The control of sucrose synthesis in leaves of Lolium temulentum L., a fructan-accumulating grass



The control of sucrose synthesis was studied in leaves of the fructan-accumulating species Lolium temulentum L. The cytoplasm it fructose-1,6-bisphosphatase (Cyt'F-1,6-BPase) was partially purified and its kinetic properties compared with the reported values for the enzyme from spinach leaves. The enzymes from the two species were similar, particularly in their inhibition by fructose-2,6-bisphosphate (F-2,6BP). Leaves were excised and illuminated or whole plants were chilled and illuminated in order to increase the accumulation of sucrose and low-molecular-weight fructan, Leaf glucose-6-phosphate (G-6-P) and fructose-6-phosphate (F-6-P) contents were estimated. The G-6-P/F-6-P ratio showed that most of the hexose phosphates were present in the cytoplasm. In excised leaves G-6-P concentrations were higher than controls after 4 h of illumination, probably reflecting the release of glucose by fructan synthesis. This extra source of substrate supply may disrupt the coordinated control of sucrose phosphate synthase (SPS) and Cyt'F-1,6-BPase. The calculated activity of Cyt'F-1,6-BPase in vivo is insufficient to catalyse the observed flux of photo assimilates to sucrose and fructan. The maximum catalytic activity of phosphofructophosphotransferase (PFP) was determined, and was found to be 12–14Ümol hexose g1 f. wt h1 in control leaves. It is proposed that, given suitable metabolite concentrations, PFP could supplement the inhibited Cyt'F-1,6-BPase activity and contribute to the flux of photo assimilates to sucrose and fructan.