A: Apópiasm C: Gytcpiasm V: Vacuole
G: Growth E; Energy
Hex: Hexoses Sue: Sucrose
IN : Invertase case that label had been given in glucose only. Prior cleavage will not be recognised in that case. On the other side, the intracellular sucrose cycling will lead to label randomization if sufficient time is allowed, even if the uptake of sucrose had taken place via the uncleaved molecule.
The experiments in Fig. 5 represent the uptake activities of tissue slices incubated with defined hexose or sucrose concentrations. In the undisturbed internodal tissue the apoplastic concentrations of sugars depend on the maturity of the internode (19). In young, immature and in ripening internodes the hexose concentration is up to half of the concentration of sucrose, therefore at these apoplastic sugar concentrations the hexose uptake rates, calculated from the in vitro activities, are expected to be much higher than sucrose uptake. In old internodes the hexose concentrations decline to very low values, whereas the sucrose concentration increases, therefore in mature tissue sucrose transport is higher than hexose transport (Fig.4). Therefore sucrose permeation from the apoplast becomes more and more important with aging of the internodes. As indicated in Fig.4, there is not only a metabolic cycling of sucrose within the symplast, but also a rapid transport cycle of sugars through plasmalemma and through the tonoplast.
A working model for sugar transport in internode parenchyma could be the following:
In young, growing internodes the apoplastic barrier around the bundle sheath may not be fully developped yet, so that partial apoplastic phloem unloading may occurr. In addition growing tissues are usually characterized by high extracellular acid invertase activity, so that active transport systems, mainly hexose transport systems, adsorb as much sugar as available for growth and cell expansion. Sucrose transport per se would be negligible in that situation. As the internode ripens, phloem unloading through the bundle sheath becomes exclusively symplastic. Hexose transporter may still function some time as a retrieval system, but with age the active transport activities decline and the "linear phase" of uptake becomes more and more prominent. If that "linear phase" represents a passive, equilibrating transport system, its major net transport direction will be from the symplast to the apoplast. As consequence an apoplastic concentration of sugars, especially of sucrose, nearly as high as in the symplast will build up, with the result of low turgor in the storage cells and further promotion of symplastic bulk flow of solution into the storage tissue. (The high apoplastic sugar concentrations in mature internode tissue do not result from a general leakiness of the parenchyma cell plasmalemma,
Fig.5: Sugar uptake by tissue slices of sugarcane internodes #0 (top) and #13 (bottom). Tissue slices had been incubated in labeled glucose or fructose or sucrose at different concentrations and the uptake rate was determined within the first hour.
sugar concentration (mM)
sugar concentration (mM)
since removal of the apoplastic solutes by washing increases immediately and persistently the cell turgor (11).
Meanwhile sugar transporter genes had been cloned from various plant species and plant organs, regrettably nearly none from sugarcane, yet (except a partial clone for a hexose transporter and a clone for sucrose transporter from leaves). The so far functionally identified hexose carrier and sucrose carrier genes from plants all code for active transport systems, no equilibrating, passive sugar transporter has been cloned from plants yet. It would be most rewardful to isolate sugar carrier genes from sugarcane stalk and to look for passive sugar transporters.
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