C3, C4, and CAM photosynthesis.

Is C4 Rice the next Green Revolution?

C3, C4, and CAM refers to the different photosynthetic pathways that are present in individual plant species and are the result of adapting to various climatic conditions.

C4 hot season growing grass, kangaroo grass, Themeda triandra. Image — Peter Miles.

C3 and C4 indicates the number of carbon atoms in the sugar molecules produced by the photosynthesis. CAM is Crassulacean acid metabolism in which carbon dioxide CO2 is fixed at night.

Generally, C3 plants are suited to cool, moist conditions, C4 to hot and dry, and CAM to arid conditions.

Terms:

Kranz anatomy or large bundle sheath cells around the veins, found in C4 plants.

Malate, malic acid, CO2 transported as malate to the bundle sheath cells in C4 plants.

Phosphoenolpyruvate (PEP) carboxylase, an enzyme used to fix CO2 in C4 plants and works well in temperatures above 15 degrees C.

Rubisco, an enzyme used in the fixing of CO2 in C3 plants and works well in temperatures below 15 degrees C.

Stomata, leaf openings which are able to regulate diffusion of water vapour, carbon dioxide and oxygen.

C4 plant, drawing of leaf anatomy:

Bundle sheath cells (circle of shaded cells around a vein) Mesophyll cells (unshaded cells surrounding bundle sheath cells) Stoma (openings in base of leaf).

Most plants have C3 photosynthesis, eg. rice, wheat, barley and oats; tropical grasses for example are C4, sorghum, sugarcane and corn (maize); and CAM plants such as pineapple, agave and prickly pear cactus are found in very dry conditions.

The terms C3 and C4 are also often used in describing grasses, such as the increasingly cultivated Australian indigenous grasses, for example the C3 cool to warm season growing species weeping rice grass, wallaby and spear grasses and C4 hot season growth kangaroo grass and windmill grass. Commonly used C4 sports turf grasses are kikuyu, buffalo and couch grasses and grow in hot conditions, while other common turf grasses rye and bent grass are C3 growing in cool conditions.

In C3 or the Calvin-Benson pathway of photosynthesis (named after the biochemists who discovered it in 1950 at the University of California, Berkeley), three carbon atoms sugars are produced in CO2 fixation. The uptake (the leaf stomata are open) and fixation of CO2 along with the production of sugars occurs during daylight, and all 3 actions occur within the mesophyll cells. In this photosynthesis process, CO2 is attached to ribulose bisphosphate, RuBP, forming Rubisco enzyme, a 6-carbon molecule which quickly splits in two, the C3. This is a temperature limited process, functioning better at lower temperatures, <1⁵⁰ C and able to cope with lower light levels, for example when competing for light in a forest.

C4 or the Hatch-Slack pathway of photosynthesis (named after scientists working in the Colonial Sugar Refinery in Brisbane in the 1960s), four carbon atom sugars are produced in CO2 fixation using the enzyme phosphoenolpyruvate (PEP) carboxylase. Carbon fixation still occurs in mesophyll cells but C4 plants also concentrate CO2 in additional cells, the bundle sheath cells. They effectively pump the CO2 as malate (malic acid) into the sheath cells, which enables more efficient respiration due to this higher concentration of CO2, where sugars are produced. Because the mesophyll cells and sheath cells are spatially separate in C4 plants, Kranz anatomy, the stomata on the leaf (pores which can open and close to permit the passage of water vapour, CO2 and oxygen) don’t have to remain open for as long allowing CO2 to enter, reducing water loss through the stomata to the outside atmosphere. The PEP also functions better at temperatures above 1⁵⁰ C. Both of these factors give C4 plants an advantage in hot, dry conditions.

