Impact of elevated cytosolic and apoplastic invertase activity on carbon metabolism during potato tuber development

During tuberization in Solanum tuberosum var. Desiree maximal catalytic act ivities of invertase(s) and sucrose synthase are inversely correlated. Duri ng the early stages, invertase activity is high and declines during maturat ion. The decrease in invertase activity is accompanied by a decrease in the hexose to sucrose ratio and an increase in sucrose synthase activity. This switch is paralleled by the onset of the storage phase as shown by the acc umulation of starch and storage proteins. Biochemical and genetic evidence suggests that sucrose synthase activity is positively correlated with sink strength. To explore the possibility of enhancing sink strength in potato t ubers by elevating the sucrolytic capacity, transgenic potato plants expres sing either cytosolic or apoplastic yeast invertase in their tubers were ma de. Surprisingly, cytosolic invertase led to a decrease and apoplastic inve rtase to an increase in tuber yield. To understand the causes of the observ ed phenotypes, carbon metabolism in tubers of transgenic and control plants was analysed during different stages of tuber development. Both cytosolic and apoplastic invertase resulted in decreased sucrose and elevated glucose contents, indicating that sucrose is accessible in both compartments. Meta bolic perturbation, however, was found to be compartment specific. Elevated cytosolic invertase activity led to increased carbon flux towards glycolys is and accumulation of phosphorylated intermediates. The phosphorylated int ermediates were not used to build up starch, In contrast, apoplastic invert ase does not lead to a significant increase in hexose phosphates compared t o untransformed controls. Thus, hexoses originating in the apoplast are not efficiently phosphorylated during potato tuber development, which might be explained by an endocytotic uptake of sucrose and/or hexoses from the apop last into the vacuole bypassing the cytosolic compartment.