Plant breeding began when early humans saved seeds and planted them. The cultural change from living as nomadic hunter-gatherers, to living in more settled communities, depended on the ability to cultivate plants for food. Present knowledge indicates that this transition occurred in several different parts of the world, about 10,000 years ago.

Today, there are literally thousands of different cultivated varieties (cultivars) of individualspecies of crop plants. As examples, there are more than 4,000 different peas (Pisum sativum ), and more than 5,000 grape cultivars, adapted to a wide variety of soils and climates.

The methods by which this diversity of crops was achieved were little changed for many centuries, basically requiring observation, selection, and cultivation. However, for the past three centuries most new varieties have been generated by deliberate cross-pollination, followed by observation and further selection. The science of genetics has provided a great deal of information to guide breeding possibilities and directions. Most recently, the potential for plant breeding has advanced significantly, with the advent of methods for the incorporation of genes from other organisms into plants via recombinant DNA-techniques. This capacity is broadly termed ‘‘genetic engineering.’’ These new techniques and their implications have given rise to commercial and ethical controversies about ‘‘ownership,’’ which have not yet been resolved.

Early selection

The plants that were eaten habitually by huntergatherer communities were palatable and non-toxic. These characteristics had been determined by trial and error. Then, by saving the largest seeds from the healthiest plants, a form of selection was practiced that provided the initial foundation of plant domestication and breeding.

Among the fruit and seed characters favored by selection in prehistoric times were cereal stalks that did not fall into separate pieces at maturity, and pods that did not open as they dried out, dispersing seeds onto the ground. Wheat or barley heads that remained unified, and pea or lentil pods that remained closed allowed easier and more efficient collection of grain and seeds.

Seed dormancy

Another seed character whose selection was favored long ago was the ability to germinate soon after planting. In cases where seed dormancy was imposed by thick, impermeable seed-coats, a selected reduction in seed-coat thickness allowed more prompt germination. Wild or semi-domesticated peas, found as carbonized remains in archeological sites throughout the Middle East, possessed thick seed-coats with a characteristic, gritty surface texture. Similarly, the seed-coats of Cicer reticulatum from Turkey, the immediate progenitor of the chick pea, account for about one-quarter of the total material in the seed. However, modern cultivars of the chick pea (Cicer arietinum ) commit only 4-9% of the seed weight to seed-coats. The seed-coats are thinner because there are fewer cells in the outermost sclereid layers. Cultivated chick peas also lack the brown and green pigments typical of wild-type seeds.

Seed dormancy imposed by natural growth regulators was also selected against in prehistoric times. For example, cultivated oats (Avena sativa ) lack the dormancy mechanisms of wild oats (Avena fatua ), and germinate soon after seasonal planting.

Quality

Among fruits and vegetables, flavor, size, shape, sweetness, texture and acidity have long been desirable characters. Trees or vines producing superior fruits were prized above those that did not. This is known from the writings of the Egyptians, Greeks, and Romans. Plant remains in the gardens of Pompeii, covered by the eruption of Mt. Vesuvius in AD 79, confirm that almond, lemon, peach, pear, grape, cherry, plum, fig, and olive were cultivated at that time. The particular varieties of onion and cabbage grown around Pompeii were highly regarded, according to the Roman author Columella (AD 50).

Climatic adaptation

Cultivars adapted to different types of climatic conditions were also selected in ancient times. In North America, various Indian tribes developed and maintained lines of maize adapted to different temperature ranges. Colonel George Morgan of Princeton, New Jersey, collected so-called ‘‘Indian corns,’’ which included the Tuscorora, King Philip, and Golden Sioux lines of field corn. An early sweet corn was also obtained from the tribes of The Six Nations (Iroquois) by U.S. General Sullivan in 1779. In July 1787, a visitor to Sullivan’s garden noted: ‘‘he had Indian corn growing, in long rows, from different kinds of seed, collected from the different latitudes on this continent, as far north as the most northern parts of Canada, and south as far as the West Indies.’’

