Evolution of the C4 photosynthetic mechanism: are there really three C4 acid decarboxylation types?

Some of the most productive plants on the planet use a variant of photosynthesis known as the C-4 pathway. This photosynthetic mechanism uses a biochemical pump to concentrate CO2 to levels up to 10-fold atmospheric in specialized cells of the leaf where Rubisco, the primary enzyme of C-3 photosynthesis, is located. The basic biochemical pathways underlying this process, discovered more than 40 years ago, have been extensively studied and, based on these pathways, C-4 plants have been subdivided into two broad groups according to the species of C-4 acid produced in the mesophyll cells and into three groups according to the enzyme used to decarboxylate C-4 acids in the bundle sheath to release CO2. Recent molecular, biochemical, and physiological data indicate that these three decarboxylation types may not be rigidly genetically determined, that the possibility of flexibility between the pathways exists and that this may potentially be both developmentally and environmentally controlled. This evidence is synthesized here and the implications for C-4 engineering discussed.