Modelling metabolic CO<inf>2</inf>evolution - A fresh perspective on respiration

Respiration is a major contributor to net exchange of CO2between plants and the atmosphere and thus an important aspect of the vegetation component of global climate change models. However, a mechanistic model of respiration is lacking, and so here we explore the potential for flux balance analysis (FBA) to predict cellular CO2evolution rates. Metabolic flux analysis reveals that respiration is not always the dominant source of CO2, and that metabolic processes such as the oxidative pentose phosphate pathway (OPPP) and lipid synthesis can be quantitatively important. Moreover, there is considerable variation in the metabolic origin of evolved CO2between tissues, species and conditions. Comparison of FBA-predicted CO2evolution profiles with those determined from flux measurements reveals that FBA is able to predict the metabolic origin of evolved CO2in different tissues/species and under different conditions. However, FBA is poor at predicting flux through certain metabolic processes such as the OPPP and we identify the way in which maintenance costs are accounted for as a major area of improvement for future FBA studies. We conclude that FBA, in its standard form, can be used to predict CO2evolution in a range of plant tissues and in response to environment. Although respiration is a major contributor to the net exchange of CO2between plants and the atmosphere, it would be desirable to take a holistic view of the network when modelling the process and to consider all the potential contributors to light-independent CO2balance. We argue that flux balance analysis of genome-scale metabolic models offers a practicable tool for predictive modelling of net CO2evolution. This is supported by the encouraging agreement that has been found between the experimental results obtained by metabolic flux analysis and the predictions of flux balance analysis. While there is scope for further improvement, particularly in the way in which flux balance analysis handles cell maintenance costs, its power is already sufficient to make useful predictions about the impact of environmental factors on CO2production by plant tissues. © 2013 John Wiley & Sons Ltd.