Photorespiration and the Evolution of C4 Photosynthesis

C4 photosynthesis is one of the most convergent evolutionary phenomena in the biological world, with at least 66 independent origins. Evidence from these lineages consistently indicates that the C4 pathway is the end result of a series of evolutionary modifications to recover photorespired CO2 in environments where RuBisCO oxygenation is high. Phylogenetically informed research indicates that the repositioning of mitochondria in the bundle sheath is one of the earliest steps in C4 evolution, as it may establish a single-celled mechanism to scavenge photorespired CO2 produced in the bundle sheath cells. Elaboration of this mechanism leads to the two-celled photorespiratory concentration mechanism known as C2 photosynthesis (commonly observed in C3–C4 intermediate species) and then to C4 photosynthesis following the upregulation of a C4 metabolic cycle.