Acclimation of photosynthetic proteins to rising atmospheric CO₂

In this review we discuss how the photosynthetic apparatus, particularly Rubisco, acclimates to rising atmospheric CO₂ concentrations (c_a). Elevated c_a alters the control exerted by different enzymes of the Calvin cycle on the overall rate of photosynthetic CO₂ assimilation, so altering the requirement for different functional proteins. A decreased flux of carbon through the photorespiratory pathway will decrease requirements for these enzymes. From modeling of the response of CO₂ uptake (A) to intracellular CO₂ concentration (c_i) it is shown that the requirement for Rubisco is decreased at elevated c_a, whilst that for proteins limiting ribulose 1,5 bisphosphate regeneration may be increased. This balance may be altered by other interactions, in particular plasticity of sinks for photoassimilate and nitrogen supply; hypotheses on these interactions are presented. It is speculated that increased accumulation of carbohydrate in leaves developed at elevated c_a may signal the 'down regulation' of Rubisco. The molecular basis of this 'down regulation' is discussed in terms of the repression of photosynthetic gene expression by the elevated carbohydrate concentrations. This molecular model is then used to predict patterns of acclimation of perennials to long term growth in elevated c_a.