C4 photosynthesis evolution: the conditional Mt. Fuji

C-4 photosynthesis implements a biochemical carbon pump to suppress photorespiration. While this mechanism allows for increased photosynthetic efficiency, it requires the ancestral C-3 state to be modified in terms of leaf anatomy, expression of metabolic genes, and enzyme kinetics. Despite the complexity of the C-4 syndrome, it evolved in more than 60 independent lineages. Because the phylogenetic distribution of these origins appears to be non-random, enabling factors that are initially unrelated to C-4 photosynthesis are assumed to be acting in certain C-3 lineages. In recent years, substantial progress has been made on firstly, the nature of enabling events and finally, quantitative models of C-4 evolution that are based on C-3-C-4 intermediate species. I discuss the synthesis of these approaches as a consensus trajectory towards C-4 photosynthesis and hypothesize on the effect of enabling factors on the fitness landscape of C-4 evolution. A complete understanding of these mechanisms will require both further experimental studies and improved quantitative models of leaf physiology.