Photorespiration and Improving Photosynthesis

Improving photosynthesis has become a strategy to increase plant productivity. Current photosynthetic targets dealing with light and CO2 capture will be briefly described and major breakthroughs dealing with photoprotection kinetics, CO2-concentrating mechanisms, and ribulose-1,5-bisphosphate regeneration will be highlighted before focusing on photorespiration. This metabolic process occurs when ribulose-1,5-bisphosphate carboxylase/oxygenase, the major CO2-assimilatory enzyme, uses O2 thereby producing toxic 2-phosphoglycolate that has to be removed. This is achieved by the photorespiratory cycle, a high energy cost pathway that competes with photosynthetic CO2 assimilation and releases both CO2 and ammonium that can be lost to the atmosphere if not re-assimilated. This wasteful metabolic pathway cannot be knocked out, as photorespiratory mutants are unable to develop normally in air and require high CO2 atmospheres that limit photorespiration for normal growth. Surprisingly, little is known about the regulation of this important metabolic cycle even though photorespiratory enzymes are associated with several post-translational modifications. Current progress in the use of photorespiratory mutants to better understand photorespiration and its interactions with other metabolic pathways, and in proteomics to identify potential regulatory mechanisms will be described before moving onto how manipulating the photorespiratory cycle has led to the improvement of photosynthesis and plant productivity. This has been achieved either by over-expressing photorespiratory proteins or by creating alternative glycolate catabolism routes within the chloroplast.