Pollination and hybridization

The genetic discoveries of Gregor Mendel with pea plants, first published in 1866, were revolutionary, although Mendel’s work remained obscure until translated from German into English by William Bateson in 1903. Nevertheless, the relationship between pollen lodging on the stigma and subsequent fruit production was realized long before Mendel’s work. The first hybrid produced by deliberate pollen transfer is credited to Thomas Fairchild, an eighteenth-century, English gardener. He crossed sweet william with the carnation in 1719, to produce a new horticultural plant.

Towards the end of that century, Thomas Andrew Knight, another Englishman, demonstrated the practical value of cross-pollination on an unprecedented scale. He produced hybrid fruit trees by cross-pollination, and then grafted shoots of their seedlings onto established, compatible root stalks. This had the effect of greatly shortening the time until fruit production, so that the horticultural success of the hybridization could be evaluated. After selecting the best fruit, the hybrid seeds could be planted, and the process of grafting the seedlings and selection could be continued. The best hybrids, which were not necessarily stable through sexual reproduction, could be propagated by grafting. Thomas Knight was also responsible for the first breeding of wrinkled-seeded peas, the kind that provided Mendel with one of his seven key characters (round being dominant, with one allele sufficient for expression; wrinkled being recessive, requiring two copies of the allele for expression).

The impact of hybridization on plant breeding in the United States

Most food plants brought from Europe to the United States in the seventeenth century failed to prosper widely. Some could not be grown successfully anywhere, because they could not adapt to the climate, or were susceptible to newly-encountered pests or diseases. At the beginning of the nineteenth century, the range of varieties available for any given plant was extremely limited. Apples, however, were an exception. This fruit crop had benefited from a number of chance varieties such as the Newtown Pippin (about 1700), the Baldwin (1742), and the Jonathan (1829). However, it was in the more typical context of low diversity that Thomas Jefferson said ‘‘the greatest service that can be rendered any country is to add a useful plant to its culture.’’

The Rural Visiter, a periodical published in Burlington, Vermont, in 1810, ran a series of extracts from Knight’s ‘‘Treatise on the Culture of the Apple and Pear.’’ Knight’s grafting methods were further described by JamesThatcherin his American Orchardist in 1822. In this way the principles behind Knight’s work became understood in the United States.

The first variety of a fruit tree to be bred in the United States was a pear produced by William Prince, around 1806. He crossed St. Germain with White Doyenne (the pollen donor), and from the seed selected a variety known as Prince’s St. Germain. Later, further improvements of the pear were made by the discovery of natural hybrids between the European pear (binomial) and the introduced Chinese sand-pear (binomial). The Kiefer, Le Conte, and Garber pears all arose in this fashion, and allowed pear cultivation to extend beyond California into the eastern and southern states.

The contribution of C. M. Hovey

C.M. Hovey produced new hybrid strawberries by 1838. The most important, Hovey’s Seedling, became the leading strawberry for more than 30 years. Unfortunately this variety was finally lost, although some derivatives were maintained. Hovey was also successful with flowers. He crossed existing yellow calceolarias (binomial) with the purple Calceolaria purpurea, imported in 1827. Flowers ranging in color from pale yellow to deep orange, and from light red to deep scarlet, were subsequently produced.

Hovey was later involved in the development of hybrid grapes. In 1844 he advocated a breeding strategy that required crossing the Isabella and Catawba, two cultivars derived from native species, with European varieties such as Golden Chasselas as pollen donors. The Delaware, named about 1850, was a chance hybrid between native and European grapes. Although many useful grape hybrids were subsequently produced by American breeders in the latter part of the nineteenth century, the grafting of European cultivars onto American rootstocks proved to be more beneficial for this crop on a worldwide scale.

