Exploiting differences in the energy budget among C4 subtypes to improve crop productivity.

C4 crops of agricultural importance all belong to the NADP-malic enzyme (ME) subtype, and this subtype has been the template for C4 introductions into C3 crops, like rice, to improve their productivity. However, the ATP cost for the C4 cycle in both NADP-ME and NAD-ME subtypes accounts for > 40% of the total ATP requirement for CO2 assimilation. These high ATP costs, and the associated need for intense cyclic electron transport and low intrinsic quantum yield Φ CO 2 , are major constraints in realizing strong improvements of canopy photosynthesis and crop productivity. Based on mathematical modelling, we propose a C4 ideotype that utilizes low chloroplastic ATP requirements present in the nondomesticated phosphoenolpyruvate carboxykinase (PEP-CK) subtype. The ideotype is a mixed form of NAD(P)-ME and PEP-CK types, requires no cyclic electron transport under low irradiances, and its theoretical Φ CO 2 is c. 25% higher than that of a C4 crop type. Its cell-type-specific ATP and NADPH requirements can be fulfilled by local energy production. The ideotype is projected to have c. 10% yield advantage over NADP-ME-type crops and > 50% advantage over C3 counterparts. The ideotype provides a unique (theoretical) case where Φ CO 2 could be improved, thereby paving a new avenue for improving photosynthesis in both C3 and C4 crops.