The Calvin-Benson cycle is one of the most important biochemical cycles for life on Earth. It is believed to be self-sufficient in that it regenerates its own substrate. A recently published report in New Phytologist challenges this classical view. The authors, in the context of a collaboration between the University of Umeå in Sweden and the Research Institute on Terrestrial Ecosystems of CNR, analysed hydrogen isotopes in starch of sunflower leaves and report evidence consistent with carbon flux through the oxidative pentose phosphate pathway. The pathway apparently replenishes Calvin-Benson cycle intermediates under certain environmental conditions. Since the pathway involves carbon dioxide release it can be expected to affect plant performance and biosphere-atmosphere carbon exchange.
The authors of this study analysed hydrogen isotope abundances in sunflower leaf starch. To acquire measurements at the highest resolution possible, they applied a protocol build around state-of-the-art Nuclear Magnetic Resonance Spectroscopy. “Our experimental approach provides data at the level of individual hydrogen positions within the glucose monomers of starch whereas conventional techniques yield an average measurement for the entire starch molecule” says Jürgen Schleucher. The researchers discovered isotope signatures consistent with carbon flux through the so-called oxidative pentose phosphate pathway. “This pathway synthesises ribulose 5-phosphate (Ru5P), the precursor of the Calvin-Benson cycle substrate, RuBP” explains Angela Augusti. Apparently, Ru5P inputs increase significantly with decreasing CO2 concentrations inside leaves, a condition often seen under drought. “Our findings suggest that the Calvin-Benson cycle is not entirely self-sufficient with respect to its substrate. This contrasts with what most scientists believe” says Thomas Wieloch. Flux through the oxidative pentose phosphate pathway is associated with the release of CO2 and the production of energy in the form of NADPH. Therefore, it can be expected to (inter alia) affect plant performance, and biosphere-atmosphere CO2 exchange and thus food security and climate change.
“Why plants would activate a pathway that releases CO2 under conditions where CO2 inputs are already low is certainly one of the most interesting questions resulting from our work” continues Thomas Wieloch. The authors hypothesise that this metabolic adjustment may support nitrogen assimilation involving photorespiration (which is also more active under low CO2 concentrations). Looking at plant metabolism from a larger (non-carbon centred) perspective, a shift from carbon towards nitrogen assimilation under carbon limited conditions might make sense in the context of long-term plant performance.
Thomas Wieloch, Angela Augusti, Jürgen Schleucher (2022). Anaplerotic flux into the Calvin-Benson cycle. Hydrogen isotope evidence for in vivo occurrence in C3 metabolism. New Phytologist. https://doi.org/10.1111/nph.17957.