Developmental changes in enzymes involved in the conversion of hexose phosphate and its subsequent metabolites during early tuberization of potato

A highly synchronized in vitro tuberization system, based on single-node cuttings containing an axillary bud, was used to investigate the activity patterns of enzymes involved in the conversion of hexose phosphates and related products during stolon-to-tuber transition of potato (Solanum tuberosum L.). At tuberization the activity of enzymes involved in glycolysis and the oxidative pentose phosphate pathway (OPPP) showed a small but clear increase. This increase reflects a higher capacity of respiratory(-related) metabolism, presumably due to the onset of rapid cell division in the apical part of the tuberizing stolon. During the phase of successive tuber growth these enzymes decreased in activity, suggesting that the concomitant massive starch accumulation is not accompanied by a large increase in respiration. A high degree of positive correlation between the activities of these enzymes could be observed, implying that the level of respiratory metabolism-related enzymes is coordinately regulated by the same mechanism of coarse control. The activity pattern of pyrophosphate:fructose-6-phosphate phosphotransferase (PFP) showed no developmental change and does not resemble the activity pattern of the enzymes participating in respiratory(-related) metabolism. Instead, its level of activity is very likely the result of metabolic regulation. The level of the content of the metabolites UDP-glucose (UDPGlc) and glucose-6 phosphate (Glc6P) decreased after the onset of tuberization. This decline indicates that tuber induction is not accompanied by an appreciable increase in the level of the cytosolic hexose phosphate (hexose-P) content but that it rather remains on a low level, which might be a prerequisite in order to maintain a high net rate of sucrose degradation during tuber development. In contrast to UDPGlc and Glc6P, the content of fructose-1,6-bisphosphate (Fru1,6bisP) showed an increase after tuber induction. The overall activities of ADP-glucose pyrophosphorylase (AGPase) and starch phosphorylase (STP) both showed a large increase after tuber initiation, which is consistent with their presumed role in the process of starch synthesis and accumulation during rapid tuber growth.