The Road to C4 Photosynthesis: Evolution of a Complex Trait via Intermediary States

C-4 photosynthesis enables high photosynthetic energy conversion efficiency as well as high nitrogen and water use efficiencies. Given the multitude of biochemical, structural and molecular changes in comparison with C-3 photosynthesis, it appears unlikely that such a complex trait would evolve in a single step. C-4 photosynthesis is therefore believed to have evolved from the ancestral C-3 state via intermediary stages. Consequently, the identification and detailed characterization of plant species representing transitory states between C-3 and C-4 is important for the reconstruction of the sequence of evolutionary events, especially since C-4 evolution occurred in very different phylogenetic backgrounds. There is also significant interest in engineering of C-4 or at least C-4-like elements into C-3 crop plants. A detailed and mechanistic understanding of C-3-C-4 intermediates is likely to provide guidance for the experimental design of such approaches. Here we provide an overview on the most relevant results obtained on C-3-C-4 intermediates to date. Recent knowledge gains in this field will be described in more detail. We thereby concentrate especially on biochemical and physiological work. Finally, we will provide a perspective and outlook on the continued importance of research on C-3-C-4 intermediates.