Evolutionary transition from C3 to C4 photosynthesis in plants
Slewinski, Thomas L. Monsanto Company, Chesterfield, Missouri.
Howell, Miya D. Monsanto Company, Chesterfield, Missouri.
- C4 photosynthesis
- Evolutionary aspect
- C3 to C4 transition
- Links to Primary Literature
- Additional Readings
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.
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