Upside Down Source of Fancy Photosynthesis
During the 1960s and 1970s, improved strains of wheat and rice resulted in a doubling of crop yields. Despite predictions of disaster from some environmentalists, the world continued to feed quickly growing human populations. This green revolution, kick-started by the research of American plant scientist Norman Borlaug and Indian rice geneticist M. S. Swaminathan, provided much higher yielding crops. However for optimum growth, these crops require the widespread application of nitrogen fertilizers and other chemicals. As a byproduct of this practice, a significant amount of fertilizer ends up in natural waterways. As a result, scientists now consider the application of such chemicals as “so last century!” (Nature October 30, 2014 p. S52). The hunt is now on for crops that do not require chemical inputs and yet produce high yields.
This story begins in the early 1960s in Brisbane, Australia where two scientists who were working at the Colonial Sugar Refining Company, set out to discover why sugar cane produces and stores so much sugar. By 1965 they had discovered and described a new biochemical process in plant leaves that results in much more efficient capture of the sun’s energy resulting in enhanced storage of sugar. This new photosynthetic process, called the Hatch-Slack pathway after its discoverers, has been discovered in about 20% of all plant species. Its occurrence however is very patchy. Some species in a taxonomic group may display this capacity and others not. These efficient plants, called C4 plants, grow best at higher temperatures and they manage with less nitrogen inputs and less water. (The terms C3 for normal plants and C4 for efficient plants, refer to the number of carbon atoms in the first product during the photosynthetic process.) Corn, millet, sorghum and sugar cane are all C4 plants, as are many other grasses. Rice however is a C3 plant. Not surprisingly, plant breeders think longingly about how nice it would be if rice were a C4 crop too. Farmers might be able to obtain 30-50% increases in yield with no increases in water, fertilizer or land. But the differences between C3 and C4 plants are major . Two additional chemical reactions are required before photosynthesis actually begins and some anatomical changes are required as well. This is no small research project.
A new initiative makes use of a highly surprising source of efficient photosynthesis. Since the 1970s, scientists have known that blue green algae, now called cyanobacteria, exhibit C4 photosynthesis. What they have since discovered is that the most important enzyme (called Rubisco) for short, exists in a much more efficient form in C4 land plants and cyanobacteria. The C4 enzyme does need a higher amount of carbon dioxide present for it to work efficiently, but there are anatomical and biochemical design features that compensate for this need.
In the cyanobacteria, we find tiny carbon concentrating mechanisms which maintain elevated carbon dioxide levels around Rubisco. Thanks to the carbon concentrating mechanisms, cyanobacteria are able to utilize a form of Rubisco that is almost three times as efficient as that found in C3 plants.
In cyanobacteria, special pumps encourage the uptake of bicarbonate ions (HCO3) and carbon dioxide into the cell. All this then enters small structure in the cell called carboxysomes which reconvert the bicarbonate back into carbon dioxide. The Rubisco which is located in the structures, acts on the carbon dioxide which eventually results in lots of sugar. What the English and American researchers want to do, for starters is to insert genes from the blue green algae/cyanobacteria into tobacco plants. This plant is popular as a subject for experiments, the plant equivalent of a guinea pig! Thus the scientists have successfully knocked out the gene for a large component of Rubisco from the tobacco plant, and replaced it with the gene for the cyanobacterial enzyme. They also inserted a gene for a “chaperone” protein which encourages the Rubisco protein to fold properly. If a protein does not fold correctly, it cannot function.
In order to achieve a successful C4 system however in plants like tobacco, scientists will need to add (in addition to the cyanobacterial form of Rubisco), proteins to form the shell of the carbon concentrating structures (carboxysomes) along with the pump proteins and also other proteins which facilitate the conversion of bicarbonate into carbon dioxide at a point adjacent to the Rubisco. These precise and complex requirements mean that scientists do not expect any successful crop plants for many years to come. The functioning system certainly has the hallmarks of intelligent design! Chance processes are not going to make the conversion happen!
It is amusing to reflect on the source of the efficient Rubisco enzyme. We can see that blue green algae/cyanobacteria are highly sophisticated organisms with a fancy photosynthetic apparatus. Yet their outward appearance is uncomplicated and most scientists have long considered that these cells are among the most “primitive” organisms that we know about. Some scientists suggest that cyanobacteria were among the first living cells to appear. It is all the more ironic that scientists would like to improve the efficiency of crops like rice, by inserting a number of genes for C4 photosynthesis from cells that supposedly come from the base of the evolutionary tree. What we actually see from all of this is that photosynthesis is an amazing process which obviously never arose by chance.
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