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Fig. 8 | BMC Plant Biology

Fig. 8

From: Systematic analysis of the sugar accumulation mechanism in sucrose- and hexose- accumulating cherry tomato fruits

Fig. 8

Schematic model of sugar accumulation of two type cherry tomato fruits. Sucrose diffuses into CCs from SEs through plasmodesmata during phloem unloading. In the hexose-accumulating cherry tomato fruit (upper), LeSUT1 located in SEs exports sucrose to the apoplast or retrieves leaked sucrose from the apoplast to SEs is not clear here. Sucrose is transported into the apoplast from SE/CCs maybe by LeSUT1 and SlSWEET12c, and then, it is broken down to glucose and fructose by acid invertase (AI). Small amount of sucrose in SE/CCs is broken down to hexose via SS, and the hexoses are used to resynthesize sucrose under the action of SPS. The hexoses may efflux from SE/CCs by SlSWEET2a. In the sucrose-accumulating cherry tomato fruit (lower), LeSUT2 located in SEs may be responsible for retrieving the leaked sucrose from the phloem apoplast space to SEs. Hydrolysis and synthesis of sucrose via SS and SPS still occur in fruit phloem SE/CCs. The hexose and sucrose are transported to PCs through plasmodesmata, or firstly transported to the apoplast space via SlSWEET1b, SlSWEET5b, SlSWEET11b, SlSWEET7a and SlSWEET14, and then transported into PCs by hexose transporters and potential sucrose transporters. Abbreviations: SE, sieve element; CC, companion cell; PC, phloem parenchyma cell; PD, plasmodesmata; AI, acid invertase; SS, sucrose synthase; SPS, sucrose phosphate synthase; H-type, hexose-accumulating cherry tomato; S-type, sucrose-accumulating cherry tomato

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