Computational modelling predicts substantial carbon assimilation gains for C3 plants with a single-celled C4 biochemical pump.

Achieving global food security for the estimated 9 billion people by 2050 is a major scientific challenge. Crop productivity is fundamentally restricted by the rate of fixation of atmospheric carbon. The dedicated enzyme, RubisCO, has a low turnover and poor specificity for CO₂. This limitation of C₃ photosynthesis (the basic carbon-assimilation pathway present in all plants) is alleviated in some lineages by use of carbon-concentrating-mechanisms, such as the C₄ cycle—a biochemical pump that concentrates CO₂ near RubisCO increasing assimilation efficacy. Most crops use only C₃ photosynthesis, so one promising research strategy to boost their productivity focuses on introducing a C₄ cycle. The simplest proposal is to use the cycle to concentrate CO₂ inside individual chloroplasts. The photosynthetic efficiency would then depend on the leakage of CO₂ out of a chloroplast. We examine this proposal with a 3D spatial model of carbon and oxygen diffusion and C₄ photosynthetic biochemistry inside a typical C₃-plant mesophyll cell geometry. We find that the cost-efficiency of C₄ photosynthesis depends on the gas permeability of the chloroplast envelope, the C₄ pathway having higher quantum efficiency than C₃ for permeabilities below 300 μm/s. However, at higher permeabilities the C₄ pathway still provides a substantial boost to carbon assimilation with only a moderate decrease in efficiency. The gains would be capped by the ability of chloroplasts to harvest light, but even under realistic light regimes a 100% boost to carbon assimilation is possible. This could be achieved in conjunction with lower investment in chloroplasts if their cell surface coverage is also reduced. Incorporation of this C₄ cycle into C₃ crops could thus promote higher growth rates and better drought resistance in dry, high-sunlight climates. [ABSTRACT FROM AUTHOR]