Acclimation to elevated CO2 affects the C/N balance by reducing de novo N-assimilation.

Plants exposed to elevated atmospheric CO2 concentrations show an increased photosynthetic activity. However, after prolonged exposure, the activity declines. This acclimation to elevated CO2 is accompanied by a rise in the carbon-to-nitrogen ratio of the biomass. Hence, increased sugar accumulation and sequential downregulation of photosynthetic genes, as well as nitrogen depletion and reduced protein content, have been hypothesized as the cause of low photosynthetic performance. However, the reason for reduced nitrogen content in plants at high CO2 is unclear. Here, we show that reduced photorespiration at increased CO2 -to-O2 ratio leads to reduced de novo assimilation of nitrate, thus shifting the C/N balance. Metabolic modeling of acclimated and non-acclimated plants revealed the photorespiratory pathway to function as a sink for already assimilated nitrogen during the light period, providing carbon skeletons for de novo assimilation. At high CO2 , low photorespiratory activity resulted in diminished nitrogen assimilation and eventually resulted in reduced carbon assimilation. For the hpr1-1 mutant, defective in reduction of hydroxy-pyruvate, metabolic simulations show that turnover of photorespiratory metabolites is expanded into the night. Comparison of simulations for hpr1-1 with those for the wild type allowed investigating the effect of a perturbed photorespiration on N-assimilation.

Subcellular reprogramming of metabolism during cold acclimation in Arabidopsis thaliana.

Metabolite changes in plant leaves during exposure to low temperatures involve re-allocation of a large number of metabolites between sub-cellular compartments. Therefore, metabolite determination at the whole cell level may be insufficient for interpretation of the functional significance of cellular compounds. To investigate the cold-induced metabolite dynamics at the level of individual sub-cellular compartments, an integrative platform was developed that combines quantitative metabolite profiling by gas chromatography coupled to mass spectrometry (GC-MS) with the non-aqueous fractionation technique allowing separation of cytosol, vacuole and the plastidial compartment. Two mutants of Arabidopsis thaliana representing antipodes in the diversion of carbohydrate metabolism between sucrose and starch were compared to Col-0 wildtype before and after cold acclimation to investigate interactions of cold acclimation with subcellular re-programming of metabolism. A multivariate analysis of the data set revealed dominant effects of compartmentation on metabolite concentrations that were modulated by environmental condition and genetic determinants. While for both, the starchless mutant of plastidial phospho-gluco mutase (pgm) and a mutant defective in sucrose-phosphate synthase A1, metabolic constraints, especially at low temperature, could be uncovered based on subcellularly resolved metabolite profiles, only pgm had lowered freezing tolerance. Metabolic profiles of pgm point to redox imbalance as a possible reason for reduced cold acclimation capacity.

Identification of a metabolic bottleneck for cold acclimation in Arabidopsis thaliana.

Central carbohydrate metabolism of Arabidopsis thaliana is known to play a crucial role during cold acclimation and the acquisition of freezing tolerance. During cold exposure, many carbohydrates accumulate and a new metabolic homeostasis evolves. In the present study, we analyse the diurnal dynamics of carbohydrate homeostasis before and after cold exposure in three natural accessions showing distinct cold acclimation capacity. Diurnal dynamics of soluble carbohydrates were found to be significantly different in cold-sensitive and cold-tolerant accessions. Although experimentally determined maximum turnover rates for sucrose phosphate synthase in cold-acclimated leaves were higher for cold-tolerant accessions, model simulations of diurnal carbohydrate dynamics revealed similar fluxes. This implied a significantly higher capacity for sucrose synthesis in cold-tolerant than cold-sensitive accessions. Based on this implication resulting from mathematical model simulation, a critical temperature for sucrose synthesis was calculated using the Arrhenius equation and experimentally validated in the cold-sensitive accession C24. At the critical temperature suggested by model simulation, an imbalance in photosynthetic carbon fixation ultimately resulting in oxidative stress was observed. It is therefore concluded that metabolic capacities at least in part determine the ability of accessions of Arabidopsis thaliana to cope with changes in environmental conditions.

Measuring whole plant CO2 exchange with the environment reveals opposing effects of the gin2-1 mutation in shoots and roots of Arabidopsis thaliana.

Using a cuvette for simultaneous measurement of net photosynthesis in above ground plant organs and root respiration we investigated the effect of reduced leaf glucokinase activity on plant carbon balance. The gin2-1 mutant of Arabidopsis thaliana is characterized by a 50% reduction of glucokinase activity in the shoot, while activity in roots is about fivefold higher and similar to wild type plants. High levels of sucrose accumulating in leaves during the light period correlated with elevated root respiration in gin2-1. Despite substantial respiratory losses in roots, growth retardation was moderate, probably because photosynthetic carbon fixation was simultaneously elevated in gin2-1. Our data indicate that futile cycling of sucrose in shoots exerts a reduction on net CO2 gain, but this is over-compensated by the prevention of exaggerated root respiration resulting from high sucrose concentration in leaf tissue.