Sucrose Metabolism

Abstract

Sucrose is one of the main products of photosynthesis in plants, and the most common form of carbohydrate transported from source to sink organs. It also functions as a storage reserve, compatible solute and signal metabolite in plants. Sucrose is synthesised via the phosphorylated intermediate sucrose‐6′‐phosphate, by sucrose‐phosphate synthase (SPS) and sucrose‐phosphatase (SPP). In sink organs, sucrose is broken down by invertase or sucrose synthase to provide carbon and energy for growth and accumulation of storage reserves, such as starch, oil and fructans. Sucrose and trehalose are the only common nonreducing disaccharides found in nature, and their metabolism is inextricably linked in plants. Trehalose‐6‐phosphate, the intermediate of trehalose synthesis, is a signal of sucrose availability in plant cells, regulating photoassimilate partitioning in leaves and the utilisation of sucrose in sink organs.

Key Concepts

  • Sucrose is one of the main products of photosynthesis and the most common transport sugar in plants.
  • Sucrose is a nonreducing disaccharide and is synthesised in the cytosol via the phosphorylated intermediate, sucrose‐6′‐phosphate.
  • In leaves, the rate of sucrose synthesis is tightly coordinated with the rates of photosynthetic carbon dioxide fixation and starch synthesis in the chloroplasts.
  • Sucrose is transported from leaves via the phloem, to provide the rest of the plant with carbon and energy for growth and storage product synthesis.
  • Sucrose is unloaded from the phloem in sink organs. It can be hydrolysed by cell wall invertases and imported into the cells as hexose sugars, or taken up intact and metabolised by intracellular invertases or sucrose synthase.
  • Fructans are soluble polymers of fructose found in about 15% of all flowering plants. Dicotyledonous plants generally make inulin‐type fructans, whereas monocotyledonous plants also make levan, inulin neoseries, levan neoseries and mixed levan (graminan)‐type fructans.
  • Fructans are synthesised from sucrose via trisaccharide intermediates – 1‐kestose, 6‐kestose or 6G‐kestose – by various fructosyltransferases in the vacuole.
  • Trehalose is a nonreducing disaccharide found in many bacteria, archaea, invertebrates and fungi, and is often used as a stress protectant, compatible solute, storage reserve or transport sugar.
  • Trehalose metabolism is ubiquitous in plants, and essential for their normal growth and development, but most flowering plants accumulate only trace amounts of trehalose.
  • Trehalose‐6‐phosphate, the intermediate of trehalose synthesis, is a signal of sucrose availability in plants, regulating photoassimilate partitioning in leaves and the utilisation of sucrose in sink organs.

Keywords: fructan; photosynthesis; starch; sucrose; trehalose; trehalose‐6‐phosphate

