Journal of Energy Bioscience 2025, Vol.16, No.5, 248-262 http://bioscipublisher.com/index.php/jeb 250 larger sugars can be synthesized, such as glucose. Some of the glucose remains in the chloroplast to form starch, while the other part is transported to the cytoplasm to synthesize sucrose (Johnson, 2016). The thioredoxin system in chloroplasts helps plants maintain a balance among light energy utilization, energy generation and sugar synthesis, enabling plants to remain stable under different light and stress conditions (Nikkanen and Rintamaki, 2019; Chauhan et al., 2023). Photosynthesis and the utilization of sugar are related to the growth, environmental adaptation and yield of plants. 3.2 Synthetic pathways of sucrose and starch The carbon produced by photosynthesis is used to synthesize sucrose and starch. Sucrose is synthesized in the cytoplasm. The three-carbon phosphate generated by chloroplasts is acted by various enzymes, such as aldolase, fructose-1, 6-diphosphatase and sucrose phosphatase synthase (SPS), and eventually sucrose is produced and transported to other tissues through the phloem (Stein and Granot, 2019; Wang et al., 2022b). Starch synthesis occurs in plastids. The enzyme ADP-glucose pyrophosphorylase (AGPase) catalyzes the formation of ADP-glucose from glucose-1-phosphate and ATP. Then, starch synthase extends the sugar chain, and branched enzymes generate amylose and amylopectin. Studies have shown that sucrose synthase (SuSy) can also generate ADP-glucose in different tissues, indicating that there are multiple pathways for starch synthesis (Baroja-Fernandez et al., 2003; Munoz et al., 2005; Li et al., 2013; Qu et al., 2018) (Figure 1). The ratio of sucrose to starch varies with the growth stage of the plant, the environment and energy requirements. Figure 1 Structural features and active site analysis of SS isoforms. Maize is shown as an example. (A) Compositions and distributions of domain structures and conserved motifs of SS proteins are marked and annotated in different colours. (B) Stereo view of the active sites of SS isoforms based on the sequence of SSI and (C) GBSS isoforms based on the sequence of GBSSI. The same site with different amino acids is marked with dots in SS isoforms. Interaction sites between SSs and ADP are shown as linked broken green lines. Interaction sites between SSSs and glucose are marked in pink. Red stars and lines shown in light pink represent catalytic sites. Amino acid sites that interact with maltopentaose are marked in blue, and these active sites are not conserved in SSIII, SSIV and SSV. Additionally, disulfide bonds were found only in SSI and GBSSI and are marked with orange stars in SSSs and as blue amino acids in GBSSI (Adopted from Qu et al., 2018) 3.3 The role of key enzymes Sucrose phosphosynthase (SPS) is responsible for transferring glucose from UDP-glucose to fructose-6-phosphate to form sucralose-6-phosphate, which subsequently forms sucrose. Its activity is regulated by signaling and phosphorylation (Rocher et al., 1989; Wang et al., 2022b).
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