Plant Breeding and Propagation

Summary

The science of plant breeding and propagation is the controlled, systematic identification and multiplication of useful plant varieties. It is critical to human survival. Of all discoveries furthering the advance of civilization, improvements in food production have arguably been the most significant. Breeding food crops for higher productivity, improved nutritional content, and greater resistance to stress and disease; preserving rare species and reintroducing them into the natural environment; and creating plant varieties that act as factories for complex pharmaceuticals are among the most active fields for the plant breeder. Genetic engineering techniques are revolutionizing the industry.

Definition and Basic Principles

Plant breeding and propagation science encompasses any systematic attempt to create or identify and select useful varieties of plants and multiply them for commercial production. Historically, the raw materials came from naturally occurring variations. Using increased knowledge of genetics and the mechanisms of inheritance, modern plant breeders crossbreed strains to produce hybrids with specific suites of character. Breeders may use radiation or chemical mutagens to increase variability. Beginning in the 1980s, genetic engineering has enabled plant scientists to insert specific genes into the genomes of cultivated plants.

Having produced and selected a desirable strain, the breeder must produce sufficient numbers of plants to test for trait stability and determine optimal conditions for commercial production. Unlike annuals that produce abundant seed, perennials with long generation times, such as forest trees, are challenging for plant breeders. For species like conifers, advances in tissue-culturing techniques have transformed production processes in both the development and the marketing phases.

The advent of genetic engineering in agriculture has produced several hotly debated issues concerning the safety, long-term environmental impact, and wisdom of introducing nonplant genes into human dietary staples on a global scale. This debate has slowed the spread of this technology.

Background and History

Humans have been selecting and propagating useful varieties of plants for more than ten thousand years. The domestication of wheat, rice, and barley in the Old World and maize and potatoes in the New World was critical to the development of the earliest civilizations. When plant breeding became systematic enough to constitute science is difficult to pinpoint. Roman agricultural science, which drew heavily on Greek and Mesopotamian antecedents, included introducing and propagating novel plant varieties. Contemporary Chinese were at least as advanced as the Romans in agronomy and horticulture, and the sophisticated agricultural techniques and variety of cultivars in pre-Columbian Mexico and Peru argue for the existence of a scientific approach in those cultures.

Agricultural science came into renewed prominence in the eighteenth century as educated landowners brought the tools of the Enlightenment to bear. Private botanical gardens became establishments for introducing, testing, and propagating exotic species and varieties of food and ornamental plants. By midcentury, breeders recognized that flowering plants have a sexual cycle and that the principles of animal breeding applied to them.

Beginning with the concept of genes pioneered by Gregor Mendel in the nineteenth century, the science of genetics progressed steadily through the twentieth century, giving plant breeders an increasing understanding of the processes underlying their work. Plant breeders have contributed a great deal to genetics in general.

How It Works

Conventional Plant Breeding. The plant breeder seeks to combine the desirable traits from different strains into a single variety. Sources for the genes producing these traits include existing commercial cultivars, field collections or germplasm banks, closely related wild plants, and spontaneous or induced mutations. Under carefully controlled conditions, the researcher cross-pollinates two varieties, plants the resulting seed, and evaluates the progeny for the desired combination of characteristics. To produce a uniform seed of the resulting hybrid for sale, a company will develop breeding stocks of the two parent strains, crossing them in production plots. A gene for male sterility is often incorporated into one of the parents, preventing self-pollination.

Tremendous improvements in methods of vegetative propagation have enabled it to be used for the commercial production of perennials with a generation time of many years. There have also been significant advances in methods for evaluating desirable characteristics at an earlier stage in a plant's life cycle. If the biochemical basis for a mature trait is known, precursors can often be detected in very young seedlings. Alternatively, if a mature trait is closely linked genetically to a trait expressed earlier in the life cycle, the presence of that marker can be used for screening.

Many plants form viable interspecific crosses. Often these are sterile, which results in seedless fruits. Seedless varieties must either be hybrids of seed-producing parents or propagated vegetatively. Sometimes breeders use colchicine to induce polyploidy, resulting in a sexually reproducing interspecific hybrid with a double chromosome complement.

Genetic Engineering. Genetic engineering and breeding differ in precision and predictability. Plants produced by scientists in a lab are created using specific genes or by manipulating particular chemical compositions. Breeding plants by traditional methods occurs outside the lab, though pollination may be done by hand. This method leaves much up to chance.

Certain viruses and virus-like plasmids invade cells and attach their DNA to that of the host. Any DNA attached to the virus or plasmid will also be incorporated. Plant genetic engineering uses a plasmid from a phytopathogenic bacterium, Agrobacterium tumefaciens, to transform plant cells. Because transformation rates are very low, geneticists incorporate an antibiotic-resistance gene to facilitate screening. Plant cells in undifferentiated tissue culture are exposed to transformed Agrobacterium plasmids and transferred to an antibiotic-containing medium. The surviving cells are then grown out as plants.

In theory, the ability to synthesize any biologically produced compound can be transferred to any plant species using this method. In practice, the absence of activators or the presence of inhibitory genes stymies many attempts.

Once engineered into a plant variety, a gene propagates normally from generation to generation and, under field conditions, into other populations of the same species. A gene for herbicide resistance, introduced into a crop to facilitate management by chemical means, can backfire if it spreads to closely related weeds. Of serious concern is so-called terminator technology, a sterility gene introduced into genetically engineered seed by seed companies to prevent patent infringement.

