023). The accumulation of glucose and fructose, facilitated by the activities of HpSuSy1 and HpINV2, directly correlates with the sweetness of the fruit (Wei et al., 2019; Wang et al., 2024). The expression of HuSWEET12a and HuSWEET13d during fruit maturation also contributes to the increased sugar content, enhancing the sweetness and overall flavor profile of pitaya (Xie et al., 2022). Furthermore, the regulatory role of Dof transcription factors in activating sugar metabolic pathway genes underscores the complex interplay between sugar metabolism and flavor development in pitaya (Mou et al., 2022). The balance between different sugars, such as the higher sucrose content in ‘YCHL’ pitaya and the dominant glucose in ‘WCHL’ pitaya, also influences the perceived sweetness and flavor nuances of different pitaya varieties. Thus, understanding the biochemical pathways and regulatory mechanisms of sugar metabolism is essential for improving the flavor and nutritional quality of pitaya fruits. 3 Organic Acid Metabolism in Pitaya 3.1 Types and content of organic acids in pitaya Pitaya fruits contain several organic acids that significantly contribute to their flavor profile. The primary organic acids identified in various pitaya species include malic acid and citric acid. For instance, Hylocereus costaricensis, H. undatus, and H. megalanthus have been reported to contain malic acid concentrations of 0.83%, 0.71%, and 0.62%, respectively, and citric acid concentrations of 0.37%, 0.36%, and 0.40%, respectively (Constantino et al., 2021). Additionally, ascorbic acid (vitamin C) content varies among different pitaya species, with some studies noting its presence in significant amounts during certain developmental stages (Xie et al., 2022). The content of organic acids in pitaya can vary significantly across different varieties and developmental stages. For example, ‘Wucihuanglong’ (Hylocereus undatus) and ‘Youcihuanglong’ (Hylocereus megalanthus) pitayas exhibit different profiles of organic acid accumulation during fruit maturation. 'Wucihuanglong' pitaya has higher ascorbic acid content compared to ‘Youcihuanglong’ pitaya (Xie et al., 2022). Furthermore, the concentration of malic and citric acids also differs, with ‘Wucihuanglong’ primarily accumulating malic acid and ‘Youcihuanglong’ accumulating citric acid. This variability is crucial for breeding programs aimed at enhancing specific flavor profiles and nutritional qualities. 3.2 Enzymatic pathways of organic acid metabolism The metabolism of organic acids in pitaya involves several key enzymes that regulate their synthesis and degradation. Enzymes such as malate dehydrogenase and citrate synthase play pivotal roles in the tricarboxylic acid (TCA) cycle, which is central to organic acid metabolism. Transcriptome analyses have identified numerous
RkJQdWJsaXNoZXIy MjQ4ODYzNA==