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 cu ttings containing an axillary bud, was used to investigate the activity pat terns of enzymes involved in the conversion of hexose phosphates and relate d products during stolen-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 increas e. This increase reflects a higher capacity of respiratory(-related) metabo lism, presumably due to the onset of rapid cell division in the apical part of the tuberizing stolen. During the phase of successive tuber growth thes e enzymes decreased in activity, suggesting that the concomitant massive st arch 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-relat ed enzymes is coordinately regulated by the same mechanism of coarse contro l. The activity pattern of pyrophosphate:fructose-6-phosphate phosphotransf erase (PFP) showed no developmental change and does not resemble the activi ty pattern of the enzymes participating in respiratory(-related) metabolism . Instead, its level of activity is very likely the result of metabolic reg ulation. The level of the content of the metabolites UDP-glucose (UDPGlc) a nd glucose-6-phosphate (Glc6P) decreased after the onset of tuberization, T his decline indicates that tuber induction is not accompanied by an appreci able increase in the level of the cytosolic hexose phosphate (hexose-P) con tent but that it rather remains on a low level, which might be a prerequisi te 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 ove rall activities of ADP-glucose pyrophosphorylase (AGPase) and starch phosph orylase (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.