Luther Burbank

The concept of ‘‘diluting’’ hybrids by crossing them back to either parent also developed in the latter part of the nineteenth century. This strategy was introduced to ameliorate undesirable characters that were expressed too strongly. Luther Burbank, based in California, became a master of this art. He bred larger walnuts from hybrids involving Juglans californica, J. regia, and J. nigra. From the 1870s onwards, he wasespecially successful with plums bred by hybridization of native American plums with a Japanese species, (Prunus tri-flora ). Burbank once found a Californian poppy (Eschscholtzia californica ) that displayed a crimson thread through one petal. By repeated selection he eventually developed an all-crimson poppy. His series of hybrids between blackberry and raspberry also produced some remarkable plants. The Primus blackberry (from western dewberry and Siberian raspberry) produced larger fruit that ripened many weeks in advance of either parent, while out-yielding both and maintaining flavor. By the turn of the century, Burbank was justly famous for having bred numerous superior cultivars of many different kinds of plants of horticultural and agricultural importance.

In genetic terms, there are two kinds of back-crossing. When one parent of a hybrid has many recessive characters, these are masked in the F₁ (first filial) hybrid generation by dominant alleles from the other parent. However, a cross of the F₁ hybrid with the recessive parent will allow the complete range of genetic variation to be expressed in the F₂ progeny. This is termed a test cross. A cross of the F₁ to the parent with more dominant characters is termed a back cross.

The goals of modern plant breeding

The broad aims of current plant breeding programs have changed little from those of the past. Improvements in yield, quality, plant hardiness, and pest resistance are actively being sought. In addition, the ability of plants to survive increasing intensities of ultraviolet radiation, due to damage in the ozone layer, and to respond favorably to elevated atmospheric concentrations of carbon dioxide are being assessed. To widen the available gene pools, collections of cultivars and wild relatives of major crop species have been organized at an international level. The United Nations’ Food and Agriculture Organization (FAO) supported the formation of the International Board for Plant Genetic Resources in 1974. However, many cultivars popular in the nineteenth century have already fallen into disuse and been lost. The need to conserve remaining ‘‘heritage’’ varieties has been taken up by associations of enthusiasts in many countries, such as the Seed Savers’ Exchange in the United States

Plant cloning and artificial hybridization

Genetically identical plants, or clones, have been propagated from vegetative cuttings for thousands of years. Modern cloning techniques are used extensively to select for cultivars with particular characteristics, since there are limits to what can be achieved through direct hybridization. Some individual species or groups of cultivars cannot be genetically crossed. Sometimes this is because of natural polyploidy, when plant cells carry extra copies of some or all of the chromosomes, or because of inversions of DNA within chromosomes. In cases where cross-fertilization has occurred, ‘‘embryo rescue’’ may be used to remove hybrid embryos from the ovules and culture them on artificial media.

Pollen mother-cells in the anthers of some species have been treated with colchicine, to generate nuclei with double the haploid chromosome number, thus producing diploid plants that are genetically-identical to the haploid pollen. The use of colchicine to induce polyploidy in dividing vegetative cells first became popular in the 1940s, but tetraploids generated from diploids tend to mask recessive alleles. Generating diploids from haploids doubles all of the existing recessive alleles, and thereby guarantees the expression of the recessive characters of the pollen source.

Somatic hybridization

In other difficult cases, the barriers to sexual crossing can sometimes be overcome by preparing protoplasts from vegetative (somatic) tissues from two sources. This involves treatment with cell-wall degrading enzymes, after which the protoplasts are encouraged to fuse by incubation in an optimal concentration of polyethylene glycol. A successful fusion of protoplasts from the two donors produces a new protoplast that is a somatic hybrid. Using tissue cultures, such cells can, in some cases, be induced to develop into new plants.

Somatic fusion is of particular interest for characters related to the chloroplast or mitochondrion. These plastids contain some genetic information in their specific, non-nuclear DNA, which is responsible for the synthesis of a number of essential proteins. In about two-thirds of the higher plants, plastids with their DNA are inherited in a ‘‘maternal’’ fashion–the cytoplasm of the male gamete is discarded after fusion of the egg and sperm cells. In contrast, in the minority of plants with biparental inheritance of plastid DNA, or when fusion of somatic protoplasts occurs, there is a mixing of the plastids from both parents. In this way, there is a potential for new plastid-nucleus combinations.

