Flux Control Analysis of the Rate of Photosynthetic CO₂ Assimilation

Metabolic flux control analysis offers an opportunity to understand how fluxes are determined and thus how they can be increased. Enzymes are assigned a control coefficient (usually between zero and one) which is a measure of the degree to which the flux is sensitive to small changes in enzyme concentration. This chapter examines research on the application of control analysis to the photosynthetic system of C₃ plants, specifically the net rate of CO₂ fixation. Control coefficients have been measured for a number of the enzymes, typically by reducing the concentration of a target enzyme in transgenic plants then comparing their flux and enzyme concentrations to those of the wild type. There have been surprisingly many difficulties with this approach. Firstly, many transgenic plants have markedly lower enzyme concentrations than the wild type, so it has been difficult to make accurate estimations of the enzyme-flux relationship in the vicinity of the wild type value. Second, natural variation, compounded by variation due to a lack of precision in measurements, has often required a very large number of replicates in order to detect significant values. Many studies have not used sufficient replication. Third, confounding variables, such as ontogenetic variation and untargeted changes in the concentration of other enzymes, have not always been identified and taken into account. As a consequence of these challenges, there are few studies in which control coefficients have been detected with a high degree of certainty. This situation is set to improve, however, with the use of transgenic plants with both an increased and decreased target enzyme concentration. Several such studies have now been published, and in two cases, estimates of the control coefficients have been markedly improved.