Transgenic approaches to manipulate the environmental responses of the C 3 carbon fixation cycle

The limitation to photosynthetic CO 2 assimilation in C 3 plants in hot, dry environments is dominated by ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) because CO 2 availability is restricted and photorespiration is stimulated. Using a combination of genetic engineering and transgenic technology, three approaches to reduce photorespiration have been taken; two of these focused on increasing the carboxylation efficiency of Rubisco either by reducing the oxygenase reaction directly or by manipulating the Rubisco enzyme by concentrating CO 2 in the region of Rubisco through the introduction of enzymes of the C 4 pathway. The third approach attempted to reduce photorespiration directly by manipulation of enzymes in this pathway. The progress in each of these areas is discussed, and the most promising approaches are highlighted. Under saturating CO 2 conditions, Rubisco did not limit photosynthesis, and limitation shifted to ribulose bisphosphate (RuBP) regeneration capacity of the C 3 cycle. Transgenic analysis was used to identify the specific enzymes that may be targets for improving carbon fixation, and the way this may be exploited in the high CO 2 future is considered.