The role of photorespiration during the evolution of C(4) photosynthesis in the genus Flaveria

C(4) photosynthesis represents a most remarkable case of convergent evolution of a complex trait, which includes the reprogramming of the expression patterns of thousands of genes. Anatomical, physiological, and phylogenetic and analyses as well as computational modeling indicate that the establishment of a photorespiratory carbon pump (termed C(2) photosynthesis) is a prerequisite for the evolution of C(4). However, a mechanistic model explaining the tight connection between the evolution of C(4) and C(2) photosynthesis is currently lacking. Here we address this question through comparative transcriptomic and biochemical analyses of closely related C(3), C(3)–C(4), and C(4) species, combined with Flux Balance Analysis constrained through a mechanistic model of carbon fixation. We show that C(2) photosynthesis creates a misbalance in nitrogen metabolism between bundle sheath and mesophyll cells. Rebalancing nitrogen metabolism requires anaplerotic reactions that resemble at least parts of a basic C(4) cycle. Our findings thus show how C(2) photosynthesis represents a pre-adaptation for the C(4) system, where the evolution of the C(2) system establishes important C(4) components as a side effect. DOI: http://dx.doi.org/10.7554/eLife.02478.001