Crop improvement, the engineering of plants for the benefit of humanity, is as old as agriculture itself. Some 10,000 years ago, primitive people made the transition from hunting and foraging to cultivating crops. With that switch began the continuous process of improving the plants on which we depend for food, fiber, and feed.
Throughout the millennia, two techniques have been used to improve crops, according to Lawrence Bogorad, a plant molecular biologist at Harvard University. The first is selection, which draws on the genetic variation inherent in plants. The earliest farmers selected plants having advantageous traits, such as those that bore the largest fruit or were the easiest to harvest. Perhaps through some rudimentary awareness that traits were passed from one generation to the next, the choicest plants and seeds were used to establish the next year’s crop. Read Pritish Kumar article for more information.
Natural selection, which determines the survival of species, was now augmented by artificial selection. By selecting and isolating choice plants for cultivation, the early farmers were in essence influencing which plants would cross-pollinate. Through selection and isolation, they were narrowing, yet controlling, the available gene pool for each crop.
Plant remains found in ancient Egypt and Mesopotamia indicate that plant cultivation was already widespread by that time. In earlier ruins of pre-Incan Indian villages in Peru, archeologists have uncovered Lima beans that have seeds nearly 100 times larger than those of wild Limas in the area. This suggests that the Incans obtained their beans from still earlier plant breeders who left no record.
The second technique was breeding. The farmers select two plants and then cross them to produce offspring having the desired traits of both parents. The process was hit or miss, however, since early plant breeders did not understand the genetic transmission of traits and could not predict the likely outcome of a particular cross. Nonetheless, valuable traits did arise that could be selected and maintained in the population.
The physical basis of inheritance—or what happens when two plants are crossed—was not understood until the early 1900s. The key was Gregor Mendel’s breeding experiments in the 1860s, though the importance of his work was not recognized until after his death. Working with peas in his monastery garden in Austria, Mendel deduced that hereditary information is stored in discrete units that we now call genes. Moreover, he reasoned that each trait, such as color, is controlled by two genes, one from the male parent and one from the female parent.
Soon after, other researchers found that genes are transmitted in blocks of 5,000 or so, rather than independently as Mendel had surmised. What Mendel did not know was that genes do not exist separately in the cell; rather, they are linked together on long chromosomes in the cell nucleus. Thus, while the gene is the unit of heredity, the chromosome is the unit of transmission. Each parent contributes half of the chromosome complement to the offspring; in humans, for instance, each parent contributes 23 chromosomes.
In the early 1900s, biologists learned how chromosomes are assorted during cell division—and how that determines the properties of the offspring. They learned how to locate genes on chromosomes, because chromosomes break and rejoin, or cross over, fairly regularly during cell division, leading to new genetic combinations. They also learned that sometimes chromosomes are present in multiple copies or reduced numbers and that this particular dosage affects gene expression.
First Biological Revolution
The foundation of Mendelian genetics enabled plant breeders to cross plants with new precision, carefully manipulating the plant genome to produce new, improved varieties. These breeding techniques have been used to develop higher-yielding varieties, including plants resistant to pests or disease. These improved varieties have contributed to a dramatic explosion in agricultural output. In the past 50 years in the United States, farm productivity has increased two-and-a-half times, while farm acreage has declined 6 percent. One of the most spectacular successes was the development of hybrid corn in the 1930s, which quickly doubled corn yields.
Breeding advances have also meant more food for the rest of the world. In the 1950s and 1960s, Norman Borlaug at the Center for Maize and Wheat Improvement in Mexico developed semidwarf wheat varieties, and the International Rice Research Institute in the Philippines developed similar improved rice varieties. When introduced in the 1960s in India and later in China, the wheat and rice varieties became the basis of the ”Green Revolution,” in which crop yields increased an estimated four to seven times. For these reasons, the introduction of applied genetics to agriculture is sometimes called the first biological revolution.
Use of markers
Selectable and scorable marker genes (SMGs) are indispensable for the selection of transformation events for the generation of GM crops. Among the most highly used selectable markers are kanamycin and hygromycin resistance genes. The major biosafety concerns that are raised regarding SMGs relate to their toxicity or allergenicity and the possibility of horizontal gene transfer (HGT) to relevant organisms and pathogens.
It has been suggested that the transfer of these marker genes to other plants, may result in the development of new unwanted weeds. Neomycin phosphotransferase II (NptII) which is the most commonly used selectable marker is most extensively evaluated for biosafety. The protein had been approved by the Food and Drug Administration (FDA) in 1994. Studies have shown that NptII is non-toxic and it is not expected to result in increased weediness or invasiveness it also does not affect the non-target organisms
Nonetheless, these productivity gains are not due to genetic advances alone. In the United States, half of this gain is generally attributed to the simultaneous improvements in farm management—in cropping practices, farm machinery, and especially in the development of new agricultural chemicals such as pesticides, fertilizers, and herbicides. Similarly, the introduction of improved wheat and rice varieties in South Asia was accompanied by heavy investment in irrigation and agricultural chemicals.
