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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.
Desirable traits for crop species 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
Plant breeding is practiced worldwide and is improtant for ensuring food security and developing practices of sustainable agriculture.
Domestication Domestication of plants is selection process conducted by humans to produce plants that meet the needs of the farmer and the consumer. Domestication of plants has been carried out for 9000 - 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 3000 years ago in the New World, essentially all our important food crops had been domesticated. In the Neolithic period domestication took a minimum of 1000 years and a maximum of 7000 years.
Cultivated crop species that have evolved from wild populations due to selective pressures from traditional farmers are called landraces, an example of landraces is 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 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 would be to introduce the mildew resistance without losing the high-yield characteristics. 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.  The Yecoro wheat (right) cultivar is sensitive to salinity, plants resulting from a hybrid cross with cultivar W4910 (left) show greater tolerance to high salinity
Classical breeding prior to World War II Intraspecific hybridization within a plant species was demonstrated by Charles Darwin andGregor Mendel, and was further developed by geneticists and plant breeders. Intraspecific hybridisaiton was the principal technique for plant breeding used prior to World War II.
Classical breeding following 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 resulting 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. Somaclonal variation 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, and some scientists 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.
Genetic engineering - 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 traints faster than classical breeding because the majority of the plants 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 labroratoy antibiotic resistance is a commonly used marker, plants that have been successfully transformed will grow on media containing antiobiotics, 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 cauliflowers.
The majority of commercially relased transgenic plants, commonly referred to as genetically modified organisms are currently limited to plants that have introduced resistanace 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 the generation of a herbicide resistance trait in tissue culture. Plants are cultured on media containg the herbicide, eventually some natural genetic mutation will arise that will enable 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.
There is debate surrounding genetic modification of plants, especially the ecological impacts of genetically modified plants. The safety of genetically modified food is also an area of current debate.
Improved agricultural practices A plants growth and yield is determined by: - the genetic characteristics of the variety and;
- environment in which the variety is grown.
Use of irrigation, fertilizers, herbicides and pesticides, crop rotation and integrated pest management (IPM) strategies increased agricultural production throughout the 20th century.
See also
References - Borojevic, S. 1990. Principles and Methods of Plant Breeding. Elserier, Amsterdam. ISBN 0444988327
- 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 (http://crop.scijournals.org/cgi/content/full/42/6/1780). Crop Science 42:1780–1790
- Origins of Agriculture and Crop Domestication - The Harlan Symposium (http://www.ipgri.cgiar.org/publications/pubfile.asp?ID_PUB=47The)
- news@nature.com. 1999 Are non-GM crops safe? (http://www.nature.com/news/1999/990923/pf/990923-3_pf.html)
- Sun, C. et al. 1998. From indica and japonica splitting in common wild rice DNA to the origin and evolution of asian cultivated rice (http://www.carleton.ca/~bgordon/Rice/papers/SUN98.htm). Agricultural Archaeology 1998:21-29
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