C4 photosynthesis in C3 rice: a theoretical analysis of biochemical and anatomical factors

Engineering C-4 photosynthesis into rice has been considered a promising strategy to increase photosynthesis and yield. A question that remains to be answered is whether expressing a C-4 metabolic cycle into a C-3 leaf structure and without removing the C-3 background metabolism improves photosynthetic efficiency. To explore this question, we developed a 3D reaction diffusion model of bundle-sheath and connected mesophyll cells in a C-3 rice leaf. Our results show that integrating a C-4 metabolic pathway into rice leaves with a C-3 metabolism and mesophyll structure may lead to an improved photosynthesis under current ambient CO2 concentration. We analysed a number of physiological factors that influence the CO2 uptake rate, which include the chloroplast surface area exposed to intercellular air space, bundle-sheath cell wall thickness, bundle-sheath chloroplast envelope permeability, Rubisco concentration and the energy partitioning between C-3 and C-4 cycles. Among these, partitioning of energy between C-3 and C-4 photosynthesis and the partitioning of Rubisco between mesophyll and bundle-sheath cells are decisive factors controlling photosynthetic efficiency in an engineered C-3-C-4 leaf. The implications of the results for the sequence of C-4 evolution are also discussed.