The molecular evolution of C4 photosynthesis: opportunities for understanding and improving the world's most productive plants.

C-4 photosynthesis is a convergent evolutionary trait that enhances photosynthetic efficiency in a variety of environmental conditions. It has evolved repeatedly following a fall in atmospheric CO2 concentration such that there is up to a 30 million year difference in the amount of time that natural selection has had to improve C-4 function between the oldest and youngest C-4 lineages. This large difference in time, coupled with the phylogenetic distance between lineages, has resulted in a large disparity in anatomy, physiology, and biochemistry between extant C-4 species. This review summarizes the myriad of molecular sequence changes that have been linked to the evolution of C-4 photosynthesis. These range from single nucleotide changes to duplication of entire genes, and provide a roadmap for how natural selection has adapted enzymes and pathways for enhanced C-4 function. Finally, this review discusses how this molecular diversity can provide opportunities for understanding and improving photosynthesis for multiple important C-4 food, feed, and bioenergy crops.