International Journal of Horticulture, 2025, Vol.15, No.5, 218-233 http://hortherbpublisher.com/index.php/ijh 220 al., 2022). A variety of SUT and SWEET genes have been identified in sugarcane, among which SWEET13c is mainly highly expressed in mature mesophyll cells, promoting the export of photosynthetic product sucrose to the phloem, while SWEET4a/4b is mainly expressed in stem tissues, participating in the unloading of sucrose from sieve tubes to surrounding storage cells. On the "sink" side, sugarcane proteins such as SUT1 and SUT4 are located on the sieve tube companion cells and parenchyma cell membranes of stem tissues, responsible for absorbing sucrose from the intercellular space into the cells. In addition to plasma membrane transport, stem medullary parenchyma cells also pump sucrose into the vacuole for storage through the sucrose/H+ antiporter on the vacuole membrane. For example, Arabidopsis vacuolar sugar transporter TMT1/2 can actively transport glucose and sucrose into the vacuole, and there is a highly homologous Tonoplast Sugar Transporter (TST) family in sugarcane that performs similar functions. These transport processes increase the sucrose concentration in the vacuole of sugarcane stem cells, which can be as high as more than 50% of the dry weight, which is the source of the high sweetness of sugarcane (Mehdi et al., 2024b). It should be pointed out that the accumulation of sucrose in the vacuole will increase the osmotic pressure of the cell. In order to maintain balance, sugarcane often increases the storage of reducing sugars such as glucose and fructose in the vacuole. Studies have found that transgenic sugarcane with increased sucrose synthase SPS activity also increases the acid invertase activity in its leaves, and the glucose and fructose content increases. This may be a compensatory mechanism: plants adjust excessive sucrose concentrations by moderately increasing sucrose decomposition to avoid feedback inhibition of photosynthesis and osmotic stress. Therefore, the long-distance transport and intracellular storage of sugars is a dynamic equilibrium process, and the sugar signal sensing and transporter expression of both the source and the sink are precisely regulated to achieve effective redistribution and efficient accumulation of sucrose in sugarcane. 2.3 Dynamic balance of sucrose, glucose, and fructose in different maturation stages The ratio of sucrose, glucose and fructose in sugarcane stalks changes significantly from the elongation stage to the maturity stage. In tender stems and elongation tissues, the invertase activity is high, and sucrose is quickly decomposed and utilized, so the content of reducing sugars (glucose, fructose) is relatively higher and the sucrose concentration is lower. As internodes mature, photosynthate input increases and stem tissue growth slows down, the activity of synthases such as SPS and SuSy reaches a peak at the mature stage, while the activity of acidic and neutral invertases decreases significantly, causing sucrose to accumulate in large quantities in mature stem cells. Especially in the late stage of sugarcane growth, the decrease in the activity of neutral invertase (NI) in the stem is considered to be one of the main reasons for the high accumulation of sucrose (Khan et al., 2021; Martins et al., 2024). The distribution pattern of neutral invertase activity in mature sugarcane internodes is closely related to the sugar gradient - in the mature zone at the base of the internode, lower NI activity is accompanied by higher sucrose concentration, while in the tender zone at the top of the internode, higher NI activity leads to an increase in the proportion of reducing sugar. Recent experimental data also support this dynamic: the high-sugar variety GT35 has a much higher sucrose content in mature stem segments than the low-sugar variety B8, while the glucose and fructose contents are lower. Correspondingly, the activities of SPS and cell wall invertase (CIN) in mature stem segments of GT35 are significantly higher than those of B8, while the activities of invertase and SuSy decomposition are lower. This indicates that in the late stage of sugarcane stalk development, the enzymes that synthesize sucrose maintain high activity, while the enzymes that decompose sucrose are feedback inhibited, allowing sucrose to accumulate continuously in the cells. In addition, the source-sink signal regulation of different varieties is also different: heat-resistant varieties can maintain high SPS and SuSy expression at high temperatures, so that sucrose can still accumulate in the late growth period, while sensitive varieties have a sharp drop in enzyme activity under heat stress, resulting in blocked sugar accumulation (Mehdi et al., 2023). It can be seen that the dynamic balance of sucrose, glucose and fructose in sugarcane stems depends on the coordinated changes of multiple enzymes under developmental stages and environmental conditions. The large accumulation of sucrose in the mature stage is the result of the combined effects of multiple metabolic regulations.
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