A scheme for C4 evolution derived from a comparative analysis of the closely related C3, C3-C4 intermediate, C4-like, and C4 species in the genus Flaveria

Key message A comparative analysis of the genus Flaveria showed a C-4 evolutionary process in which the anatomical and metabolic features of C-4 photosynthesis were gradually acquired through C-3-C-4 intermediate stages. C-4 photosynthesis has been acquired in multiple lineages of angiosperms during evolution to suppress photorespiration. Crops that perform C-4 photosynthesis exhibit high rates of CO2 assimilation and high grain production even under high-temperature in semiarid environments; therefore, engineering C-4 photosynthesis in C-3 plants is of great importance in the application field. The genus Flaveria contains a large number of C-3, C-3-C-4 intermediate, C-4-like, and C-4 species, making it a good model genus to study the evolution of C-4 photosynthesis, and these studies indicate the direction for C-4 engineering. C-4 photosynthesis was acquired gradually through the C-3-C-4 intermediate stage. First, a two-celled C-2 cycle called C-2 photosynthesis was acquired by localizing glycine decarboxylase activity in the mitochondria of bundle sheath cells. With the development of two-cell metabolism, anatomical features also changed. Next, the replacement of the two-celled C-2 cycle by the two-celled C-4 cycle was induced by the acquisition of cell-selective expression in addition to the upregulation of enzymes in the C-4 cycle during the C-3-C-4 intermediate stage. This was supported by an increase in cyclic electron transport activity in response to an increase in the ATP/NADPH demand for metabolism. Suppression of the C-3 cycle in mesophyll cells was induced after the functional establishment of the C-4 cycle, and optimization of electron transport by suppressing the activity of photosystem II also occurred during the final phase of C-4 evolution.