Plant breeding

See Agriculture for related topics and context.

Plant breeding is the purposeful manipulation of plant species in order to create desired genotypes and phenotypes for specific purposes, such as food production, forestry, or ornamental flowers. This manipulation relies on a wide range of often complementary techniques and approaches.Plant breeding often, but not always, leads to plant domestication, and complements other approaches (such as introduction of new crops, changed rotations and tillage practices, irigation, and integrated pest management) for improving crop productivitry and land stewardship.

Different Plant breeding approaches are not used in isolation. Traditional breeding programs have generated germplasm collections which are abosulutely essential for practical application of transgenic traits such as Bt-based insect tolerance created by genetic engineering methods, and conventional breeding methods are invaluable for enabling transgenic traits to be deployed after they have been introduced into crops. Molecular genetics has generated marker-assisted breeding techniques which speed up slow classical breeding, but marker-assisted breeding cannot by itself transfer valuable traits like mildew resistance if they are found outside the usual crop gene pool.

Plant breeding has been practiced for thousands of years, since near the beginning of human civilization. It is now 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 .

Range of approaches used

The diverse techniques used in plant breeding include cross-pollination, either within the species, or between related species and genera - including wide-crosses using wild relatives of domesticated plants to introduce pest resistant traits needed in domesticated varieties, creation of artificial hybrids and exploitation of hybrid vigor (heterosis), creation of mutants by irradiation or chemical treatment, embryo-rescue, colchicine treatment to create artificial polyploids, protoplast-fusion, genetic engineering to generate transgenic plants, RNA silencing (cisgenics), artificial selection of progeny and molecular-marker assisted breeding, and use of statistical principles to design field tests of new variety performance with sufficient power to detect improvement.

Genome science (chromosome sequence decoding and computer assisted dissection of gene functions and stucture) is also being bought into play to assist plant breeders. One approach is to compare gene arrangement in different species (comparative genomics) to take advantage of the greater ease of gene sequencing and faster progress with smaller more compact genomes such as those of Arabidopsis thaliana, or of rice, to provide clues for gene function and location in crop species with larger genomes.

Germplasm collections

Template:Stub It was in the 1930s that Nikolai Vavilov first called attention to the value of wild crop relatives as a source of genes for improving agriculture, and in travels over five continents amassed the largest collection of (at that time) of species and strains of cultivated plants in the world.

Vavilov's intent was to promote crop improvement but since his time other considerations have added to expansion of seed-banks. One major concern is the limited genetic diversity of crop plants, and the vulnrabilities to crop diseases that it introduces into the food supply. This genetic vulnrability was highlighted in 1970 by a severe outbreak of Southern corn leaf blight in the United States.

Domestication

Domestication of plants is an artificial selection process conducted by humans to produce plants that have fewer undesirable traits of wild plants, and which renders them dependent on artificial (usually enhanced) environments for their continued existence. 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 crops with desirable properties. Plants are crossed to introduce traits/genes from a particular variety 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 further developed. Plants may also be crossed with themselves to produce inbred varieties for breeding.

Classical breeding relies heavily on the naturally occuring plant life-cycle and homologous recombination to generate genetic diversity and to eliminate undesirable traits. It may also makes use of a variety of artificial laboratory procedures to overcome obstacles to introduction of useful traits from wild species that do not usually exchange genes with the domesticated line. These approaches include in vitro techniques such as protoplast fusion, embryo rescue or mutagenisis (see below) to generate genetic alterations and produce transgenic 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

Traits that breeders' have tried to incorporate into crop plants in the last 100 years include:

1. Increased quality and yield of the crop
2. Increased tolerance of environmental pressures (salinity, metal-ions, extreme temperature, drought)
3. Resistance to viruses, fungi and bacteria
4. Changes to plant morphology, such as dwarfing traits
5. Differences in fruit color
6. Decreased production of antinutrient chemicals
7. Increased tolerance to insect pests
8. 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 non-random 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. Heterosis 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 heterosis was widely used to produce hybrids.

Heterosis made breeders aware of the broad practical value of many genes carried in plant chromosomes even when the identity and trait specified by the paticular genes is unknown - that is that diverse plant Germplasm is generally valuable to the breeder.

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 maize plants.

The scientific use of Transgenic plants in farming gained impetus in the 1930s when a transgenic wheat variety named Hope bred by E. S. McFadden with a transgene originating in a wild grass saved American wheat growers from devastating stem rust outbreaks.

