Have you ever wondered where our agricultural crops come from? And what were they like thousands of years ago, or hundreds of years ago? Our food crops today are in fact very different from the original wild plants from which they were derived.
About 10,000 years BC, people harvested their food from the natural biological diversity that surrounded them, and eventually domesticated crops and animals. During the process of domestication, people began to select better plant materials for propagation and animals for breeding, initially unwittingly, but ultimately with the intention of developing improved food crops and livestock. Over thousands of years farmers selected for desirable traits in crops, and thus improved the plants for agricultural purposes. Desirable traits included crop varieties (also known as cultivars , from " culti vated var ieties") with shortened growing seasons, increased resistance to diseases and pests, larger seeds and fruits, nutritional content, shelf life, and better adaptation to diverse ecological conditions under which crops were grown.
Over the centuries, agricultural technology developed a broad spectrum of options for food, feed, and fiber production. In many ways, technology reduces the amount of time we dedicate to basic activities like food production, and makes our lives easier and more enjoyable. Everyone is familiar with how transportation has changed over time to be more efficient and safer (Figure 1). Agriculture has also undergone tremendous changes, many of which have made food and fiber production more efficient and safer (Figure 1). For example in 1870, the total population of the USA was 38,558,371 and 53% of this population was involved in farming; in 2000, the total population was 275,000,000 and only 1.8% of the population was involved in farming. There are negative aspects to having so few members of society involved in agriculture, but this serves to illustrate how technological developments have reduced the need for basic farm labor.
Figure 1: A timeline showing how human transportation systems have evolved.
A timeline showing how human transportation systems have evolved, from primitive, slow, and inefficient vehicles, to modern, faster, and more efficient options. Corresponding advances in agricultural biotechnology are shown below, similarly illustrating how advances changed our ability to develop new agricultural crops.
© 2012 Nature Education Courtesy of Ania M. Wieczorek and Mark G. Wright. All rights reserved. View Terms of Use
This article concentrates on how scientific discoveries and technological developments have allowed us to improve crop development in agriculture. Most people do not realize that among early agriculture developments, really at the genesis of agricultural technology, the ancient Egyptians made wine and made rising dough for bread, using fermentation. A significant event in the development of agriculture occurred in 1492 with the introduction of corn, native to the Americas, to the rest of the world, and European growers adapted the plant to their unique growing conditions. At this stage of history, crops were being transported around the world and grown under a diversity of conditions.
Agriculturalists started conducting selective breeding of crops before having a thorough understanding of the basis of genetics. Gregor Mendel's discoveries explaining how traits pass from parents to offspring shed new light on the matter. Mendel's work showed that genes separate during the formation of gametes , and unite randomly during fertilization ; he also showed that genes are transmitted independently of one another to offspring. This understanding of the way that plants and animals acquire traits form parents created the potential for people to selectively breed crops and livestock. Gregor Mendel's discovery revolutionized agriculture by launching the development of selective cross breeding with a comprehensive understanding of the underlying mechanisms of inheritance.
Selective Cross Breeding
In traditional plant breeding, new varieties are developed either by selecting plants with desirable characteristics or by combining qualities from two closely related plants through selective breeding. These features may for example be resistance to a particular pest or disease, or tolerance to climatic conditions. Pollen with the genes for a desired trait is transferred from plants of one crop variety to the flowers of another variety with other desirable traits. Eventually, through careful selection of offspring, the desired trait will appear in a new variety of plants. Traditional plant breeding has produced numerous highly successful new varieties of crops over the centuries. There have also been many less than successful crosses made. In traditional breeding, crosses are often made in a relatively uncontrolled manner. The breeder chooses the parents to cross, but at the genetic level, the results are unpredictable. DNA from the parents recombines randomly, and desirable traits such as pest resistance may be bundled with undesirable traits, such as lower yield or poor quality. The parent plants must be closely related to produce offspring. Traditional breeding programs are time-consuming, often taking decades to produce new viable crop varieties, and labor-intensive. A great deal of effort is required to separate undesirable from desirable traits, and this is not always economically practical. Many potential benefits are lost along the way, as plants that fail to demonstrate the introduced characteristics are discarded. Traditional plant breeding takes on average 12-15 years to produce a new crop variety.
