Rapid CO₂ effects on protein-related metabolism during photosynthetic gas exchange

It is common practice to vary atmospheric CO₂ mole fraction (C_a) during gas exchange experiments to manipulate photorespiration and explore photosynthetic properties such as maximum carboxylation velocity. In doing so, it is assumed that changing C_a essentially alters the carboxylation-to-oxygenation ratio, with negligible effects on other metabolic pathways. There is now substantial evidence that this assumption is incorrect, with strong modifications in day respiratory metabolism, N and S assimilation and cytosolic translational activity. In principle, these modifications should reconfigure protein content and composition which might in turn affect photosynthetic activity. To address this point, we carried out gas exchange experiments on sunflower leaves under varying CO₂, either using a portable system or a special leaf chamber designed for ¹³C-labelling. Non-targeted analysis of metabolites, proteins and isotopic features was performed to look at metabolites, protein synthesis rate and proteins affected by CO₂ mole fraction within 1 to 3 h. Proteogenic amino acids and amino acid derivatives were amongst the most affected metabolites. Protein synthesis rate increased with C_a, except at very high CO₂. Accordingly, proteins affected by C_a included proteins involved in translation (ribosomal proteins, initiation factors) and proteases, showing a change in protein turn-over. Also, C_a impacted some proteins of the photosynthetic machinery. Taken as a whole, our results demonstrate that CO₂ mole fraction has broad short-term effects on leaf protein-related metabolism. In principle, this effect should have consequences on several aspects of photosynthesis, such as day respiration or photosynthetic capacity itself.