Though agriculture has profited immensely from the improved breeding practices developed from Mendelian genetics, the technology does have its limitations. One problem, as Bogorad described at the convocation, is time. It may take generations and generations to develop the desired strain through selection and breeding. The greatest limitation, however, is simply the available supply of genetic diversity.
As Darwin discovered more than 100 years ago, new species evolve through natural selection. If part of a breeding population becomes isolated, its gene pool becomes more and more distinct from that of the parent population. Often, biological barriers arise that prevent the two populations from interbreeding. Consequently, within each distinct species, genetic variation among individuals decreases.
Because of such natural breeding barriers, the plant breeder in search of useful new variants is confined to members of the same species or closely related species. Compounding the problem, many major crops have been under cultivation for thousands of years, which has led to an increasingly homogeneous gene pool. In some cases, the desired trait is simply not available in the breeding population. “You can breed and breed and breed, and never get the trait you are looking for,” Bogorad said.
For this reason, in many essential crops, we may be reaching what Lowell N. Lewis, director of the California Agricultural Experiment Station, called a “biological roadblock” in the drive for greater productivity. “Yields have started to level off, and in some cases are declining. For these crops, it is no longer simply a matter of sprinkling on a little more fertilizer.”
To Vernon Ruttan, an agricultural economist at the University of Minnesota, the closest analogy to the current situation is the closing of the land frontier in the United States in the 1890s. As land became scarce and expensive, farmers could no longer increase output by simply extending their existing techniques to a new land. Instead, the increased agricultural output became dependent upon improved varieties and agricultural chemicals, which came into use over the years through the work of plant breeders and agricultural scientists.
In essence, these agricultural chemicals became a substitute for land. Now farmers in the more developed countries are beginning to exhaust the potential of these chemical technologies as well. For instance, the application of nitrogen fertilizer once assured a sizeable boost in yield. Now the gains come harder. Corn is one of many examples. From 1954 to 1960, the use of nitrogen fertilizer increased corn yields by two bushels per acre per year. From 1971 to 1980, fertilizer added only half a bushel.
Demand for Food
Meanwhile, as the increase in agricultural productivity slows, the demand for food continues to rise. The United States still produces a surplus of grain, but as Orville Bentley, assistant secretary for science and education of the U.S. Department of Agriculture, stated, “for countries that can produce more than their needs, there are many more that are experiencing food shortages.” According to W. David Hopper, vice president of the South Asia Region of the World Bank, although there has not been as massive famine as the one in India in 1943, hunger and malnourishment are still pervasive worldwide. The Food and Agriculture Organization of the United Nations estimates that some 500 million people are severely undernourished.
The contribution of nitrogen fertilizer to U.S. corn yields during the periods 1954–1960, 1961–1970, and 1971–1980. From W. B. Sundquist et al., “A Technology Assessment of Commercial Corn Production in the United States,” Minnesota.
If the world population continues to grow at 1.8 percent annually, food production will have to at least double in the next 40 years to keep pace with demand. Hopper suspects that the demand for food will double in 30 years; as people become more affluent, they will seek a greater and more varied diet. Moreover, said Bentley, “not only will an increasing number of people need to be fed, but that food must be produced from inferior soil under poor climatic and deteriorating biological conditions.” Existing biological and chemical technologies may not be adequate for the task.
In the short term, for the next 10 years, Hopper predicted that world food production can keep pace with demand if there is a substantial investment in these biological and chemical technologies. In India and China, for instance, improved varieties of wheat and rice are fairly well distributed. These nations, like other developing nations, now need what Hopper called a steady accumulation of the “betters”—better use of irrigation, better use of pesticides and fertilizer, and better agronomic practices.
If both improved biological varieties and supporting technologies are provided, these nations should experience a surge in productivity. But within 20 to 30 years, Hopper warned, the developing nations will also begin to exhaust the potential of these technologies. Unless new biological materials—new varieties—are introduced, there will not be enough food for the world’s population.
Faced with growing populations, many Third World countries urgently need to increase their agricultural output. Yet, introducing new agricultural practices is not a simple task, according to W. David Hopper, vice president of the South Asia Region of the World Bank. Moreover, increased yields will not come simply from introducing improved varieties or cropping practices. There must also be social and economic incentives for the farmer to adopt these new agricultural technologies. In short, the new agricultural technologies must be profitable, even for farmers who practice collective agriculture.
There must also be an organizational structure in the country that will support the adoption of new practices. For example, there must be a source of fertilizers, pesticides, and farm equipment, and irrigation must be available. The farmer must also have a market where he can sell his product. And there must be a transportation system linking all of these.
In the 1950s, international development agencies tended to neglect one or more of these components, Hopper said. Yet all three—the agricultural technologies, economic incentives, and infrastructure—coalesced in the 1960s in India and China, Hopper said. Norman Borlaug’s improved wheat varieties were introduced, as were the advanced rice varieties of the International Rice Research Institute. These were accompanied by a major investment by the development agencies that allowed the expansion of irrigation systems and the widespread use of agricultural chemicals. The result was the “Green Revolution.”
The rest of the world has not been so fortunate, Hopper said. Many African nations, for example, are still “desperately short” of techniques for working with their soils. Genetic engineering can play a major role in developing new varieties suited for these conditions and could direct the future course of agriculture in developing nations. Yet attention must also be given to the supporting technologies that will make these new varieties more productive than the traditional techniques.
