An Assyrian relief carving from 870 B.C. showing artificial pollination of date palms.

What is Plant Breeding? For several thousand years, farmers have been altering the genetic makeup of the crops they grow. Human selection for features such as faster growth, larger seeds or sweeter fruits has dramatically changed domesticated plant species compared to their wild relatives. Remarkably, many of our modern crops were developed by people who lacked an understanding of the scientific basis of plant breeding.

Despite the poor understanding of the process, plant breeding was a popular activity. Gregor Mendel himself, the father of genetics, was a plant breeder, as were some of the leading botanists of his time. Mendel's 1865 paper (http://www.MendelWeb.org/Mendel.html) explaining how dominant and recessive alleles could produce the traits we see and could be passed to offspring was the first major insight into the science behind the art. The paper was largely ignored until 1900, when three scientists working on breeding problems rediscovered it and publicized Mendel's findings.

Major advances in plant breeding followed the revelation of Mendel's discovery. Breeders brought their new understanding of genetics to the traditional techniques of self-pollinating and cross-pollinating plants.

Plant breeder Sally Clayshulte collects pollen

Corn breeders, particularly, tried numerous strategies to capitalize on the insights into heredity. Corn plants that had traditionally been allowed to cross-pollinate freely were artificially self-pollinated for generations and crossed to other self-pollinated lines in an effort to achieve a favorable combination of alleles. The corn we eat today is the result of decades of this strategy of self-pollination followed by cross-pollination to produce vigorous hybrid plants. Information on the history of corn breeding is available in an article written by L.W. Kannenberg for the Ontario Corn Producers Association (http://www.ontariocorn.org/ ocpmag/dec99feat.html).

The art of recognizing valuable traits and incorporating them into future generations is very important in plant breeding. Breeders have traditionally scrutinized their fields and traveled to foreign countries searching for individual plants that exhibit desirable traits. Such traits occasionally arise spontaneously through a process called mutation, but the natural rate of mutation is too slow and unreliable to produce all the plants that breeders would like to see. In the late 1920s, researchers discovered that they could greatly increase the number of these variations or mutations by exposing plants to X-rays. "Mutation breeding" accelerated after World War II, when the techniques of the nuclear age became widely available. Plants were exposed to gamma rays, protons, neutrons, alpha particles, and beta particles to see if these would induce useful mutations. Chemicals, too, such as sodium azide and ethyl methanesulphonate, were used to cause mutations. Examples of plants that were produced via mutation breeding are given in the table below.

Crop	Cultivar Name	Method Used to Induce Mutation
rice	Calrose 76	gamma rays
wheat	Above	sodium azide
	Lewis	thermal neutrons
oats	Alamo-X	X-rays
grapefruit	Rio Red	thermal neutrons
	Star Ruby	thermal neutrons
burmuda grass	Tifeagle	gamma rays
	Tifgreen II	gamma rays
	Tift 94	gamma rays
	Tifway II	gamma rays
lettuce	Ice Cube	ethyl methanesulphonate
	Mini-Green	ethyl methanesulphonate
common bean	Seafarer	X-rays
	Seaway	X-rays
lilac	Prairie Petite	thermal neutrons
St. Augustine grass	TXSA 8202	gamma rays
	TXSA 8212	gamma rays

Quite a few flower cultivars have been developed via mutation breeding, among them some of the cultivars of Alstroemeria, begonia, carnation, chrysanthemum, dahlia, and snapdragon. Mutation breeding was particularly popular in the United States during the 1970s. Although interest has waned somewhat in recent years, occasional varieties continue to be produced using these methods. For example, the new herbicide-resistant wheat variety Above (http://wheat.colostate.edu/above.html) was developed using exposure to sodium azide. Mutation breeding efforts continue around the world today. Of the 2,252 officially released mutation breeding varieties, 1,019, or almost half, have been released during the last 15 years. For more information on mutation breeding, go to the International Atomic Energy Agency's site at http://www-infocris.iaea.org/MVD/ and click first on "introduction" and then on "FAO/IAEA Mutant Variety Database."

A rose grown in tissue culture. Photo: USDA

Another method for increasing the number of mutations in plants is tissue culture. Tissue culture is a technique for growing cells, tissues, and whole plants on artificial nutrients under sterile conditions, often in small glass or plastic containers. Tissue culture was not developed with the intention of causing mutations, but the discovery that plant cells and tissues grown in tissue culture would mutate rapidly increased the range of methods available for mutation breeding. More information on tissue culture of plants is available at http://www.jmu.edu/biology/biofac/facfro/cloning/cloning.html. A lesson on the basics of tissue culture http://croptechnology.unl.edu/viewLesson.cgi? LessonID=957885612 is available at the Crop Technology web site maintained by the University of Nebraska at Lincoln.

