The triose-hexose phosphate cycle and the sucrose cycle in carrot (Daucus carota L.) cell suspensions are controlled by respiration and PPi: Fructose-6-phosphate phosphotransferase

Short-term labelling was applied to two different lines of Daucus carota L. cell suspensions by feeding [1-C-13]-glucose. The A10-line, containing 10 % proembryogenic masses (PEMs) and 90 % large, vacuolated cells (VACs), sho wed a 2 times higher label exchange from C-1 to C-6 carbons within sucrose and hexoses than the A+-line, containing 80% PEMs. This label exchange is k nown to be caused by cycling of carbon from hexose phosphates to triose pho sphates and vice versa, in which ATP-dependent phosphofructokinase (PFK, EC 2.7.1.11) catalyses the glycolytic reaction and PPi-dependent phosphofruct okinase (PFP, EC 2.7.1.90) the gluconeogenetic reaction. The ratio of extra ctable PFP/PFK was 3 times higher in the A10-line compared to the A+-line. However, PEMs and VACs from one line showed identical PFP/PFK ratios and id entical label exchange. It is concluded that the level of PPP is geneticall y determined and that this level influences the amount of label exchange fr om C-1 to C-6 carbons in hexoses and sucrose in Daucus cells. High levels o f the reversible enzyme PFP might give plants the advantage to respond adeq uately to quickly changing demands for substrates for either glycolytic or gluconeogenetic reactions. Both triose-hexose phosphate cycling and respiration were higher when suspe nsions were aerated with 100 % O-2 instead of 6 % O-2. It is concluded that high respiratory activity stimulated both the flow of hexose phosphates in to the respiratory pathway and the back-flow from triose to hexose phosphat es. However, total labelled sucrose was at: least: two times higher at 6% O -2 than at 100 % O-2, indicating that more hexose phosphates were available for sucrose synthesis at 6% O-2.