Plants using C4 photosynthesis possess a number of anatomical, cellular, and biochemical adaptations that distinguish them from C3 ancestors. These include decreased vein spacing (A) and enlarged bundle sheath (BS) cells, which lie adjacent to veins (B). Together, these adaptations decrease the ratio of mesophyll (M) to BS cell volume. C4 metabolism is generated by the increased abundance and M or BS-specific expression of multiple enzymes (shown in purple), which are expressed in both M and BS cells of C3 leaves. Abbreviations: ME–Malic enzymes, RuBisCO—Ribulose1-5,Bisphosphate Carboxylase Oxygenase, PEPC–phosphoenolpyruvate carboxylase, PPDK–pyruvate,orthophosphate dikinase.