It was during the 1970s also that haploid breeding was heavily utilized. Spontaneously occurring haploid plants, those having half the normal number of chromosomes, were discovered in the 1920s, but haploid breeding was not a practical technique until methods for the controlled production of haploid plants were developed. Once a haploid plant has been obtained, its chromosomes are artificially doubled to return the plant to the normal number of chromosomes. Such a plant is valuable because the chromosomes that were created by artificial doubling are exact copies of the chromosomes that were present in the haploid plant. Haploids have been used in creating cultivars of barley, maize, tobacco, asparagus, strawberries, and tall fescue grass. Often these plants are more useful in basic research than in commercial applications, but the haploid-derived barley cultivar Tangangara was released for commercial production in Australia in 1996. A list of haploid-derived barley lines that are being tested for commercial value is available at http://www.regional.org.au/au/abts/2001/t4/broughton.htm. A diagram of the haploid breeding process is provided at http://barleyworld.org/NABGMP/QTLFIG.HTM. A description of how this technique is being used to create new barley cultivars in Australia, and how it differs from genetically engineering the barley, is available at http://www.wintv.com.au/science/barley.shtml.

While most breeders cross-pollinate plants of a single species, some breeding methods rely on crosses that can be made between two species within the same genus. A cross between Musa acuminata and Musa balbisiana, both members of the genus Musa, produced the bananas with which we are familiar. Less commonly, the cross is between members of two different genera. A cross between wheat, Triticum aestivum, and rye, Secale cereale, produced the grain called triticale, which contains a copy of all the chromosomes from both species..

Photo: USDA

A variation on the wide crossing procedure is to select plants that have single chromosomes or chromosome arms substituted from one species into another. Many modern wheat cultivars, for example, contain a chromosome arm from rye, which adds resistance to several diseases. A list of wheat cultivars that contain a chromosome arm from rye is available at http://wheat.pw.usda.gov/ggpages/1rscom.html .

Transgenic technology provides the means to make even more distant "crosses" than were previously possible. Organisms that have until now been completely outside the realm of possibility as gene donors can be used to donate desirable traits to crop plants. These organisms do not provide their complete set of genes, but rather donate only one or a few genes to the recipient plant. For example, a single insect-resistance gene from the bacterium Bacillus thuringiensis can be transferred to a corn plant to make Bt corn. A description of Bt corn is available on our Current Transgenic Products page. Transgenic plants were first created in the early 1980s by four groups working independently at Washington University in St. Louis, Missouri, the Rijksuniversiteit in Ghent, Belgium, Monsanto Company in St. Louis, Missouri, and the University of Wisconsin. On the same day in January 1983, the first three groups announced at a conference in Miami, Florida, that they had inserted bacterial genes into plants. The fourth group announced at a conference in Los Angeles, California, in April 1983 that they had inserted a plant gene from one species into another species. The Washington University group, headed by Mary-Dell Chilton, had produced cells of Nicotiana plumbaginifolia, a close relative of ordinary tobacco, that were resistant to the antibiotic kanamycin (Framond et al., 1983). Jeff Schell and Marc Van Montagu, working in Belgium, had produced tobacco plants that were resistant to kanamycin and to methotrexate, a drug used to treat cancer and rheumatoid arthritis (Schell et al., 1983). Robert Fraley, Stephen Rogers, and Robert Horsch at Monsanto had produced petunia plants that were resistant to kanamycin (Fraley et al, 1983a). The Wisconsin group, headed by John Kemp and Timothy Hall, had inserted a bean gene into a sunflower plant. These discoveries were soon published in scientific journals. The Schell group's work appeared in Nature in May (Herrera-Estrella et al., 1983) and the Chilton group's work followed in July (Bevan et al., 1983). The Monsanto group's work appeared in August in Proceedings of the National Academy of Sciences (Fraley et al, 1983b). The Hall group's work appeared in November in the journal Science (Murai et al., 1983). These early transgenic plants were laboratory specimens, but subsequent research has developed transgenic plants with commercially useful traits such as resistance to herbicides, insects, and viruses. The rest of this web site discusses the methods for creating transgenic plants, the plants that have been created, their evaluation and regulation, and the many issues that have arisen as a result of this new phase in the history in plant breeding. For one view comparing crop domestication, traditional plant breeding, and genetic engineering, see the review by Gepts, 2002.