Plant breeding has been practiced for thousands of years. Domestication, classical plant breeding and genetic engineering are all processes that alter the genome of a plant to enhance its qualities as a crop.
Plant breeding is practiced worldwide by government institutions and commercial enterprises. International development agencies believe that breeding new crops is important for ensuring food security and developing practices of sustainable agriculture through the development of crops suitable for their environment 2.
Domestication of plants is a selection process conducted by humans to produce plants that meet the needs of the farmer and the consumer. The practice is estimated to date back 9,000-11,000 years. Many crops in present day cultivation are the result of domestication in ancient times, about 5,000 years ago in the Old World and 3,000 years ago in the New World. In the Neolithic period, domestication took a minimum of 1,000 years and a maximum of 7,000 years. Today, all of our principal food crops come from domesticated varieties.
A cultivated crop species that has evolved from wild populations due to selective pressures from traditional farmers is called a landrace. Landraces, which can be the result of natural forces or domestication, are plants (or animals) that are ideally suited to a particular region or environment. An example are the landraces of rice, Oryza sativa subspecies indica, which was developed in South Asia, and Oryza sativa subspecies japonica, which was developed in China.
Classical plant breeding
Classical plant breeding uses deliberate interbreeding (crossing) of closely or distantly related species to produce new cops with desirable properties. Plants are crossed to introduce traits/genes from one species into a new genetic background. For example, a mildew resistant pea may be crossed with a high-yielding but susceptible pea, the goal of the cross being to introduce mildew resistance without losing the high-yield characteristics. Progeny from the cross would then be crossed with the high-yielding parent to ensure that the progeny were most like the high-yielding parent, (backcrossing), the progeny from that cross would be tested for yield and mildew resistance and high-yielding resistant plants would be futher developed. Plants may also be crossed with themselves to produce inbreed varieties for breeding.
Classical breeding relies on homologous recombination of two genomes to generate genetic diversity. It also makes use of a number of molecular techniques to generate diversity and produce plants that would not exist in nature.
Traits that breeders' have tried to incorporate into crop plants in the last 100 years include:
- Increased quality and yield of the crop
- Increased tolerance of environmental pressures (salinity, extreme temperature, drought)
- Resistance to viruses, fungi and bacteria
- Increased tolerance to insect pests
- Increased tolerance of herbicides
Before World War II
Intraspecific hybridization within a plant species was demonstrated by Charles Darwin and Gregor Mendel, and was further developed by geneticists and plant breeders. In the early 20th century, plant breeders realized that Mendel's findings on the nonrandom nature of inheritance could be applied to seedling populations produced through deliberate pollinations to predict the frequencies of different types.
In 1908, George Harrison Shull described heterosis, also known as hybrid vigor. Hetrosis describes the tendency of the progeny of a specific cross to outperform both parents. The detection of the usefulness of heterosis for plant breeding has lead to the development of inbred lines that reveal a heterotic yield advantage when they are crossed. Maize was the first species where hetrosis was widely used to produce hybrids.
By the 1920s, statistical methods were developed to analyze gene action and distinguish heritable variation from variation caused by environment. In 1933, another important breeding technique, cytoplasmic male sterility (CMS), developed in maize, was described by Marcus Morton Rhoades. CMS is a maternally inherited trait that makes the plant produce sterile pollen, enabling the production of hybrids and removing the need for detasseling.
These early breeding techniques resuted in large yield increase in the United States in the early 20th century. Similar yeild increases were not produced elsewhere until after World War II, the Green Revolution increased crop production in the developing world in the 1960s.
After World War II
Following World War II a number of techniques were developed that allowed plant breeders to hybridize distantly related species, and artificially induce genetic diversity.
When distantly related species are crossed, plant breeders make use of a number of plant tissue culture techniques to produce progeny from other wise fruitless mating. Interspecific and intergeneric hybrids are produced from a cross of related species or genera that do not normally sexually reproduce with each other. The cereal triticale is a wheat and rye hybrid. The first generation created from the cross was sterile, so the cell division inhibitor colchicine was used to double the number of chromosomes in the cell. Cells with an uneven number of chromosomes are sterile.
Failure to produce a hybrid may be due to pre- or post-fertilization incompatibility. If fertilization is possible between two species or genera, the hybrid embryo aborts before maturation. When the cross is incompatible after fertilization, the embryo resulting from an interspecific or intergeneric cross can be rescued and cultured to produce a whole plant. This technique has been used to produce new rice for Africa, an interspecific cross of Asian rice (Otyza sativa) and African rice (Otyza glaberrima).
Hybrids may also be produced by a technique called protoplast fusion. In this case protoplasts are fused, usually in an electric field. Viable recombinants can be regenerated in culture.
Chemical mutagens like EMS and DMSO, radiation and transposons are used to generate mutants with desirable traits to be bred with other cultivars. Classical plant breeders also generate genetic diversity within a species by exploiting a process called somaclonal variation, which occurs in plants produced from tissue culture, particularly plants derived from callus. Induced polyploidy, and the addition or removal of chromosomes using a technique called chromosome engineering may also be used.
