Abstract
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 13C signal (i.e. systematic 13C/12C 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 13C 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 13C discrimination at glucose C-4. Overall, 13C-4 signal analysis may enable an improved understanding of leaf carbon and energy metabolism.
Highlights
Relative abundances of stable carbon isotopes (13C/12C ratios) in plant metabolites convey information about physiological processes and associated environmental controls (Farquhar et al, 1982; McCarroll and Loader, 2004; Schmidt et al, 2015)
We propose the signal is introduced by glyceraldehyde-3-phosphate dehydrogenases in the cytosol of leaves
In line with expectations related to cytosolic oxidation-reduction (COR) cycling, we found a positive c correlation between air vapour pressure deficit and 13C discrimination at glucose C-4
Summary
Relative abundances of stable carbon isotopes (13C/12C ratios) in plant metabolites convey information about physiological processes and associated environmental controls (Farquhar et al, 1982; McCarroll and Loader, 2004; Schmidt et al, 2015). Photosynthetic GAP has two fates, leaf-cytosolic 3PGA or fructose 1,6te bisphosphate (FBP) metabolism with the latter supplying tree-ring glucose synthesis (Fig. 1). Leaf-cytosolic 3PGA is partly reimported into chloroplasts, and chloroplast metabolism can be expected to transfer part of the GAPDH fractionation d signal to other glucose carbon positions (see SI 1 in Wieloch et al, 2021, Preprint).
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