Anaplerotic flux into the Calvin-Benson cycle. Hydrogen isotope evidence forin vivooccurrence in C3metabolism

Summary - As the central carbon uptake pathway in photosynthetic cells, the Calvin-Benson cycle is among the most important biochemical cycles for life on Earth. A carbon flux of anaplerotic origin (i.e., through the chloroplast-localised oxidative branch of the pentose phosphate pathway) into the Calvin-Benson cycle was proposed recently. - Here, we measured intramolecular deuterium abundances in leaf starch of Helianthus annuus grown at varying ambient CO 2 concentrations, C a . Additionally, we modelled deuterium fractionations expected for the anaplerotic pathway and compared modelled with measured fractionations. - We report deuterium fractionation signals at H 1 and H 2 of starch glucose. Below a C a change point, these signals increase with decreasing C a consistent with modelled fractionations by anaplerotic flux. Under standard conditions ( C a =450 ppm corresponding to intercellular CO 2 concentrations, C i , of 328 ppm), we estimate negligible anaplerotic flux. At C a =180 ppm ( C i =140 ppm), more than 10% of the glucose 6-phosphate entering the starch biosynthesis pathway 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.