Table 2. Functions of key enzymes in polysaccharide synthesis.

Key enzyme	Role in carbohydrate synthesis	Other physiological functions	Influencing factors of activity	Reference
Sucrose synthase (SUS)	It catalyzes the decomposition and synthesis of sucrose, that is, sucrose is decomposed into UDP-glucose and fructose in the presence of UDP, and its reverse reaction	Sucrose is transported to multiple pathways to provide the precursor UDP-glucose for the biosynthesis of cell wall polymers and starch; It helps the growth, development and metabolism of sink organs, and help plants adapt to abiotic stress environments such as hypoxia and cold	Fructose and UDPG inhibited degradation activity, UDP inhibited synthesis activity, and glucose inhibited synthesis and degradation; inhibited by Zn2+, Hg2+, Gu2+, Fe2+, Ni2+ and Co2+	Sheng et al. (2023)
Sucrose invertase (INV)	Irreversibly catalyze the hydrolysis of sucrose into glucose and fructose	It regulates the distribution and utilization of assimilates in the sink organ, for example, in the sink tissue, the cell wall sucrose invertase hydrolyzes sucrose into hexose, and the hexoglycoprotein transports hexose into the cells of the sink tissue, thereby reducing the sucrose concentration outside the sink tissue and driving the extracellular phloem of the sink organ to unload; delaying leaf senescence; early defense in disease resistance and plant-symbiont system	By transcription, translation, post-translational modification; induced by low temperature, stress and maturation	Coculo & Lionetti (2022)
Sucrose phosphateSynthase (SPS)	SPS catalyzes the conversion of UDPG and fructose-6-phosphate (F6P) to sucrose-6-phosphate (S6P) in plants. In the ripening stage of fruit, the expression and activity of SPS were up-regulated, which promoted the synthesis of sucrose and increased the sweetness of fruit	By regulating sucrose synthesis to cope with cold, drought, water shortage and other environmental stress pressure	Phosphate and sucrose-6-phosphate are SPS inhibitors. Fructose-6-phosphate and 1,5-anhydroglucitol-6-phosphate could activate SPS activity. Histidine participates in the catalytic reaction; under the regulation of post-translational modification, SPS is phosphorylated in the dark and its activity is inhibited. Under light, SPS is phosphorylated and its activity is restored, indicating that phosphorylation and dephosphorylation can regulate the activity of SPS in plants	Liao et al. (2022)
Hexokinase (HXK)	Phosphorylation of several hexoses, including d-glucose (Glc), d-fructose (Fru), d-mannose (Man) and d-galactose (Gal)	as a hexose sensor, it plays a variety of roles in regulating plant growth, sugar sensors, regulating sugar signal transduction, and cooperating with plant hormones	Glucose excess can cause HXK sugar sensing pathway, T6P pathway and rapamycin (TOR) kinase target pathway to play a role.	Dou et al. (2022)
Fructokinase (FRK)	Phosphorylation of free fructose with high substrate specificity and affinity	It is specifically expressed in the anther during the late stage of pollen development and pollen germination, and regulates the acquisition of carbohydrates required for cell wall synthesis during pollen development; plays a role in long-term developmental processes in vascular development.	Only regulated by its own substrate fructose	Fan et al. (2022)
UDP-glucose pyrophosphorylase (UGPase)	UDPG was synthesized by the reaction of glucose 1-phosphate with UTP	Involved in carbohydrate metabolism, cell wall biosynthesis and protein glycosylation metabolism; regulating plant cell apoptosis in chloroplasts	It is regulated by Suc (the major transport form of carbon in plants). It was strongly up-regulated by low temperature and down-regulated by drought and flood conditions.	Xu et al. (2022)
Glycosyl transferases (GTs)	Catalytically activated glycosyl donors are transferred to specific receptor molecules to form glycosidic bonds	The glycosylation process can increase the polarity and water solubility of antibiotics, so that they can reach the ideal effective concentration inside or outside the cell, and the presence of glycosylation can also enhance the chemical stability of antibiotics. Glycosylation can specifically recognize biological targets and play a key role in bacteriostasis.	N-glycosylation strongly affects GTs activity and Golgi localization.	Kurze et al. (2022)