These early breeding techniques resulted in large yield increase in the United States in the early 20th century. Similar yield increases were not produced elsewhere until after World War II, the Green Revolution increased crop production in the developing world in the 1960s.

Success stories like Hope and hybrid-vigor made it clear that genetic divesity present in wild-species was of great potential value to plant breeders, and eventially lead to the establisment of Germplasm collections consisting of seed-banks devoted to preservation of potentially useful uncharacterised traits for posterity.

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. These crosses are referred to as Wide crosses. 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 may abort before maturation. If this does occur the embryo resulting from an interspecific or intergeneric cross can sometimes be rescued and cultured to produce a whole plant. Such a method is referred to as Embryo Rescue. This technique has been used to produce new rice for Africa, an interspecific cross of Asian rice (Oryza sativa) and African rice (Oryza 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 also found uses.

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 (See backcrossing ). 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 poison solanine was accidentally re-introduced into varieties of potato through plant breeding.

Genetic modification (Post-1975)

See main article on Transgenic plants.

Genetic modification 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 plants resulting from adding a gene are often referred to as transgenic plants. Plants in which RNAi is used to silence genes are now starting to be called Cisgenic plants. Genetic modification can produce a plant with the desired trait or traits faster than classical breeding because the majority of the plant's genome is not altered.

To genetically modify 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 transcription of the new gene, and the gene of genes of interest must 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. In some instances markers for selection are removed by backcrossing with the parent plant prior to commercial release.

The construct can be inserted in the plant genome by genetic 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 and related species. Another limitation of viral vectors is that the virus is not usually passed on the progeny, so every plant has to be inoculated.

The majority of commercially released transgenic plants, 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, the cotton bollworm, a common cotton pest, feeds on Bt cotton it will ingest the toxin and die. Herbicides usually work by binding to certain plant enzymes and inhibiting their action. The enzymes that the herbicide inhibits are known as the herbicides target site. Herbicide resistance can be engineered into crops by expressing a version of target site protein that is not inhibited by the herbicide. This is the method used to produce glyphosate resistant crop plants (See Glyphosate)

Genetic modification of plants that can produce pharmaceuticals (and industrial chemicals), sometimes called pharmacrops, is a rather radical new area of plant breeding.

Twenty first century plant breeding

Template:Stub The scope of plant breeding continues to expand in the twenty first century. Genomics, marker-assisted breeding, and RNA interferance (RNAi, siRNA, cisgenics) are increasingly effective in accellerating commercial breeding, identifying the functions of physiologically relevant genes, and in allowing traits to be modified. Recent work with identifying wheat genes that infuence protein content illustrates how RNAi and marker assisted breeding come together in providing faster methods for crop improvement, although it needs to be borne in mind that improved protein quality and crop yield represent a trade-off.

Issues and concerns

Modern plant breeding, whether classical or through genetic engineering, comes with issues of concern, particularly with regard to food crops.

Surveys of changes in American foods 1950-1999 have suggested there may be decreases in nutitional quality of many garden crops over this time period, possibly because of breeding for higher yield

This is not a new issue though. Recent studies have revealed that at the begining of agriculture, a gene was lost from wheat that mobilizes nutrients from leaves, causing better yields at the expense of protein content. It has long been known that among the many varieties of wheat used in modern times, there is an inverse relationship between yield and protein content.. There is also increasing emphasis on breeding crops for nutritional improvement .

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.

General Bibliography

• Borojevic, S. 1990. Principles and Methods of Plant Breeding. Elserier, Amsterdam. ISBN 0-444-98832-7
• Chrispeels, M.J.,and Sadava, D.E. 2003 Editors. Plants, Genes, and Crop Biotechnology. 2nd Edition. Jones and Bartlett/American Society of Plant Biologists ISBN 0-7637-1586-7
• 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
• Fedoroff, N. V. and Brown, N. M. 2004 Mendel in the Kitchen: A Scientist's View of Genetically Modified Food. National Academy Press. ISBN 0-3090-9205-1
• McCouch, S. 2004. Diversifying Selection in Plant Breeding. PLoS Biol 2(10): e347.
• news@nature.com. 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

External links

• Making genetically engineered plants
• Adoption of Genetically Engineered Crops in the U.S.(1996-2006) ERS USDA
• ISAAA Briefs 34-2005: Global Status of Commercialized Biotech/GM Crops: 2005
• Biotech Crops Reduce Pesticide Use, Greenhouse Gas Emissions Planting of these crops generates additional US$27.5 billion in global farm income 2005
• 2006 Update of Impacts on US Agriculture of Biotechnology-Derived Crops Planted in 2005]