Control of intracellular sucrose concentration by soluble acid invertase is plausible, however there is also neutral invertase present in the cells, which catalyses the same reaction. Obviously intracellular compartmentation of sucrose also has to be taken into account. Most of sucrose in storage cells has to reside in the vacuole simply because of the fact that the vacuole comprises 90% of the cells'volume. If the sucrose concentration is the same in cytosol and vacuole (as was found in suspension cells (26), the concentration of intracellular sucrose will be determined mostly by the fate of intravacuolar sucrose and by the intravacuolar enzymes, namely the soluble acid invertase. This will be true, however, only if the permeation of sucrose through the tonoplast is small enough, otherwise sucrose will undergo a rapid cycle of transfer into the cytosol and subsequent hydrolysis by cytosolic enzymes (neutral invertase and sucrose synthase). There is indirect evidence that indeed the tonoplast-residing sucrose transport system may exert an additional, decisive control on sucrose concentration in storage cells: Sugarcane suspension cells accumulate sucrose more than suspension cultures from other plant sources, still, however, their final, intracellular sucrose concentration achieved is far lower than that in storage cells of ripe internodes. Analysis of enzymes of sucrose metabolism revealed definitely no correlation between soluble acid invertase and sucrose concentration (27). If at all a weak correlation may be seen to neutral invertase (Fig. 7). What is the reason for this discrepancy between suspension cells and internode cells? From both sources vacuoles had been isolated and tested for sucrose transport capacity (28-29). In both cases no evidence for energetic coupling to protonmotive potential nor indication of substrate saturation was observed. However, the permeation rates for sucrose in the two preparations were
Was this article helpful?