Tweaking photosynthesis
(appeared in Sep 2014)

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The same numbers of plants that we have could be nudged to produce more food, says S.Ananthanarayanan.

Burgeoning population, pollution, land degradation and encroachment, all point to crises of food supply in coming decades. Better methods, better technology, are hesitantly suggesting solutions. Spacing of plants, crop rotation, control of fertilizer, getting plants to grow for longer hours, would all do their bit. But revving up the process of photosynthesis, the engine of plants to produce food in the first place, would multiply the benefits of the other ways to do better.

Myat T. Lin, Alessandro Occhialini, P. John Andralojc, Martin A. J. Parry and Maureen R. Hanson, from Cornell University and Rothamsted Research, UK, report in the journal, Nature, that they have made a change in the genetic factor which is the key to photosynthesis in the tobacco plant, a change that makes the plant more efficient in using the carbon in the air to make food, with the help of sunlight. Tobacco is a typical plant often used for research and there are prospects of transferring higher efficiency in tobacco to other, food producing plants.

Efficiency of photosynthesis

The capacity of plants to generate carbohydrates from CO2 and water, and releasing oxygen at the same time, with the help of sunlight, has come to plants from ancient precursor cells, called cyanobacteria. The very early atmosphere of the earth was highly deficient in oxygen and was not suitable for life-forms, as we know them, to arise. The life forms that flourished were many that are oxygen intolerant, and then there were the single-celled cyanobacteria, which had the apparatus for photosynthesis. It is thought that it was this group of organisms that added to the atmosphere its content of oxygen, by pulling the carbon out of CO2. In the early earth, any free oxygen created was absorbed by iron, to form oxides, or by organic matter and oxygen intolerant organisms could survive. About 2.3 billion years ago, these oxygen sinks got saturated and free oxygen began to build up. Many life-forms got extinct, except for a few which survive only in the harshest, oxygen free corners of the earth - but with the rise in available oxygen, oxygen consuming organisms multiplied and thrived. Great biodiversity and complexity emerged and continued to rise till there was a balance in the level of free oxygen.

In this growth of oxygen dependant life forms, like present day plants, the cells of the new organisms made use of portions of the existing cyanobacteria to generate an enzyme called Rubisco, which gives the cells the capacity of photosynthesis. But a problem with Rubisco, which plants picked up, is that the mechanism had evolved in an oxygen-free environment, and the enzyme does not have a way to tell the difference between oxygen (O2) and CO2. Thus, while the useful function of photosynthesis in plants is to loosen the C fromCO2 and form carbohydrates, in the presence of oxygen, plants also expend resources in separating the two Os in O2.This makes the process of photosynthesis in plants, while astonishing in the sophistication of light gathering and optimizing energy, rather wasteful in using some of the energy in a counterproductive way.

Two ways have evolved for plants to get around this problem. One is with a version of Rubisco which does discriminate between O2 and CO2, but this version is 30% less efficient at fixing CO2.The other way is to keep the first version of rubisco, but with a device called a CO2 concentrating mechanism, CCM, to increase the level of CO2 around the enzyme. Most plants which are regarded as crops have followed the first way, and thus they have a built-in limitation. One way out may be to manipulate the CO2 levels around the leaf, which may be something like a CCM. Plant biologists are hence engaged in trying to get a CCM into crops, which would bring about a direct jump in the plant output.

But in some plants, mostly weeds, microalgae and in cyanobacteria, CCMs have evolved on their own and these organisms maintain high efficiency of photosynthesis. Cyanobacteria, in fact, may be the most successful microorganisms in the world. They are found nearly everywhere and in large quantity. They cover thousands of square kilometers in the sea, as blue-green algae, and account for most of the cleaning of the atmosphere of CO2. They are in the soil and because they also fix atmospheric nitrogen, which is inert and useless for plants, into the reactive form that plants need, they are the largest source of natural fertilizer for all vegetation.

The way CCMs work in cyanobacteria is by modifying the environment within the cell, so that CO2 concentrates near where the Rubisco enzyme is found. In this way, action on oxygen is greatly eliminated and Rubisco hence works efficiently. With this mechanism in place, cyanobacteria have not needed to make any changes in the structure of Rubisco and they retain the ancient form, which is almost three times more efficient than the form found in crops.

Efforts to introduce CCMs, as are found in some plants and also cyanobacteria, have not been successful. The current team used genetic engineering to get modify the DNA of tobacco plant cells involved in photosynthesis, so that the cells created a basic form of the cyanobacteria Rubisco. The team carried this out with the refinement, which had not been tried in earlier, unsuccessful attempts, of also introducing proteins that are involved in the assembly of Rubisco. They found that using particular helper proteins like this seemed to created structures, within the bodies that are active in photosynthesis, that were related to the factors that caused concentration of CO2 in cyanobacteria.

The resulting tobacco plants showed clear higher competence in photosynthesis, equally with both kinds of supporting proteins. But the method, basically of introducing the higher output enzyme, appears to have effectively increased the capacity of the tobacco plant to generate carbohydrates using sunlight. The tobacco is typical of plants that are the major crop plants and further research promises more effective CCM, and later transfer to more economically important plants.

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