Evolutionary transition from C₃ to C₄ photosynthesis in plants

Article By:

Slewinski, Thomas L. Monsanto Company, Chesterfield, Missouri.

Howell, Miya D. Monsanto Company, Chesterfield, Missouri.

Last reviewed:2014

DOI:https://doi.org/10.1036/1097-8542.YB150914

Photosynthesis in terrestrial plants is undergoing a dramatic transformation. Photosynthesis originated in bacterial species in a prehistoric environment under high carbon dioxide (CO₂) and low oxygen (O₂) conditions, which was an environment very different from the recent past and current conditions on Earth. Most plant species today still utilize the ancestral C₃ photosynthetic mechanism acquired through symbiosis with chloroplast (cell plastid)–transformed bacteria. In the C₃ system, Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase, which is the primary enzyme that feeds carbon into the metabolic Calvin cycle) directly fixes CO₂ from the atmosphere to produce phosphoenolpyruvate, a three-carbon compound. However, Rubisco is an inefficient catalyst for CO₂ fixation, only turning over about twice per second when at maximum capacity. Additionally, Rubisco has oxygenase activity leading to a competing reaction, thereby producing a toxic by-product phosphoglycolate when the enzyme processes O₂ instead of CO₂; this process is referred to as photorespiration. Photorespiration undermines the efficiency of photosynthesis in the current high O₂ atmosphere and becomes exacerbated as temperatures increase, dramatically limiting the efficiency of photosynthetic production. Thus, some plants have evolved new methods to reduce this competing oxygenase reaction by excluding O₂ and concentrating CO₂ around Rubisco in order to run the enzyme at full capacity and full carbon fixation efficiency.

The content above is only an excerpt.