Anaplerotic flux into the Calvin-Benson cycle. Isotope evidence for in vivo occurrence in Helianthus annuus

- As the central carbon uptake pathway in photosynthetic cells, the Calvin-Benson cycle is among the most important biochemical cycles for life on Earth. Recently, anaplerotic carbon flux (through the chloroplast-localised oxidative branch of the pentose phosphate pathway) into this cycle was proposed. - Here, we measured intramolecular deuterium abundances in leaf starch of Helianthus annuus grown at varying ambient CO2 concentrations, Ca. Additionally, we modelled deuterium fractionations expected for the anaplerotic pathway and compared modelled with measured fractionations. - We report deuterium fractionation signals at starch glucose H1 and H2. Below a response change point, these signals increase with decreasing Ca consistent with modelled fractionations by anaplerotic flux. Under normal growth conditions (Ca[≥]450 ppm corresponding to intercellular CO2 concentrations, Ci, [≥]328 ppm), we estimate negligible anaplerotic flux. At Ca=180 ppm (Ci=140 ppm), we estimate that of the glucose 6-phosphate entering the starch biosynthesis pathway more than 11.5% is diverted into the anaplerotic pathway. - In conclusion, we report evidence consistent with anaplerotic carbon flux into the Calvin-Benson cycle in vivo. We propose the flux may help to (i) maintain high levels of ribulose 1,5-bisphosphate under source-limited growth conditions to facilitate photorespiratory nitrogen assimilation required to build-up source strength and (ii) counteract oxidative stress. Supporting information Deuterium fractionation by glucose-6-phosphate dehydrogenase