Figure 1. Sucrose metabolism in plants. (a) Sucrose is synthesised via the phosphorylated intermediate sucrose‐6′‐phosphate (Suc6P), by the sequential activity of sucrose‐phosphate synthase (SPS) and sucrose‐phosphatase (SPP). The crystal structure of the SPS (top left; PDB accession 2R68) shows Suc6P bound in the active site of the enzyme. The crystal structure of the sp. PCC 6803 SPP (top right; PDB accession 1TJ5) shows hydrolysis of Suc6P to sucrose and Pi and movement of the Mg2+ ion cofactor (green) within the active site. (b) There are two routes of sucrose breakdown in plants, hydrolysis by invertase and cleavage by sucrose synthase.
Figure 2. Domain architecture of sucrose‐phosphate synthase (SPS) and sucrose‐phosphatase (SPP) from bacteria and plants. There are three types of SPS in bacteria. The SPS (Figure) belongs to type (I), which contains only the glucosyltransferase domain (blue), whereas the other two types of SPS have an additional SPP‐like domain (red). This domain contains three motifs associated with phosphatase activity (orange) and is catalytically active in type (II), but not in type (III) SPSs. Species with type (I) or type (III) SPSs have a separate SPP enzyme. Plant SPSs have a noncatalytic SPP‐like domain and an ‐terminal extension (black) containing the light/dark regulatory phosphorylation site (P1). Two other phosphorylation sites involved in 14‐3‐3 protein binding (P2) or osmotic stress activation (P3) are present in the A, B and C families, but not the D family. The latter also lacks the highly variable linker region (yellow) between the glucosyltransferase and SPP‐like domains. The plant SPP has a catalytic domain (red) that closely resembles the sp. PCC 6803 SPP (Figure), with a ‐terminal extension (grey) that might be involved in dimerisation.
Figure 3. Photosynthetic sucrose synthesis. During photosynthesis, carbon dioxide (CO2) is fixed in the chloroplasts via the Calvin–Benson cycle, providing the substrates for starch synthesis in the chloroplasts and sucrose synthesis in the cytosol. Triose‐phosphates are exported from the chloroplast via the triose‐phosphate translocator (TPT), in exchange for Pi. In the cytosol, the triose‐phosphates are equilibrated by triose‐phosphate isomerase (TPI) and then converted in a series of reactions to UDPglucose (UDPGlc) and fructose 6‐phosphate (Fru6P), the substrates for the synthesis of sucrose by sucrose‐phosphate synthase (SPS) and sucrose‐phosphatase (SPP). Most of the sucrose is exported from the leaf via the phloem, but some may be stored in the leaf for metabolism at night. The rate of sucrose synthesis is tightly co‐ordinated with the rates of CO2 fixation and starch synthesis in the chloroplasts by allosteric and posttranslational regulation of the cytosolic fructose‐1,6‐bisphosphatase (FBPase), SPS and ADPglucose pyrophosphorylase (AGPase). Abbreviations: ADPGlc, ADPglucose; Glc1P, glucose 1‐phosphate; Glc6P, glucose 6‐phosphate; PGI, phosphoglucose isomerase; PGM, phosphoglucomutase; RuBP, ribulose‐1,5‐bisphosphate; Suc6P, sucrose‐6′‐phosphate and UGPase, UDPglucose pyrophosphorylase.
Figure 4. Fructan synthesis in plants. Fructans are soluble polymers of fructose that are found in about 15% of all flowering plants. Plants contain five types of fructan, which are all synthesised from sucrose in the vacuole, but vary in conformation, degree of polymerisation and chain branching. One pathway proceeds via synthesis of the trisaccharide 1‐kestose, which contains a fructose residue (shown in green) attached by a β(2,1) glycosidic linkage to the fructosyl moiety of sucrose. Addition of further fructose residues via β(2,1) linkages generates the inulin series of fructans, which are the most common type of fructan in dicotyledonous plants. The trisaccharide, 6G‐kestose, is synthesised from 1‐kestose and contains a fructose residue (shown in red) attached to the glucosyl moiety of sucrose. Addition of fructose residues to 6G‐kestose via β(2,1) or β(2,6) linkages produces the inulin neoseries and levan neoseries of fructans, respectively. The second major pathway of fructan synthesis proceeds via the trisaccharide 6‐kestose, which has the second fructose residue (shown in blue) attached by a β(2,6) linkage to the fructosyl moiety of sucrose. Chain extension via β(2,6) linkages generates the levan series of fructans found in many monocotyledonous plants. The branched, mixed‐levan type of fructan, found in wheat and barley, contains fructose residues attached via β(2,1) and via β(2,6) linkages. The synthesis of this type of fructan is not yet fully resolved, but probably involves 1‐kestose as a precursor. Reactions are catalysed by the following enzymes: (1) 1‐sucrose:sucrose fructosyltransferase (1‐SST); (2) 6‐sucrose:fructan fructosyltransferase (6‐SFT); (3) 6G‐fructan:fructan fructosyltransferase (6G‐FFT) and (4) 1‐fructan:fructan fructosyltransferase (1‐FFT).
Figure 5. Trehalose metabolism in plants. Trehalose and sucrose are the only two common nonreducing disaccharides found in nature. Trehalose is synthesised via the phosphorylated intermediate trehalose‐6‐phosphate (Tre6P), by the sequential activity of trehalose‐phosphate synthase (TPS) and trehalose‐phosphatase (TPP). This pathway closely resembles the synthesis of sucrose (Figure), and the enzymes, TPS and TPP, have many similarities with SPS and SPP. Trehalose is hydrolysed to glucose by trehalase, in a reaction comparable to the hydrolysis of sucrose by invertase.
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Further Reading

Heldt HW and Heldt F (2005) Polysaccharides, Plant Biochemistry, 3rd edn, pp. 243–273. Burlington, CA: Elsevier Academic Press.

Kandler O and Hopf H (1980) Occurrence, metabolism, and function of oligosaccharides. In: Preiss J (ed) The Biochemistry of Plants – A Comprehensive Treatise. Vol. 3 Carbohydrates – Structure and Function, pp. 221–270. New York: Academic Press.

Taiz L and Zeiger E (2010) Translocation in the Phloem, Plant Physiology, 5th edn, pp. 271–303. Sunderland, MA: Sinauer Associates Inc.

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Lunn, John E(Apr 2016) Sucrose Metabolism. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021259.pub2]