C4 photosynthesis: principles of CO2 concentration and prospects for its introduction into C3 plants

C4 photosynthesis has a number of distinct properties that enable the capture of CO2 and its concentration in the vicinity of Rubisco, so as to reduce the oxygenase activity of Rubisco, and hence the rate of photorespiration. The aim of this review is to discuss the properties of this CO2‐concentrating mechanism, and thus to indicate the minimum requirements of any genetically‐engineered system. In particular, the Kranz leaf anatomy of C4 photosynthesis and the division of the C4‐cycle between two cell types involves intercellular co‐operation that requires modifications in regulation and transport to make C4 photosynthesis work. Some examples of these modifications are discussed. Comparisons are made with the C4‐type photosynthesis found in single‐celled C4‐type CO2‐concentrating mechanisms, such as that found in the aquatic plant, Hydrilla verticillata and the single‐celled C4 system found in the terrestrial chenopod Borszczowia aralocaspica. The outcome of recent attempts to engineer C4 enzymes into C3 plants is discussed.