For chloroplasts, one application of plastid fusion is in the breeding of resistance to the effects of triazine herbicides. For mitochondria, an application relevant to plant breeding is in the imposition of male sterility. This is a convenient character when certain plants are to be employed as female parents for a hybrid cross. The transfer of male-sterile cytoplasm in a single step can avoid the need for several years of backcrosses to attain the same condition. Somatic hybridization has been used successfully to transfer male sterility in rice, carrot, rapeseed (canola), sugar beet, and citrus. However, this character can be a disadvantage in maize, where male sterility simultaneously confers sensitivity to the blight fungus, Helminthosporium maydis. This sensitivity can lead to serious losses of maize crops.

Somaclonal variation

Replicate plant cells or protoplasts that are placed under identical conditions of tissue culture do not always grow and differentiate to produce identical progeny (clones). Frequently, the genetic material becomes destabilized and reorganized, so that previously-concealed characters are expressed. In this way, the tissue-culture process has been used to develop varieties of sugar cane, maize, rapeseed, alfalfa, and tomato that are resistant to the toxins produced by a range of parasitic fungi. This process canbeusedrepeatedlytogenerateplantswith multiple disease resistance, combined with other desirable characters.

Genetic engineering

The identification of numerous mutations affecting plant morphology has allowed the construction of genetic linkage maps for all major cultivated species. These maps are constantly being refined. They serve as a guide to the physical location of individual genes on chromosomes.

DNA sequencing of plant genomes has shown that gene expression is controlled by distinct ‘‘promoter’’ regions of DNA. It is now possible to position genes under the control of a desired promoter, to ensure that the genes are expressed in the appropriate tissues. For example, the gene for a bacterial toxin (Bt) (from Bacillus thuringiensis ) that kills insect larva

KEY TERMS

Allele— Any of two or more alternative forms of a gene that occupy the same location on a chromosome.

Antibiotic— A compound produced by a microorganism that kills other microorganisms or retards their growth. Genes for antibiotic resistance are used as markers to indicate that successful gene transfer has occurred.

Biolistics— The bombardment of small pieces of plant tissue with tungsten microprojectiles coated with preparations of DNA.

Colchicine— An alkaloid compound derived from seeds and corms of the autumn crocus (Colchicum autumnale ). Colchicine has the ability to disrupt the cell cycle, causing a doubling of chromosome numbers in some plant cells.

Cultivar— A distinct variety of a plant that has been bred for particular, agricultural or culinary attributes. Cultivars are not sufficiently distinct in the genetic sense to be considered to be subspecies.

Cytoplasmic inheritance— The transmission of the genetic information contained in plastids (chloroplasts, mitochondria, and their precursors). In most flowering plants this proceeds through the egg cell alone, i.e., is maternal.

Diploid— Possessing two complete sets of homologous chromosomes (double the haploid number n, and designated as 2n).

Dormancy— The inability to germinate (seeds) or grow (buds), even though environmental conditions are adequate to support growth.

Electroporation— The induction of transient pores in the plasmalemma by pulses of high voltage electricity, in order to admit pieces of DNA.

Gametes— Specialized cells capable of fusion in the sexual cycle; female gametes are termed egg cells; male gametes may be zoospores or sperm cells.

Gene— A discrete unit of inheritance, represented by a portion of DNA located on a chromosome. The gene is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.

Haploid— Nucleus or cell containing one copy of each chromosome. (designated n), as in the gametes of a plant that is diploid (2n).

Hybrid— A hybrid plant is derived by crossing two distinct parents, which may be different species of the same genus, or varieties of the same species. Many plant hybrids are infertile and must therefore be maintained by vegetative propagation.

Plasmid— A specific loop of bacterial DNA located outside the main circular chromosome in a bacterial cell.

Polyploidy— The condition where somatic cells have three or more sets of n chromosomes (where n is the haploid number). Functional ploidy is unusual in plants above the level of tetraploid (4n).

Transgenic plant— A plant that has successfully incorporated a transferred gene or constructed piece of DNA into its nuclear or plastid genomes.

Zygote— The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

might be placed next to a leaf-development promoter sequence, so that the toxin will be synthesized in any developing leaf. Although the toxin might account for only a small proportion of the total protein produced in a leaf, it is capable of killing larvae that eat the genetically-modified leaves.