When a desirable trait has been bred into a species, a number of crosses to the favoured parent are made to make the new plant as similar as the parent as possible. Returning to the example of the mildew resistant pea being crossed with a high-yielding but susceptible pea, to make the mildew resistant progeny of the cross most like the high-yielding parent, the progeny will be crossed back to that parent for several generations. This process removes most of the genetic contribution of the mildew resistant parent. Classical breeding is therefore a cyclical process.
It should be noted that with classical breeding techniques, the breeder does not know exactly what genes have been introduced to the new cultivars. Some scientists therefore argue that plants produced by classical breeding methods should undergo the same safety testing regime as genetically modified plants. There have been instances where plants bred using classical techniques have been unsuitable for human consumption, for example the nerve toxin solanine was accidentally re-introduced into varieties of potato.
- See main article on Transgenic plants.
Genetic engineering of plants is achieved by adding a specific gene or genes to a plant, or by knocking out a gene with RNAi, to produce a desirable phenotype. The resulting plants are often referred to as transgenic plants. Genetic engineering can produce a plant with the desired trait or traits faster than classical breeding because the majority of the plant's genome is not being altered.
To genetically engineer a plant, a genetic construct must be designed so that the gene to be added or knocked-out will be expressed by the plant. To do this, a promoter to drive transcription and a termination sequence to stop transciption of the new gene must also be introduced to the plant. A marker for the selection of transformed plants is also included. In the laboratory, antibiotic resistance is a commonly used marker: plants that have been successfully transformed will grow on media containing antibiotics, plants that have not been transformed will die. Markers for selection are removed by mating (backcrossing) with the parent plant prior to commercial release.
The construct can be inserted in the plant genome by recombination using the bacteria Agrobacterium tumefaciens or A. rhizogenes, or by direct methods like the gene gun or microinjection . Using plant viruses to insert genetic constructs into plants is also a possibility, but the technique is limited by the host range of the virus. For example, Cauliflower Mosaic Virus (CaMV) only infects cauliflower.
The majority of commercially relased transgenic plants, commonly referred to as genetically modified organisms, are currently limited to plants that have introduced resistance to insect pests and herbicides. Insect resistance is achieved through incorporation of a gene from Bacillus thuringiensis (Bt) that encodes a protein that is toxic to some insects. For example, if cotton pest the cotton bollworm feeds on Bt cotton it will ingest the toxin and die. Herbicide resistance, particularly to the herbicide Roundup, is achieved through tissue culture. Plants are cultured on media containing the herbicide, and eventually some natural genetic mutation will arise that enables the plant to survive in the presence of the herbicide. The gene is then located (mapped) by crossing with susceptible species, and once identified can be introduced into other species.
Genetic engineering of plants that can produce pharmaceuticals (and industrial chemicals), sometimes called pharmacrops , is a rather radical new area of plant breeding.
Issues and concerns
Modern plant breeding, whether classical or through genetic engineering, comes with issues of concern, particularly with regard to food crops. The question of whether breeding can have a negative effect on nutritional value is central in this respect. Although relatively little direct research in this area has been done, there are scientific indications that, by favoring certain aspects of a plant's development, other aspects may be retarded. A study published in the Journal of the American College of Nutrition in 2004, entitled Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999, compared nutritional analysis of vegetables done in 1950 and in 1999, and found substantial decreases in six of 13 nutrients measured, including 6% of protein and 38% of riboflavin. Reductions in calcium, phosphorus, iron and ascorbic acid were also found. The study, conducted at the Biochemical Institute, University of Texas, concluded in summary: "We suggest that any real declines are generally most easily explained by changes in cultivated varieties between 1950 and 1999, in which there may be trade-offs between yield and nutrient content.3"
The debate surrounding genetic modification of plants is huge, encompassing the ecological impact of genetically modified plants and the safety of genetically modified food.
Plant breeders' rights is also a major and controversial issue. Today, production of new varieties is dominated by commercial plant breeders, who seek to protect their work and collect royalties through national and international agreements based in intellectual property rights. The range of related issues is complex. In the simplest terms, critics of the increasingly restrictive regulations argue that, through a combination of technical and economic pressures, commercial breeders are reducing biodiversity and significantly constraining individuals (such as farmers) from developing and trading seed on a regional level. Efforts to strengthen breeders' rights, for example, by lengthening periods of variety protection, are ongoing.
- Borojevic, S. 1990. Principles and Methods of Plant Breeding. Elserier, Amsterdam. ISBN 0444988327
- McCouch, S. 2004. Diversifying Selection in Plant Breeding. PLoS Biol 2(10): e347.
- Briggs, F.N. and Knowles, P.F. 1967. Introduction to Plant Breeding. Reinhold Publishing Corporation, New York.
- Gepts, P. (2002). A Comparison between Crop Domestication, Classical Plant Breeding, and Genetic Engineering. Crop Science 42:1780–1790
- Origins of Agriculture and Crop Domestication - The Harlan Symposium
- [email protected] 1999 Are non-GM crops safe?
- Sun, C. et al. 1998. From indica and japonica splitting in common wild rice DNA to the origin and evolution of asian cultivated rice. Agricultural Archaeology 1998:21-29
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