Evolutionary Switch and Genetic Convergence on rbcL following the Evolution of C4 Photosynthesis

Rubisco is responsible for the fixation of CO2 into organic compounds through photosynthesis and thus has a great agronomic importance. It is well established that this enzyme suffers from a slow catalysis, and its low specificity results into photorespiration, which is considered as an energy waste for the plant. However, natural variations exist, and some Rubisco lineages, such as in C-4 plants, exhibit higher catalytic efficiencies coupled to lower specificities. These C-4 kinetics could have evolved as an adaptation to the higher CO2 concentration present in C-4 photosynthetic cells. In this study, using phylogenetic analyses on a large data set of C-3 and C-4 monocots, we showed that the rbcL gene, which encodes the large subunit of Rubisco, evolved under positive selection in independent C-4 lineages. This confirms that selective pressures on Rubisco have been switched in C-4 plants by the high CO2 environment prevailing in their photosynthetic cells. Eight rbcL codons evolving under positive selection in C-4 clades were involved in parallel changes among the 23 independent monocot C-4 lineages included in this study. These amino acids are potentially responsible for the C-4 kinetics, and their identification opens new roads for human-directed Rubisco engineering. The introgression of C-4-like high-efficiency Rubisco would strongly enhance C-3 crop yields in the future CO2-enriched atmosphere.