Enhancing vacuolar sucrose cleavage within the developing potato tuber has only minor effects on metabolism

Modification of tuber carbohydrate metabolism by the tuber-specific expression of a yeast invertase targeted to the cytosol or apoplast has previously been demonstrated to have diverse effects on tuber growth and metabolism. In the current study, we generated plants exhibiting tuber-specific expression of the same enzyme targeted to the vacuole. Enzymatic analysis of the carbohydrate levels of the tuber revealed dramatic decreases in sucrose content coupled with large increases in the levels of glucose and hexose phosphates, but unaltered starch content in the transformants. Analysis of the key enzyme of glycolysis suggests that this pathway is down-regulated in the transformants. Despite these changes in metabolite pools and enzyme activity, few consistent changes could be observed in the estimated metabolic fluxes following incubation of isolated tuber discs in labelled glucose. The analysis of the relative levels of a wide range of metabolites using a gas chromatography-mass spectrometry (GC-MS)-based metabolite profiling method revealed large changes in the levels of fructose and decreases in a range of other sugars, but very few changes in the contents of organic and amino acids. This metabolic profile is remarkably consistent with that obtained following expression of the invertase in the apoplastic compartment, providing circumstantial evidence for the endocytotic trafficking of sugars within potato tuber parenchyma. Finally, the results of this study are compared with those from other plant species and the relative roles of the vacuolar isoform of the enzyme are contrasted.