Journal of Energy Bioscience 2025, Vol.16, No.5, 248-262 http://bioscipublisher.com/index.php/jeb 249 Manipulating sugar metabolism offers promising strategies to boost agricultural productivity, enhance crop quality, and increase stress tolerance (Patrick et al., 2013; Julius et al., 2017; Ji et al., 2022; Nägele et al., 2022). Emerging gene-editing tools enable precise modulation of sugar transporters such as SWEET and SUT family proteins, thereby optimizing carbohydrate allocation and storage (Breia et al., 2021; Gautam et al., 2022). Moreover, efficient conversion and utilization of sugars position plants as vital feedstocks for bioenergy and biobased materials (Tang et al., 2023). This study reviews recent research on sugar metabolism in plants. It focuses on: (1) sugar production and breakdown; (2) the role of sugars in energy and structure; (3) sugar metabolism under environmental and nutrient conditions; and (4) applications of sugar metabolism engineering in crop improvement and the bioeconomy. Finally, it points out research gaps and looks ahead to future directions. 2 Types and Functions of Plant Sugar 2.1 Classification: monosaccharides, disaccharides and polysaccharides Monosaccharides are the simplest sugars, such as glucose, fructose and galactose. They are soluble in water and can provide energy directly or participate in the synthesis of other substances (Qi and Tester, 2019; Niyigaba et al., 2021; Zhu et al., 2024). Disaccharides such as sucrose, maltose and lactose are composed of two monosaccharides. Sucrose is the most common. After synthesis in leaves, it is transported to roots and fruits and then decomposed into glucose and fructose for energy supply (Koch, 2004; Liu et al., 2025b). Polysaccharides are composed of many monosaccharides. Starch and cellulose are homopolysaccharides, while pectin and hemicellulose are heteropolysaccharides. They are respectively responsible for energy storage and structural support (Mohammed et al., 2021). 2.2 Main sugars: glucose, sucrose, fructose and starch Glucose is produced during photosynthesis, providing energy for cells and also serving as a carbon source for the synthesis of molecules such as amino acids (Couee et al., 2006; Qi and Tester, 2019). Fructose often coexists with glucose and is also a component of sucrose. Sucrose is synthesized in leaves and transported through the phloem to roots, fruits and seeds, where it is decomposed for energy supply or storage (Koch, 2004; Gautam et al., 2022; Liu et al., 2025b). Starch is composed of amylose and amylopectin and is the main energy storage substance. Synthesis during the day and decomposition for energy supply at night to maintain energy balance (Dong and Beckles, 2019; Cho and Kang, 2020). 2.3 Functions and roles: energy supply, osmotic regulation and stress response Glucose and sucrose produce ATP during respiration to provide energy for growth (Couee et al., 2006; Qi and Tester, 2019). The transport of sugar is accomplished by proteins such as SWEET, SUT and MST, ensuring normal nutrient distribution (Koch, 2004; Saddhe et al., 2020; Gautam et al., 2022). Under drought, high temperature or salt stress, plants accumulate soluble sugars, such as sucrose and trehalose, to retain moisture, stabilize membrane structure and reduce oxidative damage (Couee et al., 2006; Afzal et al., 2021; Nagele et al., 2022). In addition, sugar regulates gene expression and hormone balance, and participates in growth and stress response together with hormones such as abscisic acid (Liu et al., 2025b). 3 The molecular mechanism of carbohydrate biosynthesis 3.1 Photosynthesis and carbohydrate production In chloroplasts, light energy is used to produce ATP and NADPH. These two molecules enter the Calvin cycle, converting carbon dioxide (CO2) into a small sugar molecule called 3-phosphoglyceraldehyde (GAP). After GAP,
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