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 [1-13C]-glucose. The A10-line, containing 10 % proembryogenic masses (PEMs) and 90 % large, vacuolated cells (VACs), showed 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 known to be caused by cycling of carbon from hexose phosphates to triose phosphates and vice versa, in which ATP-dependent phosphofructokinase (PFK, EC 2.7.1.11) catalyses the glycolytic reaction and PPi-dependent phosphofructokinase (PFP, EC 2.7.1.90) the gluconeogenetic reaction. The ratio of extractable 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 identical label exchange. It is concluded that the level of PFP is genetically determined and that this level influences the amount of label exchange from C-1 to C-6 carbons in hexoses and sucrose in Daucus cells. High levels of the reversible enzyme PFP might give plants the advantage to respond adequately to quickly changing demands for substrates for either glycolytic or gluconeogenetic reactions. Both triose-hexose phosphate cycling and respiration were higher when suspensions were aerated with 100% O2 instead of 6% O2. It is concluded that high respiratory activity stimulated both the flow of hexose phosphates into the respiratory pathway and the back-flow from triose to hexose phosphates. However, total labelled sucrose was at least two times higher at 6% O2 than at 100% O2, indicating that more hexose phosphates were available for sucrose synthesis at 6% O2.