Anaplerotic flux into the Calvin-Benson cycle. Hydrogen isotope evidence for in vivo occurrence in C₃ metabolism

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 CO₂ 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 H¹ and H² 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 CO₂ 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.