Biotechnology: Its Application to Plants

Photosynthesis: Improving Conversion of Sun Energy

David W. Krogmann, professor, Department of Biochemistry, Purdue University, West Lafayette, IN.

The process of photosynthesis is of supreme importance to all of us in that it is the basis for agriculture and the source of biological fuel and building blocks for virtually all living creatures. In addition to providing the food and fiber of this year's crops for this year's needs, photosynthesis has provided, in ancient times, the vast amounts of green plant material that became trapped in the earth and were converted into coal and oil the fossil fuels which provide so much of our energy supply today. Furthermore, photosynthesis has, over the long history of our planet, supplied the oxygen in the atmosphere which makes life possible for us.

Characteristics of Photosynthesis

Several characteristics of the process of photosynthesis should be emphasized. The first is that photosynthesis is a process of enormous magnitude on the global scale. About one hundred billion tons of carbon from the carbon dioxide gas in the atmosphere are converted each year by green plants into the organic molecules which become the substance of newly grown plants.

In addition to uniqueness in magnitude, photosynthesis is unique in the efficiency with which it converts light energy into useful chemical products. Green plants are far more efficient than the photocells which power pocket computers and power some of the devices in satellites and space probes.

Finally, there is a uniqueness in the uniformity of photosynthesis in that the chemical machinery in the leaves of grass and the leaves of oaks is nearly identical. This is of great practical importance since if we learn to regulate the process in some very simple plant like a microscopic alga, we will be able to regulate the process in complex crop plants like corn.

Photosynthetic Energy Conversion

Light energy is collected by an array of pigment molecules which act like a radio or a TV antenna in collecting radiant energy which falls on its surface. The energy is transferred at enormous speed to a processing center.

In your house, antenna collection and energy processing result in sound or a picture. In the plant, the collected energy is used to do chemical work that ultimately results in more plant material. The energy processing occurs in what is called a "reaction center," a complex of proteins and pigments. Light energy collected in the antenna and delivered to the "reaction center" is used to make a chemical change to push an electron away from one molecule into a neighboring molecule. This happens with dazzling speed in less than a pico second (10^-12) which is one millionth of one millionth of a second. A quick reaction occurs when light strikes many molecules, but the usual result is for the electron to bounce back to its point of origin and no chemical change is accomplished.

Photosynthesis is unique in that the energized electron is captured and stabilized with high efficiency. part of the efficiency may come from the series of slightly slower steps that move the electron on to other neighboring molecules. Only recently, with the development of laser technology, could we measure changes that occur so rapidly. Now the pico second measurements are allowing new understanding of the result of light absorption by a molecule. Part of the efficiency may depend on the way in which the complex molecules of the "reaction center" are fitted together.

In 1985, it became clear that we could know the precise architecture of the "reaction center" from breakthroughs in x-ray crystal structure analysis. This analysis will pinpoint the locations of the many thousands of individual atoms in the complex molecules that make up the "reaction center." Like a child looking inside a watch, we can now see all the pieces and how they fit together. When we understand how each piece works, we will not only know why photosynthesis is unique in photoconversion of energy, but we also may hope to improve on the efficiency of human devices like photocells.

In recent years, herbicide-resistant varieties of weeds have appeared. With knowledge of which polypeptide interacts with the herbicide, we have begun, with the new techniques of molecular biology and genetic engineering, to study this polypeptide in more detail. By learning more about how the herbicide poisons this protein, we can design new herbicides to poison the new resistant varieties of weeds. Resistance can be genetically engineered into crop plants to more easily rid them of susceptible weeds.

This is but a single example of the frequent experience of the last few decades. Precise knowledge of the structure and function of individual Molecules gives us great power to Manipulate nature to the best advantage.

Recent research into the process of photosynthesis is helping scientists genetically engineer superior crops and improve crop productivity.

The diagram outlines a pathway to stable products of photosynthetic energy conversion which are used in yet another complex process of conversion of carbon dioxide from the atmosphere into sugar molecules. These molecules can then be converted into all the other kinds of molecules proteins, nucleic acids, fats, and so forth that are the substance of the new plant.

The first step in this CO₂ fixation process is made possible by the enzyme ribulose bis phosphate carboxylase. It is the limiting reagent of photosynthetic CO₂ fixation. If there is more enzyme or if the enzyme works more efficiently, there is more CO₂ fixation and more plant production. It catalyzes the series of reactions indicated in the upper pathway which also use compounds produced by light and which ultimately fix the CO₂ into the stable products of a growing plant. The alternative is for the enzyme to react with O₂ instead of CO₂ as shown in the lower pathway, and the result is a lowering of CO₂ fixation and a decrease in productivity. This undesirable alternative often occurs in crop plants.

Presently we are learning the precise structure and the chemical details of functioning of this enzyme. There is a good possibility that the powerful tools of biotechnology molecular biology and genetic engineering will allow us to regulate the choice between these two alternative reactions and so improve crop productivity at its most fundamental level.