Frehner and coworkers (38) proposed that large central vacuole in the fructan storing cells appears to function like a sponge able to absorb large quantities of water and storage carbohydrates or water or both. This property of the vacuole depends on the water demand that can be withdrawn most elegantly without affecting the water potential of cells simply playing with DP of vacuolar fructans in order to control its amount and osmotic potential. To achieve this, the activities of the vacuolar hydrolases and transferases as well as cytosolic sucrose synthesis must cooperate (38).
1-FFT redistributes fructose residues from high DP inulin polymers to low DP oligomers by transferring fructosyl residues on to sucrose (formed from the fructose released by FEH) without accumulation of free fructose (37, 64). Such changes in the average DP of the inulin oligosaccharide were well reflected in HPAE elution chromatography of water soluble carbohydrate extract from developing and sprouting tubers of H. tuberosus.
The tubers of developing phase contained inulin polymers up to DP 50 whereas the tubers of sprouting period had increased concentration of low molecular weight oligosaccharides apparently at the expense of high molecular weight polymers.
In sprouting tubers the sucrose concentration was still high and would be expected to inhibit FEH. In dormant tubers about 80% of the sucrose together with FEH is located in the vacuole (38). However, when tubers are sprouting, sucrose is exported to support the growth of the developing shoot. In the cells of storage parenchyma, sucrose is transferred from vacuole to compartments dedicated to export (cytoplasm of parenchyma cells of vascular bundles). As a consequence, sucrose concentration in the vacuole in vivo might be much lower than that of a tissue extract. Hence the in vivo activity of FEH in sprouting tubers may rise because (a) it is less inhibited than in dormant tubers and (b) higher ambient temperatures allow for higher metabolic rates (83).
Sucrose appears to play a dual role in fructan synthesis. GF2 synthesis by SST is dependent on availability of sucrose. Therefore, high sucrose concentration indirectly favours the rate of first polymerization step by FFT and the synthesis of GF3 from GF2 by elevating GF2 concentrations. It is possible that the equilibrium concentrations of sucrose and G-Fn at the site of fructan synthesis reflect the optimal conditions for the collective action of SST and FFT at a given sucrose availability (49).
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