Plant improvement 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
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 10 000 years, many crops in present day cultivation are the result of domestication in ancient times. These include: wheat and legumes.
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.
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).
Chemicals like EMS and radiation 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.
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, they 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 the breeder does not know the 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.
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 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 succerssfully 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 emample 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 and genetically modified food
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.
Gepts, P. (2002). A Comparison between Crop Domestication, Classical Plant Breeding, and Genetic Engineering. Crop Science 42:1780–1790