Best practice procedures for the establishment of a C4 cycle in transgenic C3 plants

C-4 plants established a mechanism for the concentration of CO2 in the vicinity of ribulose-1,5-bisphosphate carboxylase/oxygenase in order to saturate the enzyme with substrate and substantially to reduce the alternative fixation of O-2 that results in energy losses. Transfer of the C-4 mechanism to C-3 plants has been repeatedly tested, but none of the approaches so far resulted in transgenic plants with enhanced photosynthesis or growth. Instead, often deleterious effects were observed. A true C-4 cycle requires the co-ordinated activity of multiple enzymes in different cell types and in response to diverse environmental and metabolic stimuli. This review summarizes our current knowledge about the most appropriate regulatory elements and coding sequences for the establishment of C-4 protein activities in C-3 plants. In addition, technological breakthroughs for the efficient transfer of the numerous genes probably required to transform a C-3 plant into a C-4 plant will be discussed.