Evolution of the C4 photosynthetic pathway: events at the cellular and molecular levels

The biochemistry and leaf anatomy of plants using C-4 photosynthesis promote the concentration of atmospheric CO2 in leaf tissue that leads to improvements in growth and yield of C-4 plants over C-3 species in hot, dry, high light, and/or saline environments. C-4 plants like maize and sugarcane are significant food, fodder, and bioenergy crops. The C-4 photosynthetic pathway is an excellent example of convergent evolution, having evolved in multiple independent lineages of land plants from ancestors employing C-3 photosynthesis. In addition to C-3 and C-4 species, some plant lineages contain closely related C-3-C-4 intermediate species that demonstrate leaf anatomical, biochemical, and physiological characteristics between those of C-3 plants and species using C-4 photosynthesis. These groups of plants have been extremely useful in dissecting the modifications to leaf anatomy and molecular biology, which led to the evolution of C-4 photosynthesis. It is now clear that great variation exists in C-4 leaf anatomy, and diverse molecular mechanisms underlie C-4 biochemistry and physiology. However, all these different paths have led to the same destination-the expression of a C-4 CO2 concentrating mechanism. Further identification of C-4 leaf anatomical traits and molecular biological components, and understanding how they are controlled and assembled will not only allow for additional insights into evolutionary convergence, but also contribute to sustainable food and bioenergy production strategies.