Modern Methods of Plant Propagation. A key feature of modern plant propagation is the use of explants and their proliferation under sterile conditions to produce large numbers of genetically identical, pathogen-free plant starts. Undifferentiated plant cells are grown in liquid or solid media, promoting rapid growth. Subcultures are subjected to growth regimes promoting differentiation and maturation to produce a mature plant. For biopharmaceutical production, plant cells may be grown indefinitely in liquid-culture bioreactors and harvested in the same manner as microbial populations, often heterotrophically.

Applications and Products

Seed. The primary use of plant breeding is to improve the productivity, cultivation characteristics, stress tolerance, and nutritional content of food crops, particularly cereal grains. The Green Revolution stressed productivity under modern agricultural methods. Later, more attention was paid to tailoring crops to specific environmental and social conditions. An example is yellow rice, genetically engineered for high vitamin A content, to combat vitamin A deficiency in rice-dependent populations.

Preservation of Biodiversity. Tissue culture methods for propagation have been a great boon for the proliferation of medicinal plants, heritage varieties, and stock for habitat restoration. Orchids and cycads, which are difficult to propagate from seed, are under tremendous pressure from collectors who encourage poaching in nature preserves. Modern nursery propagation helps protect these vulnerable species.

Molecular Farms. The use of genetically engineered plants to produce exotic organic compounds is still in the development phase but shows promise. Researchers have successfully produced a strain of Arabis (a mustard) that synthesizes hirudin, an anticlotting agent, from leeches. Molecular-farmed pharmaceuticals of plant origin are safer than those extracted from animals or human plasma. Another promising line of development is edible vaccines—food plants synthesizing proteins that provoke an antigen response to a particular disease-causing bacterium.

Reforestation. With tissue culture and rapid mass multiplication of stocks of woody plants, reforestation following disturbance has become more targeted. The forester can readily access seedlings adapted to the site, with built-in insect and disease resistance. Species such as the American chestnut, eliminated from most of its original range by blight in the early 1900s, are being successfully reintroduced as highly selected resistant strains.

Virus-Indexed Plants. Producing disease-free stocks of potatoes and bananas has always been a challenge. To produce commercial quantities of plants, viral pathogens must be eliminated and prevented. A small amount of tissue, called the test, is grafted to the indicator plant. If the plant develops symptoms of the virus in question, its presence is confirmed. This greatly reduces the spread of viruses and other pathogens in these vegetatively propagated crops.

Careers and Course Work

In the United States, a person intending to go into the production end of the plant propagation industry as a contract seed grower or operator of a large retail nursery will probably need a four-year degree in agronomy or horticulture, with a strong dose of business management course work as well as preparation in plant science. An advanced degree is usually necessary for employment in corporate research and development or a university or government laboratory. Individuals intending to do genetic engineering will need extensive graduate-level coursework in genetics, molecular biology, and computerized data management. They also may need to complete a dissertation and obtain a doctoral degree in genetic engineering. As with most scientific fields, obtaining the credentials is no guarantee of permanent employment at a living wage. Most university positions in the United States are filled temporarily with graduate students or recent graduates classified as interns and research fellows, and the situation in government laboratories is not much better. Corporate employment is typically better paid.

Social Context and Future Prospects

Advances in plant breeding have provoked many controversies, the most pressing of which are the contribution of improved crop varieties to unsustainable population growth, ownership of rights to germplasm and the products of genetic engineering, the safety of transgenic plants as human food, the risks of the unplanned spread of modified genes, and the adverse effects on traditional agriculture in developing countries. Safety concerns have led to a patchwork of national laws that inhibit but fail to effectively regulate global commerce in products of genetic engineering.

The relationship of increased crop productivity to exponential population growth, already noted by William Malthus in 1798, was a feature of the Green Revolution in the twentieth century. Gains from improved technology or new crops are quickly canceled out unless population growth slows. Rural populations may end up worse off because improved technology favors large-scale operations and displaces farmers. Institutes in developing countries, such as the International Maize and Wheat Improvement Center (Centro Internacional de Mejoramiento de Maíz y Trigo, or CIMMYT), are working to ensure that the new round of agricultural technology will have a more beneficial human impact.

New varieties of plants under patent can be propagated only under license from the originator, which prevents farmers from saving seed. This increases costs and forces small operations to use older, less productive strains. When genetically engineered strains cross with crops in adjacent fields, the farmer of the traditional crop may be unable to sell it for human consumption. There are conflicting claims over the ownership of rights to improved varieties whose base stock came from field collections in developing nations.

If these objections can be overcome, the potential for the new technology in genetically modified plants is tremendous. Greater productivity and improved nutritional content in staple crops could be immensely beneficial. Incorporating resistance to disease and stress should lower the need for pesticides and herbicides and make farming on marginal lands more environmentally friendly. Using genetically engineered green plants to produce pharmaceuticals and other complex organic chemicals may lower costs and increase availability. The ability to propagate a species from vegetative portions of a few individuals and reintroduce it into its native habitat will undoubtedly be a boon in preserving endangered species, restoring native vegetation, and promoting sustainable, ecologically friendly landscaping.

Bibliography

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