What’s C4 photosynthesis?
Energy conversion in eukaryotes is strictly regulated. In C4 plants, solar energy is converted to chemical energy, driving the carbon fixation engine of C4 photosynthesis—a complex network of finely tuned biochemical reactions, tightly regulated transport networks and underlying regulatory mechanisms . It evolved from the classical C3 pathway, which solely uses ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) as the primary carboxylase for CO2 fixation . During C4 photosynthesis (Fig. 1), CO2 is initially fixed by phosphoenolpyruvate carboxylase (PEPC) in mesophyll cells producing a four-carbon compound that is subsequently transported and decarboxylated in bundle sheath cells, concentrating CO2 around Rubisco, thus maintaining a favourable concentration of CO2 for suppressing rates of photorespiration .
Firstly, it separates two very important and opposite reactions: 1) the carboxylation of the C3 compound, which ‘adds’ CO2 and 2) the decarboxylation of the C4 compound, which ‘removes’ CO2.
Compartmentalisation in C4 plants allows for compounds to be recycled more readily than in C3 plants, thus increasing the efficiency and reducing the costs of metabolism. Secondly, CO2 is placed directly near Rubisco, reducing the unfavourable side reaction with O2. This means that rates of photorespiration are lower in C4 plants and C4 plants have tighter control of water usage and gas exchange. Finally, C4 photosynthesis allows plants to respond better in hotter and drier environments, maintaining high rates of carbon assimilation. In such environments, the efficiency of C4 photosynthesis allows C4 plants to outperform C3 plants.
So what’s the significance of this phenotype?
Why are we interested?
Let’s not jump into conclusions too quickly: C3 plants are not at a complete disadvantage. In fact, we depend on C3 plants, like rice or wheat, for crop production. Their roles in the ecosystem are still highly important and evolution will not be so quick to wipe them out. When C3 photosynthesis first evolved, the environment was completely different to what we know today. With lower levels of O2 in the air, Rubisco wouldn’t interact with O2 so often; but as reactions with CO2 increased, the availability of O2 increased. Steadily shifting a predominantly toxic CO2 environment into the O2 rich air we breathe today. Perhaps evolution didn’t think so far ahead, as it never does.
About 30 million years ago , selection pressures were just right, allowing some C3 plants to evolve C4 photosynthesis. However, not all plant lineages were that ‘lucky’. Rice, for example, is one of the most consumed and the most important crop plant on the planet (maize and wheat trailing right behind). Unlike maize, which is C4, rice is a C3 plant that lacks the modernised capabilities of the former. But what if we could bypass evolution and introduce a C4-augmented rice superplant? Consider a rice field (Fig. 2). Typically a certain amount of rice biomass could be produced when using minimal resources such as fertilisers, water and nutrients. If we increase the efficiency of rice by engineering C4 photosynthesis, given the same minimal resources, we could grow more crop plants without drastically increasing the amount of resources used. It’s like having two aircraft designs: 1) a traditional metal frame aircraft carrying a maximum of 250 PAX with Ntotal mass and 2) a composite aircraft with increased aerodynamics and fuel-efficient engines , carrying the same maximum mass as aircraft 1. Although flight time would not change, assuming equal navigational rules, aircraft 2 will consume less fuel, because the aircraft components allows the plane to be more fuel-efficient in the air. In relation to C4-augmented C3 crops, the aim is so find the best ratio between growing costs, production yields and profit. If we look back at the airplane example, a more efficient plane may be adapted to carry more passengers or cargo for the same route or pushed to travel longer distances, as long as the ratio between operational costs and net gain remain profitable.
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