International Journal of Horticulture, 2025, Vol.15, No.5, 218-233 http://hortherbpublisher.com/index.php/ijh 219 (such as SuSy, SPS, invertase), sugar transport systems (such as SWEET, SUT), cell wall polysaccharide structure and modification enzymes (such as expansin, XTH, pectinase), etc., focusing on the gene expression regulation mechanism affecting sweetness and texture formation, including the spatiotemporal expression pattern of key genes, the role of transcription factors such as NAC/MYB/bZIP, and the regulatory role of plant hormones and environmental factors (such as light and temperature). Finally, through comparative studies among typical varieties, attempts are made to reveal the molecular basis of the formation of different types of sugarcane quality traits from the perspective of transcriptome, in order to provide theoretical support for molecular breeding and precise improvement of high-quality fresh sugarcane. 2 Sugar Metabolism and Accumulation 2.1 Key enzymes: roles of SuSy, SPS, and invertases in sucrose synthesis and breakdown Sugarcane is famous for its rich sucrose content in its stems, and its sweetness essentially depends on the high concentration of sucrose accumulated in the pith cells of the stems. This process is strictly regulated by a series of enzymatic reactions, among which sucrose phosphate synthase (SPS), sucrose synthase (SuSy), and invertase are the key enzymes for sucrose synthesis and decomposition. SPS catalyzes the synthesis of uridine diphosphate glucose (UDP-Glc) and fructose-6-phosphate produced by photosynthesis into sucrose-6-phosphate, which is then dephosphorylated by sucrose-6-phosphate phosphatase to produce sucrose. It is the rate-limiting enzyme in the sucrose biosynthesis pathway (Khan et al., 2023). Studies have shown that the SPS enzyme activity level in different sugarcane varieties is significantly positively correlated with their final sucrose content, and high-sucrose varieties tend to show stronger SPS activity. SuSy can reversibly catalyze between sucrose and UDP-Glc. On the one hand, it works with SPS in the direction of sucrose synthesis, and on the other hand, it provides substrates for cellular respiration and the construction of polysaccharides such as cellulose in the direction of sucrose decomposition. There are usually multiple SuSy isoenzymes in crops such as sugarcane, with spatiotemporal expression patterns to adapt to the needs of sucrose synthesis or decomposition at different developmental stages. In contrast, invertases are responsible for irreversibly hydrolyzing sucrose into glucose and fructose, including acid invertases in the vacuole and neutral invertases in the cytoplasm. High invertase activity often means that sucrose is continuously broken down into reducing sugars, which provides energy and carbon skeletons in rapidly growing young tissues, but excessive invertase activity reduces the net accumulation rate of sucrose in the stem (Mehdi et al., 2024b). Comparative tests of high-sugar and low-sugar sugarcane varieties confirmed this difference in key enzyme activity: the SPS activity in the stems of high-sugar varieties (such as GT35) was significantly higher than that of low-sugar varieties, while the invertase activity was relatively low, so the sucrose content in the stem cells was significantly higher than that of low-sugar varieties. On the contrary, in low-sugar varieties, neutral and acid invertase activities were stronger, resulting in more sucrose being decomposed into glucose and fructose, and sucrose accumulation in the stems was limited. This shows that the efficient expression of synthases such as SuSy and SPS and the moderate decrease in invertase activity are conducive to the accumulation of sucrose in sugarcane stems (Niu et al., 2019). On the other hand, the sucrose decomposition enzyme system cannot be too low, otherwise the stem cells will lack sufficient respiratory substrates and metabolic regulation, which will inhibit plant growth. Therefore, sugarcane achieves a carbon distribution balance between the source (leaf photosynthetic products) and the sink (stem sugar storage) by finely regulating the expression of sucrose synthesis and decomposition enzymes, so as to meet growth needs and maximize sugar storage (Mehdi et al., 2024a). 2.2 Sugar transport and storage: function of SWEET, SUT, and tonoplast transporters The sucrose produced by photosynthesis needs to be transported through sieve tube assimilates to reach "sink" organs such as stems and stored in intracellular vacuoles, which is an important mechanism for the accumulation of high sugar content in sugarcane. In the long-distance transport of sucrose, SWEET and SUT family transporters play a key role. SWEET is a class of sucrose transmembrane export proteins that mediate the release of sucrose from source tissue cells into the intercellular space, and then is actively loaded by sucrose transporters (SUTs) in phloem companion cells and sieve tubes, thereby realizing the transport of sucrose from leaves to stems (Chen et
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