Potential metabolic mechanisms for inhibited chloroplast nitrogen assimilation under high CO2

Abstract

Improving photosynthesis is considered a major and feasible option to dramatically increase crop yield potential. Increased atmospheric CO2 concentration often stimulates both photosynthesis and crop yield, but decreases protein content in the main C3 cereal crops. This decreased protein content in crops constrains the benefits of elevated CO2 on crop yield and affects their nutritional value for humans. To support studies of photosynthetic nitrogen assimilation and its complex interaction with photosynthetic carbon metabolism for crop improvement, we developed a dynamic systems model of plant primary metabolism, which includes the Calvin-Benson cycle, the photorespiration pathway, starch synthesis, glycolysis-gluconeogenesis, the tricarboxylic acid cycle, and chloroplastic nitrogen assimilation. This model successfully captures responses of net photosynthetic CO2 uptake rate (A), respiration rate, and nitrogen assimilation rate to different irradiance and CO2 levels. We then used this model to predict inhibition of nitrogen assimilation under elevated CO2. The potential mechanisms underlying inhibited nitrogen assimilation under elevated CO2 were further explored with this model. Simulations suggest that enhancing the supply of α-ketoglutarate is a potential strategy to maintain high rates of nitrogen assimilation under elevated CO2. This model can be used as a heuristic tool to support research on interactions between photosynthesis, respiration, and nitrogen assimilation. It also provides a basic framework to support the design and engineering of C3 plant primary metabolism for enhanced photosynthetic efficiency and nitrogen assimilation in the coming high-CO2 world.

Figure 1

The structure of the systems model of primary metabolism in a C₃ mesophyll cell. In the C₃ primary metabolism model, the cell was divided into four compartments including the chloroplast, the mitochondria, the peroxisome and cytosol. All abbreviations used in the figure were defined in the Supplemental Table S1.

Figure 2

Responses of net photosynthetic CO₂ uptake (A) to environmental changes. (A and D) and (B and E) represent responses of A to CO₂ concentration under different O₂ and light conditions, respectively. (C and F) represent responses of A to photosynthetic photon flux density (PPFD) under different CO₂ conditions. Solid lines (A, B, and C) represent simulation results; and dots and dotted lines (D, E, and F) represent experimental data, which were the unpublished data gained from Dr Florian Busch in previous studies (Busch et al., 2018). Data in parts (D, E, and F) are mean ± sd (n = 7 individual plants, the extra measurements at 400 ppm are double measurements on the same plants).

Figure 3

The changes of modeled and measured metabolite profiles (Arrivault et al., 2009) under different CO₂ conditions. The height of the bars represents the relative levels of metabolite in reference to that at the lowest CO₂ condition. The blue, red, and dark yellow bars represent relative metabolite levels under CO₂ levels of about 100, 200, and 500 ppm, respectively. The light density was 1,500 μmol m⁻² s⁻¹ in simulation and was 150 μmol m⁻² s⁻¹ in experimental treatment (A. thaliana). Simu, simulated metabolite levels; Meas, measured metabolite levels from Arrivault et al. (2009). All abbreviations used in the figure were defined in the Supplemental Table S1.

Figure 4

The predicted light and CO₂ response curves of Rd, PR,NA. A–C represent CO₂ response curves of PR, Rd, and NA, respectively. D–F represent light response curve of PR, Rd, and NA, respectively. The PPFD was 1,500 μmol m⁻² s⁻¹ in (A–C). The ambient CO₂ was 400 ppm in (D–F). The dashed line in (A–C) corresponds to the ambient CO₂ level of 400 ppm.

Figure 5

The influence of elevated CO₂ on nitrogen assimilation rate under different photosynthetic photon flux densities. The number of the x-axis represents PPFD during simulation with a unit of “μmol m⁻² s⁻¹”. The blue bars and red bars represent either concentrations of metabolites or particular ratio (e.g. NADH/NAD) values under CO₂ concentrations of 400 ppm and 800 ppm, respectively. The “cyt” and “chl” in the subscripts represent their concentration (or ratio) in cytosol and chloroplast, respectively.

Figure 6

Model simulated metabolic fluxes of net 2-OG production to provide carbon skeletons as nitrogen assimilation in the C₃ primary metabolism model, under three light levels and two CO₂ conditions. The color in the bar graph represents the different reactions generating 2-OG, as shown in the left part of the figure. Substrates ICIT, glyoxylate (GOX), GLU, GLY, SER, PYR, ALA, ASP, OAA, and 2-OG represent iso-citrate, GOX, glutamate, GLY, serine, pyruvate, alanine, ASP, OAA, and 2-oxoglutarate, respectively. Enzymes GPT and GOT represent glutamate: pyruvate transaminase and glutamate: OAA transaminase, respectively.

Figure 7

Chloroplastic FDH₂ consumptions through different metabolic pathways in under three light conditions and two CO₂ levels. The left and right parts represent the fluxes and percentage of fluxes of FDH₂ consumptions, respectively. The color of purple, yellow, orange, and blue represent the pathway of GS–GOGAT system, NO₂-reduction (NiR), chloroplastic OAA/malate shuttle (OMT), and the CBC.

Figure 8

Changes in the ratio of fixed C to N (C/N) under different CO₂ and light levels. The O₂ concentration used in the simulation was 21%. The C/N was calculated by the equation C/N = (12*A)/(14*NA), where A and NA represent net photosynthetic CO₂ uptake rate and nitrogen assimilation rate, respectively; 12 and 14 represent the molecular weights of C and N, respectively.