CAM, Crassulacean acid metabolism, photosynthesis produces 4 carbon atom sugars as in the C4 pathway but the CO2 is able to be fixed and stored at night. The CO2 is fixed forming malate in the mesophyll cells and then stored in vacuole compartments in those same cells, until day light when it released into the cytoplasm of the cell and used by the chloroplasts to produce sugars by the C3 Calvin-Benson cycle. The CAM plant’s ability to open its stomata at night to allow entry of CO2 greatly reduces water loss from the plant, as night is when the atmosphere water vapor is at its highest level.

Understanding whether a plant has a C3, C4 or CAM photosynthetic pathway enables species selection in human manipulated environments, such as from establishing Australian Native Grass pastures with both hot and cool season growing grasses to provide for more continuous grazing throughout the year, to an amenity landscape planting with species which will flower or look their best at alternative times, or selecting appropriate agricultural crops for the conditions at hand.

Many field sports, for example football are played in cold winter conditions when the C4 grasses such as kikuyu aren’t actively growing and may be looking a little yellow, in this case the curators will often over sow with C3 rye grass seed, which when germinated gives the oval a bright green look in cool conditions. As an aside, the broad stripes in the turf seen on ovals are made by the large ride-on lawn mowers and the rollers behind the blades. These rollers push the upright rye grass over as they pass, pushing the grass over in alternating directions gives the striped appearance.

C4 plants are able to make 50% more use of sunlight than C3 plants due to their ability to concentrate CO2 and to withstand bright sunlight whilst reducing transpiration of moisture.

If the C4 leaf bundle sheath physical characteristics and biochemical production malate characteristics could be transplanted into a food producing plant such as C3 rice, this would make a significant increase in rice farmers yield around the world.

The C4 Rice Project https://c4rice.com is an international consortium of universities and research organisations attempting to create a new rice, in order to provide small farmers around the world with a means to increase production with which to feed a growing population.

Specifically, they are attempting to introduce Kranz anatomy genes into rice with genome editing tools to edit the DNA and for this the regulatory genes still need to be identified. The biochemistry of the C4 enzyme genes have been identified but more work is needed in turning them all on at the same time.

This is an incredibly complex task but plants have evolved in nature from C3 to C4 photosynthetic pathways independently over 60 times. Researchers are taking encouragement that it must be possible once the mechanism is understood.

A high yielding C4 rice is of such importance to feeding a growing world population it has the potential to be the second green revolution.

The first green revolution, or the third agricultural revolution, occurred from the 1930s and 1960s with an increase in production associated with planting monocultures of high yielding grain varieties, the introduction of chemical fertilizers and pesticides, growing more than one crop each year, along with increased farm mechanization and more irrigation.

The second agricultural revolution occurred in Britain and other countries over the 1700s for many reasons including the discovery of natural deposits of mineral fertilizers (in addition to animal manures as fertilizers) such as sodium nitrate in Chile, Chile saltpeter, a nitrogen fertilizer, the mining of seabird guano containing phosphate and potassium, and the mining of deposits of coprolites containing phosphates. The change in cultural practices from fallow to rotation using clover to add nitrogen to the soil, it is a legume with rhizobium bacteria within root nodules that fix atmospheric nitrogen, and turnips, a deep-rooted vegetable that brought valuable nutrients up to the surface from deeper soil. A legislative change of ownership of land from commons to ownership by an individual helped to increase productivity.

The first agricultural revolution, of course, was the gradual change from a nomadic hunting and gathering lifestyle to settled agriculture, which is thought to have happened around 10,000BC.

The understanding of 3 types of photosynthesis has the potential for many improvements in our horticultural and agricultural practices.

References:

Ermakova, M., Danila, F. R., Furbank, R. T., & Von Caemmerer, S. (2020). On the road to C4 rice: advances and perspectives. The Plant Journal, 101(4), 940–950. Google Scholar.

Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.

Miller, G.T. and Spoolman, S.E. (2016). Living in the Environment, (Ed.19) Canada.: Cengage Learning. Book.

Peter Miles B.Env.Sc. 45 years in Environmental Science, specializing in Wildlife and Conservation Biology. Writes about Animals, Revegetation & Climate Change.

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