Classical Breeding with Induced Mutation
Mutations (Figure 2) are changes in the genetic makeup of a plant. Mutations occur naturally and sometimes result in the development of new beneficial traits. In 1940, plant breeders learned that they could make mutations happen faster with a process called mutagenesis. Radiation or chemicals are used to change the plant's DNA, thebasic molecular system of all organisms' genetic material. The goal is to cause changes in the sequence of the base pairs of DNA, which provide biochemical instructions for the development of plants. Resultant plants may possess new and desirable characteristics through this modification of their genetic material. During thisprocess, plant breeders must grow and evaluate each plant from each seed produced.
Figure 2: The effects of genetic mutations in carrots.
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More than 2,500 plant varieties (including rice, wheat, grapefruit, lettuce and many fruits) have been developed using radiation mutagenesis (FAO/IAEA, 2008) . Induced mutation breeding was widely used in the United States during the 1970's, but today few varieties are produced using this technique. As our understanding of genetics developed, so new technologies for plant variety development arose. Examples of these that are used today include genetic marker assisted breeding, where molecular markers associated with specific traits could be used to direct breeding programs, and genetic engineering. Some of the significant steps leading to the current state of the art are explained below.
1. Discovery by Watson and Crick: structure of DNA, 1953: Another milestone in the development of understanding of genetics and how genes function, was the discovery of the structure of DNA (the basis of genes), and how DNA works. Two scientists, James Watson and Francis Crick made this discovery (Pray 2008), considered to be one of the most significant scientific works in biology, largely through synthesis of the work of other scientists. Their work contributed significantly to understanding what genes were.
2. Discovering genes that move (transposons): Transposons are sections of DNA-genes-that move from one location to another on a chromosome . Transposons have been referred toas "jumping genes", genes that are able to move around. Interestingly, transposons may be manipulated to alter the DNA inside living organisms. Barbara McLintock (1950) discovered an interesting effect of transposons. She was able to show how the changes in DNA caused by transposons affected the color of maize kernels.
3. Tissue culture and plant regeneration: Another significant development in technology that was important for plant breeding was the development of micropropagation techniques, known as tissue culture (Thorpe 2007). Tissue culture permits researchers to clone plant material by excising small amounts of tissue fromplants of interest, and then inducing growth of the tissue on media, to ultimately form a new plant. This new plant carries the entire genetic information of the donor plant. Exact copies of a desired plant could thus be produced without depending on pollinators, the need for seeds, andthis could all be done quickly.
4. Embryo rescue: Often when distantly related plant species are hybridized are crossed, the embryos formed following fertilization will be aborted. The development of embryo rescue technology permitted crop breeders to make crosses among distantly related varieties, and then to save the resulting embryos and then grow them into whole plants through tissue culture.
5. Protoplast fusion: Protoplasts are cells that have lost their cell walls. The cell wall can be removed either by mechanical means, or by the action of enzymes. They are left with only a cell membrane surrounding the cell. Protoplasts can be manipulated in many ways that can be used inplant breeding. This includes producing hybrid cells (by means of cell fusion) and using protoplasts to introduce new genes into plant cells, which can then be grown using tissue culture techniques (Thorpe 2007).
6. Genetic engineering: Building on the above discoveries into the 1980s, advances in the field of molecular biology provided scientists with the potential to purposefully transfer DNA between organisms, whether closely or distantly related. This set the stage for potentially extremely beneficial advancement in crop breeding, but hasalso been very controversial.
Genetic Engineering of Organisms
The basic structure of DNA is identical in all living things. In all organisms, different characteristics are determined by the sequence of the DNA base pairs. Biotechnology has developed to the point where researchers can take one or more specific genes from nearly any organism, including plants,animals, bacteria, or viruses, and introduce those genes into the genome of another organism. This is called recombinant DNA technology (Watson etal . 1992). In 1978, the first commercial product arising from the use of recombinant DNA technology gene transfer was synthetic insulin . Pig and cattle pancreatic glands were previously the only way of producing insulin for human use. In 1988, chymosin (known as Rennin) was the first enzyme produced from a genetically modified source-yeast-to be approved for use in food. Previously this enzyme for cheese production was obtained from cows' stomach linings.