Vectors for gene transfer

Agrobacterium tumefaciens and A. rhizogenes are soil bacteria that infect plant roots, causing crown gall or ‘‘hairy roots’’ diseases. Advantage has been taken of the natural ability of Agrobacterium to transfer plasmid DNA into the nuclei of susceptible plant cells. Agrobacterium cells with a genetically-modified plasmid,containing a gene for the desired trait and a marker gene, usually conferring antibiotic resistance, are incubated with protoplasts or small pieces of plant tissue. Plant cells that have been transformed by the plasmid can be selected on media containing the antibiotic, and then cultured to generate new, transgenic plants.

Many plant species have been transformed by this procedure, which is most useful for dicotyledonous plants. The gene encoding Bt, as well as genes conferring resistance to viral diseases, have been introduced into plants by this method.

Direct gene transfer

Two methods have been developed for direct gene transfer into plant cells—electroporation and biolistics. Electroporation involves the use of high-voltage electric pulses to induce pore formation in the membranes of plant protoplasts. Pieces of DNA may enter through these temporary pores, and sometimes protoplasts will be transformed as the new DNA is stably incorporated (i.e., able to be transmitted in mitotic cell divisions). New plants are then derived from cultured protoplasts. This method has proven valuable for maize, rice, and sugar cane, species that are outside the host range for vector transfer by Agrobacterium.

Biolistics refers to the bombardment of plant tissues with microprojectiles of tungsten coated with the DNA intended for transfer. Surprisingly, this works. The size of the particles and the entry velocity must be optimized for each tissue, but avoiding the need to isolate protoplasts increases the potential for regenerating transformed plants. Species that cannot yet be regenerated from protoplasts are clear candidates for transformation by this method.

Genetically modified plants

In 1992, a tomato with delayed ripening became the first genetically modified (GM) commercial food crop. More than 40 different GM crops are now being grown commercially. GM corn and cotton contain bacterial genes that kill insects and confer herbicide-resistance on the crops. GM squash contains viral genes that confer resistance to viruses. Potatoes carry the Bt gene to kill the Colorado potato beetle and a viral gene that protects the potato from a virus spread by aphids. Mauve-colored carnations carry a petunia gene required for making blue pigment. In many cases, GM crops result in increased yields and reduced use of pesticides. New research is focused on producing GM foods containing increased vitamins and human or animal vaccines.

GM crops are very controversial. There is concern that the widespread dissemination of the Bt gene will cause insects to become resistant. It has been reported that pollen from Bt corn is toxic to the caterpillars of monarch butterflies. It also is possible that GM crops will interbreed with wild plants, resulting in ‘‘super-weeds’’ resistant to herbicides. There is also concern that the antibiotic-resistance genes, used as markers for gene transfer, may be passed from the plants to soil microorganisms or bacteria in humans who eat the food. Finally, the possibility of allergic reactions to the new compounds in food exists. Many countries have banned the production and importation of GM crops.

Resources

BOOKS

McMahon, M., A.M. Kofranek, and V.E. Rubatzky. Hartmann’s Plant Science: Growth, Development and Utilization of Cultivated Plants. 4th ed. Englewood Cliffs, NJ: Prentice-Hall, 2006.

Murray, David R., ed. Advanced Methods in Plant Breeding and Biotechnology. Oxford: C.A.B. International, 1991.

Plant Breeding: The Arnel R. Hallauer International Symposium, edited by Michael Lee and Arnel R. Hallauer. Malden, MA: Blackwell Publishing, 2006.

PERIODICALS

Adams, K.L., et al. ‘‘Repeated, Recent and Diverse Transfers of a Mitochondrial Gene to the Nucleus in Flowering Plants.’’ Nature 408 (2000): 354-357.

Palmer, J. D., et al. ‘‘Dynamic Evolution of Plant Mitochondrial Genomes: Mobile Genes and Introns and Highly Variable Mutation Rates.’’ Proceedings of the National Academy of Sciences of the United States of America 97 (2000): 6960-6966.

David R. Murray

The Gale Encyclopedia of Science