C4 Rice: The rice of future

C₄ Rice: The rice variety of future

a-General about C₃, C₄ and CAM plants

In the nature, plants or trees have three types of Photosynthesis reactions. Depending on type of photosynthesis plants or trees are called C₃ Photosynthesis plants, C₄ Photosynthesis plants and CAM Photothesis plants

C₃ Photosynthesis plants or C₃ plants

Plants which use only the Calvin cycle for fixing the carbon dioxide from the air are known as C3 plants. In the first step of the cycle CO₂ reacts with RuBP to produce two 3-carbon molecules of 3-phosphoglyceric acid (3-PGA). This is the origin of the designation C3 or C₃ in the literature for the cycle and for the plants that use this cycle.

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Calvin circle

This is the most common form of photosynthesis seen in nature, and most plants regularly undergo this process. Under normal conditions of light, moisture and temperature, this is the type of photosynthesis that is most common and produces energy in the most efficient manner. RUBISCO is the enzyme in plants that is responsible for getting the carbon dioxide out from the atmosphere, and in the case of C₃

photosynthesis,  RUBISCO collects the CO₂ during the daytime. Around 75% of the carbon dioxide captured is effectively converted into energy here, and the name is derived from the fact that the carbon dioxide is converted into a 3-carbon compound. 

About 85% of plant species are C₃ plants. They include the cereal grains: wheat, rice, barley, oats. Peanuts, cotton, sugar beets, tobacco, spinach, soybeans, and most trees are C₃ plants. Most lawn grasses such as rye and fescue are C₃ plants.

C₃ plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration. When the CO₂ concentration in the chloroplasts drops below about 50 ppm, the catalyst rubisco that helps to fix carbon begins to fix oxygen instead. This is highly wasteful of the energy that has been collected from the light, and causes the rubisco to operate at perhaps a quarter of its maximal rate.

All rice varieties on the world now are C₃ plants. They have lower yield of bio mass and calories than C₄ plants such as corn and shorgum.

C₄ Photosynthesis plants or C4 plants

Alternately, in this case the carbon dioxide is converted into a 4-carbon compound, hence the name. Since this photosynthesis occurs in conditions of extreme light and heat, most desert plants and shrubs in hot regions use this method to create energy. Since water is scarce in the desert, the Water Use Efficiency (WUE) ratio is better in such plants. The underlying concept here is that carbon dioxide is collected at a faster rate, so the transpiration levels are also low. This occurs due to the presence of PEP Carboxylase, an enzyme that such plants release for this very purpose itself. The stomata are open during the day in this case as well, and the special enzyme then delivers the CO₂ to the RUBISCO. 

C4 Photosynthesis

CAM Photosynthesis plants or CAM plants

This is the last type and the process is a very different one in this case. CAM implies Crassulacean Acid Metabolism, and in this process the carbon dioxide is actually stored as an acid before it is used for the process of conversion into energy. In this case, the PEP Carboxylase enzyme collects CO₂ in the night, so that the level of transpiration is as low as possible. The most recognizable plant that uses this type of photosynthesis is the cactus, and the enzymes store the carbon dioxide as an acid during the night to prevent water loss, and then carry out the conversion in the daylight. The control over the stomata is also very precise in this case, so they can keep their pores closed whenever they want. 

CAM plants only open the stomata at night, reducing their intake of CO₂ . Because of this, these plants take in CO₂ and incorporate it into C₄ organic acids using a PEP carboxylase enzyme. These acids are stored in vacuoles until needed in the morning, when stomata close and CO₂ is released into the C₃ Calvin cycle for carbon fixation and the production of glucose.

C4 and CAM plants

So these were the 3 different types of photosynthesis reactions that are carried out in nature. Each of them are carried out by plants that have adapted to different weather and temperature conditions around them, and have learned to make the most efficient use of their resources. This is the main reason why plants can survive in any condition and perform photosynthesis to create energy for them to survive.

b-C₄ Rice: the rice of future

The C₄ rice project is about using cutting edge science to discover the genes that will supercharge photosynthesis, boost food production and improve the lives of billions of poor people in the developing world.

The C₄ consortium is a group of multidisciplinary scientists from advanced institutions around the world. The Bill and Melinda Gates Foundation provides funds to the International Rice Research Institute to lead the consortium on this exciting, but urgent voyage of discovery.

Problem and Urgency

Currently, a billion people live on less than a dollar a day and spend half their income on food, 854 million people are hungry and each day about 25000 people die from hunger-related causes. 

