Plants fix carbon dioxide (CO2) into sugar using sunlight as the source of energy. This fixed carbon makes up the bulk of the plant itself – roots, stems, leaves, flowers - and the sugars or starches that are stored in the seeds or fruits that we harvest for food.
In the majority of plants, including rice, CO2 is first fixed into a compound with three carbons (C3) by the photosynthetic enzyme ribulose bisphosphate carboxylase oxygenase (Rubisco)—this is known as C3 photosynthesis.Rubisco is inherently inefficient because it can also catalyze a reaction with oxygen from the air, in a wasteful process known as photorespiration (rather than photosynthesis). At temperatures above 20°C, there is increasing competition by oxygen (O2), with a dramatic reduction in CO2 fixation and photosynthetic efficiency. While all this is happening, water is escaping from the leaves while the CO2 is diffusing in. Thus, in the hot tropics where most rice is grown, photosynthesis becomes very inefficient.
C3 Photosynthesis: Rubisco
C4 Photosynthesis: PEP Carboxylase then Rubisco
C4 plants are more efficient in carbon dioxide concentration that results in increased efficiency in water and nitrogen use and improved adaptation to hotter and dryer environments.
In nature, this has occurred more than 50 times in a wide range of flowering plants, indicating that, despite being complex, it is a relatively easy pathway to evolve.
Kranz (C4) anatomy arose before the C4 biochemistry within the bundle sheath cell, in response to photorespiration. Therefore, strategies to engineer C4 photosynthesis should first address the introduction of Kranz anatomy into C3 plants.
Improvement on existing crops
Our calculations show that the cost-benefit ratio of C4 rice is likely to be of the same order as the “dwarf-cultivars” produced in the first Green Revolution bringing benefits to hundreds of millions of people in the poorer parts of the world. Inserting the C4 photosynthetic pathway into rice should increase rice yield by 50%, double water-use efficiency, and use less fertilizer to achieve those improvements. No other evolutionary mechanism exists that could be added to C3 rice that could deliver that superior combination of benefits.
Poverty alleviation would be further magnified if the C4 syndrome were added to other C3 crops, such as wheat, growing in the hot countries of the developing world.
Increased water use efficiency. C4 rice would need less water because water loss will be reduced and the water used more efficiently. C4 plants would have the pores in the leaves (stomata) partially closed during the hottest part of the day. Also C4 plants absorb more CO2 per unit of water lost. C4 plants are able to do this because of the compartmentalization and concentration of CO2 that occurs in the bundle sheath cells.
Increased nitrogen use efficiency. C4 rice would increase nitrogen-use efficiency by 30% because the plant will need lower amounts of Rubisco, an abundant enzyme that fixes CO2 into sugars. By requiring less Rubisco for the same amount of CO2 fixed, C4 rice can achieve the same productivity with fewer enzymes, which means less nitrogen. (enzymes and proteins contain 15% nitrogen).
Yield benefits. Models show that increased water and nitrogen use efficiencies and other characteristics would support yield increases of 30% to 50% based on comparative studies between rice and maize.