Working with existing gene pools, plant breeders must somehow develop varieties that are higher yielding, more nutritious, adapted to harsh environments, less costly to farm, and perhaps resistant to pests and disease. That is where molecular biology and genetic engineering hold great promise.
Genetic engineering enables molecular biologists to reshuffle genes in combinations not possible in nature, opening up a vast new source of genetic diversity for crop improvement. “One of the most remarkable achievements of genetic engineering and molecular biology is that we now operationally have a kind of world gene pool,” Bogorad explained. “Darwin aside, speciation aside, we can now envision moving any gene, in principle at least, out of any organism and into any organism.”
In some cases, gene transfer will entail combining the genes of two plants, as do today’s plant breeders—but without the limitations of working with the whole plant. Although Mendelian genetics eliminated much of the guesswork in classical breeding, there is still an element of trial and error: when two entire genomes are combined in a sexual cross, the breeder cannot be certain of the outcome. He may be breeding for one trait, controlled by one gene, but the hundreds of thousands of other genes in each plant complicate the task.
By contrast, the molecular genetic engineer can pluck that single gene from the donor plant and insert it into the recipient, leaving the extraneous genes behind. That specificity also brings a saving in time. Through gene transfer, an improved variety can be created in a single experiment, in one generation. Yet, using conventional techniques, it takes repeated backcrosses to eliminate the unnecessary genes and thus many generations and several years to create an improved variety.
Moreover, the genetic engineer in search of a gene for pest resistance, heat tolerance, or another trait is no longer constrained by the natural breeding barriers—he can select from any species. Eventually, the genetic engineer may also select from outside the plant kingdom, borrowing genes from animals or bacteria. A recent experiment demonstrated that such transfers are indeed possible: a gene for antibiotic resistance was transferred from a bacterium into a petunia plant, where it conferred resistance on the plant. Eventually, it may be possible to transfer the genes for nitrogen fixation from bacteria to plants, thereby reducing the need for fertilizer.
Molecular genetic engineering is still in its infancy. It is too early to gauge the impact it will have on agriculture and crop improvement. As Ruttan explained, “It took 30 years to make the transition from getting most of our productivity growth by bringing new land into production to beginning to get it from the old biological technology—the first biological revolution. The question of whether and when the second biological or biotechnological revolution will reverse the current productivity decline is still unanswered.”
Some of the simpler new techniques, based on the ability to regenerate plants from cells in culture, are already offering a shortcut in selection and breeding for some plants. These techniques are generally known as somatic cell genetics, as they involve the manipulation of cells, as opposed to genes or whole plants, for crop improvement.
Transfer of genetics
Gene-transfer techniques are far less accessible than somatic cell genetics. Their successful application will depend upon breakthroughs in the understanding of gene expression and regulation, as well as increased knowledge of plant physiology, biochemistry, and development. It is also too early to judge how plants will respond to such manipulation. For those reasons, Ruttan and others have predicted that, even with the much-needed increase in research, the impact of biotechnology will be small until the late 1990s.
Though their specific applications cannot be predicted, these new genetic engineering techniques seem likely to become powerful adjuncts to conventional breeding practices. Ultimately, their success will depend on how well they can be integrated with conventional technologies. Molecular biologists will need to work closely with plant breeders to identify promising projects for genetic engineering.
When a new variety is developed in the laboratory, it will face the same scrutiny as any new variety; it will need to undergo lengthy evaluations in the field. It must perform, offering an advantage in quality, yield, time, or cost, if the farmer is to adopt it. For many major crops, sophisticated and effective breeding strategies already exist; it is unlikely that these new gene-transfer techniques will supplant them. Instead, they may offer the greatest advantage in the engineering of crops that are difficult to manipulate by conventional techniques.
The new genetic technologies will undoubtedly aid agriculture in ways that cannot be anticipated now. Cell culture techniques, for instance, are providing a valuable supply of genetic diversity that was unexpected when work began a few years ago.
Conclusion and research
The greatest impact of these new technologies, however, may be in elucidating the basic biology of plants. Though this work is just beginning, gene-transfer techniques are already proving an invaluable tool for exploring the structure, function, and control of genes. This new knowledge can then be used to devise more direct, and thus quicker, breeding strategies—either at the whole plant, cellular, or molecular level.
The vast potential of genetic engineering does not diminish the need for other advanced research. As Lowell N. Lewis pointed out, increasing agricultural output in developing nations will require more research on the biology and ecology of tropical food plants, as well as the pests and diseases that plague them. It will entail bringing underutilized plants into production and a continued search for new, valuable germplasm.
Thus, it can be concluded that sustainable integration of conventional agricultural practices with modern biotechnology can enable the achievement of food security for present and future generations. However, the performance of a GM crop must be closely scrutinized for several generations under field conditions and goes through rigorous bio-safety assessments on a case-by-case basis, before being released for commercial cultivation. GM crops are going to be an essential part of our life and the enormous potential of biotechnology must be exploited to the benefit of humankind.