Sixty percent of the world’s population lives in Asia where each hectare of land used for rice production currently provides food for 27 people, but by 2050 that land will have to support at least 43 people Climate change will likely result in more extreme variations in weather and cause adverse shifts in the world’s existing climatic patterns. Water scarcity will grow. The increasing demand for biofuels will result in competition between grain for fuel and grain for food, resulting in price increases. Furthermore, more than 75% of the world’s people will live in cities, whose populations will need to be largely supported by a continuous chain of intensive food production and delivery. However, growth in production is slowing. The elite rice cultivars, which dominate the food supplies of the millions of poor people in Asia, have approached a yield barrier, plant breeding seems to have exploited all of the intrinsic high yield-linked genes Ultimately insufficient yields of rice produce food insecurity, unsustainable agricultural practices, environmental degradation and social unrest This vicious cycle must be replaced by a virtuous cycle where raised productivity improves food security so that investments in sustainable agriculture are attractive; then the environment is protected.

The Solution

What technology could simultaneously solve those problems and prevent the bleak future outlined above from becoming a reality? Innovative research at IRRI suggested that the solution to the challenges ahead for rice would require solar energy to be used more efficiently in photosynthesis.Fortunately, there is one example from evolution of a supercharged photosynthetic mechanism; the C₄ system. 
Converting the photosynthetic system in rice to the more efficient, supercharged C₄ one used by maize would increase rice yields while using scarce resources (land, water, fertilizer) more effectively. However a technological innovation of this magnitude requires the skills and technologies of a global alliance of multidisciplinary partners from advanced institutions. In 2008, IRRI formed the International C₄ Rice Consortium.

What is C4 Rice?

Agriculture is the indispensable base of human society and the nature and productivity of agriculture is determined by land, water, climate, management and agricultural research. Only 29% of the earth’s surface is land and only a little over a third of that is suitable for agriculture; the rest is ice, desert, forest or mountain and is unsuitable for farming. More simply stated, only 10% of the surface of the earth has topographical and climatic conditions suitable for producing the food requirements of human beings. Today, 75% of the world’s 6.6 billion people live in the developing world where most of the world’s existing poverty is concentrated.

Currently, about a billion people live on less than a dollar a day and spend half their income on food; 854 million people are hungry and each day about 25,000 people die from hunger-related causes. Sixty percent of the world’s population lives in Asia, where each hectare of land used for rice production currently provides food for 27 people, but by 2050 that land will have to support at least 43 people. Nonetheless, the area for rice cultivation is continually being reduced by expansion of cities and industries, to say nothing of soil degradation. Climate change will likely result in more extreme variations in weather and cause adverse shifts in the world’s existing climatic patterns.

Water scarcity will grow; the increasing demand for biofuels will result in competition between grain for fuel and grain for food resulting in price increases. Furthermore, more than 75% of the world's people will live in cities, the populations of which will need to be largely supported by a continuous chain of intensive food production and delivery.

All of these adverse factors are growing now, at a time when the growth in rice production has slowed as efficient farmers have approached yield limits. Research shows that current maximum rice yields are close to a fundamental yield barrier shaped by the efficiency of solar energy conversion.

How will the required increases in yield be achieved? Solar energy captured in photosynthesis over the duration of a crop gives it the capacity to grow. There is now a growing body of scientific opinion, that the only way to achieve the rice harvests needed for the future is to change the biophysical structure of the rice plant, making it a much more efficient user of energy from the sun. Plants use solar radiation to grow—to develop leaves, roots, stems, flowers, and seeds in a process known as photosynthesis. Rice has what is known as a C₃ photosynthetic pathway, less efficient than that of maize, which has a C₄ pathway. Taking a lesson from evolution and converting a plant from C₃ to C₄ would involve a rearrangement of cellular structures within the leaves and more efficient expression of various enzymes related to the photosynthetic process. However, all the components for C₄ photosynthesis already exist in the rice plant, but they are distributed differently and are not as active.

The costs of the project were estimated to be about $5m per year. In October 2008, The Bill & Melinda Gates Foundation awarded IRRI a grant of $11.1M to begin research on the first 3 year phase of the C₄ Rice Project.

Science of C₄ Rice:  C₄ Photosynthesis

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.

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

IRRI 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.
Value proposition:

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.

Preferences:

1-C4-Rice- http://irri.org/partnerships/networks/c4-rice

2-C4 Rice irri.org/partnerships/networks/c4-rice 

3-Rice Policy - C4 Rice Project solutions.irri.org/index.php?option=com...

4-International C4 Rice Consortium www.csiro.au/en/...asp...