Carbon flux around leaf-cytosolic glyceraldehyde-3-phosphate dehydrogenase introduces a 13C signal in plant glucose

Within the plant and Earth sciences, stable isotope analysis is a versatile tool conveying information (inter alia) about plant physiological and paleoclimate variability across scales. Here, we identify a C-13 signal (i.e. systematic C-13/C-12 variation) at tree-ring glucose C-4 and report an experimentally testable theory on its origin. We propose the signal is introduced by glyceraldehyde-3-phosphate dehydrogenases in the cytosol of leaves. It conveys two kinds of (potentially convoluted) information: (i) commitment of glyceraldehyde 3-phosphate to 3-phosphoglycerate versus fructose 1,6-bisphosphate metabolism; and (ii) the contribution of non-phosphorylating versus phosphorylating glyceraldehyde-3-phosphate dehydrogenase to catalysing the glyceraldehyde 3-phosphate to 3-phosphoglycerate forward reaction of glycolysis. The theory is supported by C-13 fractionation modelling. Modelling results provide the first evidence in support of the cytosolic oxidation-reduction (COR) cycle, a carbon-neutral mechanism supplying NADPH at the expense of ATP and NADH, which may help to maintain leaf-cytosolic redox balances. In line with expectations related to COR cycling, we found a positive correlation between air vapour pressure deficit and C-13 discrimination at glucose C-4. Overall, C-13-4 signal analysis may enable an improved understanding of leaf carbon and energy metabolism.

Automated summary

Stable isotope analysis is a versatile tool in the plant and Earth sciences, conveying information about plant physiology and paleoclimate variability. A C-13 signal at tree-ring glucose C-4 is identified, believed to be introduced by glyceraldehyde-3-phosphate dehydrogenases, conveying information about metabolism and catalysis. The theory is supported by C-13 fractionation modelling, providing evidence for a carbon-neutral mechanism known as the cytosolic oxidation-reduction (COR) cycle.