Integration of cytosolic and plastidic carbon metabolism by fructose 2,6-bisphosphate

Abstract

Abstract The aim of this report is to assess the role of fructose 2,6-bisphosphate (Fru-2,6-P2) in integration of metabolism between the cytosol and chloroplast during photosynthesis. Transgenic plants expressing modified copies of the rat liver bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase were used to obtain lines differing specifically in their Fru-2,6-P2 concentrations. Such plants differed in the quantum requirement for photosynthesis, rate of oxygen evolution, and the extent of fluorescence quenching at the onset of illumination. Increasing Fru-2,6-P2 resulted in a progressive increase in the period and number of oscillations in oxygen evolution and fluorescence following dark-light transition in saturating carbon dioxide. At the beginning of the photoperiod, transgenic lines with increased amounts of Fru-2,6-P2 showed greater starch accumulation and reduced fluxes to soluble sugars, organic acids and amino acids. In contrast, transgenic lines with reduced Fru-2,6-P2 concentrations accumulated sucrose more rapidly than untransformed plants, while the rate of starch synthesis was lower. The flux response coefficients for Fru-2,6-P2 on sucrose and starch synthesis were — 0.43 and 0.33, respectively. These results are interpreted as evidence that Fru-2,6-P2 makes a significant contribution to the potential limitation of photosynthesis by sucrose production in the cytosol.