In agricultural biotechnology, changes are made directly to the plant's genome. Once the gene that determines a desirable trait is identified, it can be selected, extracted, and transferred directly into another plant genome (Figure 3). Plants that have genes from other organisms are referred to as transgenic. The presence of the desired gene, controlling the trait, can be tested for at any stage of growth, such as in small seedlings in a greenhouse tray. A breeder can thus quickly evaluate the plants that are produced and then select those that best express the desired trait. Producing new varieties of cropsthrough genetic engineering takes about 10 years on average.
Figure 3: The process of genetic engineering in papaya.
The process of genetic engineering in papaya. This is the first fruit tree ever genetically engineered (for resistance to a virus that kills papaya plants), conducted by Land Grant Universities (University of Hawaii and Cornell University) in the USA, and licensed to the Hawaii papaya industry for use and distribution.
© 2012 Nature Education Reprinted with permission: Land Grant Universities (University of Hawaii and Cornell University) All rights reserved. View Terms of Use
The applications of genetic engineering through recombinant DNA technology increased with time, and the first small scale field trials of genetically engineered plant varieties were planted and in the USA and Canada in 1990, followed by the first commercial release of genetically engineered crops in 1992. Since that time, adoption of genetic engineered plants by farmers has increased annually. While the benefits of genetically engineered crop varieties have been widely recognized, there has been extensive opposition to this technology, from environmental perspectives, because of ethics considerations, and people concerned with corporate control of crop varieties.
Comparing Classical Breeding and Crop Breeding Through Genetic Engineering
Crops produced through genetic engineering are sometimes referred to as genetically modified organisms. The term genetic modification, and so-called genetically modified organisms (GMOs) is frequently misused. All types (organic, conventional) of agriculture modify the genes of plants so that they will have desirable traits. The difference is that traditional forms of breeding change the plant's genetics indirectly by selectingplants with specific traits, while genetic engineering changes the traits by making changes directly to the DNA. In traditional breeding, crosses are made in a relatively uncontrolled manner. The breeder chooses the parents to cross, but at the genetic level, the results are unpredictable. DNA from the parents recombines randomly. In contrast, genetic engineering permits highly targeted transfer of genes, quick and efficient tracking of genes in new varieties, and ultimately increased efficiency in developing new crop varieties with new and desirable traits.
Conclusions: Technology, Progress, Opposition, and Risk Assessment
Many different tools are available for increasing and improving agricultural production. These tools include methods to develop new varieties such as classical breeding and biotechnology. Traditional agricultural approaches are experiencing some resurgence today, with renewed interest in organic agriculture; an approach that does not embrace the use of genetically engineered crops. The role that genetic engineering stands to play in sustainable agricultural development is an interesting topic for the future. As with the development of any new technology there are concerns about associated risks, and agricultural biotechnology is no exception. All crops developed using genetic engineering are subjected to extensive safety testing before being released for commercial use. Risk assessments are conducted for these new varieties, and only those that are safe for human use are released. Some concerns arise through people not fully understanding the reporting of risk. Many consider any level of risk unacceptable. Some prefer the application of the precautionary principle when releasing new technology, but this is not a realistic interpretation of what risk assessments tell us (See information presented by Land Grant Universities of the USA).
Extensive risk assessment and safety testing of crops developed through the use of genetic engineering has shown that there are no varieties in use that pose risks to consumers. This is not to say that new varieties should not be carefully examined for safety; each case should be considered on its unique merits.
References and Recommended Reading
American Association for the Advancement of Science. Annual meeting (2011). ( link )
Land Grant Universities (2011). ( link )
NERC. Can GM crops harm the environment? (2011). ( link )
McLintock, B. The origin and behavior of mutable loci in maize. Proceedings of the National Academy of Sciences of the United States of America 36 , 344–355 (1950).
Pray, L. A. Discovery of DNA structure and function: Watson and Crick. Nature Education Knowledge 1 , (2008). ( link )
Thorpe, T. A. History of plant tissue culture. Molecular Biotechnology 37 , 169–180 (2007).
Watson, J. D. et al . Recombinant DNA , 2nd ed. New York, NY: W. H. Freeman, 1992.
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