The metabolic origins of non-photorespiratory CO2 release during photosynthesis: a metabolic flux analysis

Abstract

Respiration in the light (RL) releases CO2 in photosynthesizing leaves and is a phenomenon that occurs independently from photorespiration. Since RL lowers net carbon fixation, understanding RL could help improve plant carbon-use efficiency and models of crop photosynthesis. Although RL was identified more than 75 years ago, its biochemical mechanisms remain unclear. To identify reactions contributing to RL, we mapped metabolic fluxes in photosynthesizing source leaves of the oilseed crop and model plant camelina (Camelina sativa). We performed a flux analysis using isotopic labeling patterns of central metabolites during 13CO2 labeling time course, gas exchange and carbohydrate production rate experiments. To quantify the contributions of multiple potential CO2 sources with statistical and biological confidence, we increased the number of metabolites measured and reduced biological and technical heterogeneity by using single mature source leaves and quickly quenching metabolism by directly injecting liquid N2; we then compared the goodness-of-fit between these data and data from models with alternative metabolic network structures and constraints. Our analysis predicted that RL releases 5.2 μmol CO2 g−1 FW hr−1 of CO2, which is relatively consistent with a value of 9.3 μmol CO2 g−1 FW hr−1 measured by CO2 gas exchange. The results indicated that ≤10% of RL results from TCA cycle reactions, which are widely considered to dominate RL. Further analysis of the results indicated that oxidation of glucose-6-phosphate to pentose phosphate via 6-phosphogluconate (the G6P/OPP shunt) can account for >93% of CO2 released by RL.

Anaplerotic flux into the Calvin-Benson cycle. Hydrogen isotope evidence forin vivooccurrence in C3metabolism

“…Third, Xu et al . (2021) analysed 13 C enrichment patterns of central metabolites in Camelina sativa leaves by 13 C isotopically nonstationary metabolic flux analysis.…”

“…Third, Xu et al (2021) analysed 13 C enrichment patterns of central metabolites in Camelina sativa leaves by 13 C isotopically nonstationary metabolic flux analysis. They reported estimates for anaplerotic flux (4.6 μmol CO2 g -1 FW hr -1 , ≈3% of Rubisco carboxylation) and day respiration (5.2 μmol CO2 g -1 FW hr -1 ).…”

Catalase protects against non-enzymatic decarboxylations during photorespiration

“…The CO 2 release associated with these pathways was proposed to have potential impact on the amount of CO 2 release per oxygenation, but it is difficult to evaluate this claim without quantitative flux estimates. Future labeling work combined with formal flux estimates should help resolve this source most conclusively (Xu et al 2021), but is outside of the scope of the current work.…”

Pentose Phosphate Pathway Reactions in Photosynthesizing Cells

“…There is no evidence that cytosolic G6PDH enzymes in the cytosol are turned off in the light and there is a significant pool of G6P in the cytosol in leaves in the light [ 42 , 43 , 44 , 45 ]. Both static [ 33 ] and dynamic [ 46 ] analyses of 13 CO 2 feeding results have found that the operation of the oxidative PPP as a shunt bypassing much of the Calvin–Benson cycle is strongly supported. It is estimated to proceed at approximately 5% of the rate of net CO 2 assimilation.…”