plant breeding, science of altering the genetic pattern of plants in order to increase their value. Increased crop yield is the primary aim of most plant-breeding programs; advantages of the hybrids and new varieties developed include adaptation to new agricultural areas, greater resistance to disease and insects, greater yield of useful parts, better nutritional content of edible parts, and greater physiological efficiency. Other goals are adaptation of crops to modern production techniques such as mechanical harvesting and improvement in the market quality of the product.
Traditionally, plant breeders have made genetic changes in crops by using various crossing and selection methods, creating desirable hybrids; breeders also now attempt to induce favorable genetic mutations by the use of ultraviolet light, gamma radiation, or chemicals. With the development of genetic engineering, plant breeders have increasingly used its techniques to introduce desirable traits (i.e., genes), often from other species, into cultivated plants.
See also breeding.
"plant breeding." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (May 24, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/plant-breeding
"plant breeding." The Columbia Encyclopedia, 6th ed.. . Retrieved May 24, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/plant-breeding
The vast majority of the human population and most civilizations have depended on the productivity of plant-based agriculture for sustenance, vitality, and quality of life during the preceding five to fifteen millenia. During this time, and especially since the rediscovery of Gregor Mendel's principles of heredity in the late nineteenth century, the genetic content (genome) of crop plants has become a more important resource for crop management and production. The dynamic genomes of crop plants contain tens of thousands of genes that interact with themselves and the environment to determine the many traits affecting crop productivity. Gradually, humans have learned that the genomes of crop species and their relatives contain a range of genetic variation for many traits; how the genomes are transmitted from parent to progeny; a few of the myriad relationships among genomes, genes, the environment, and traits; some methods and mechanisms for maintaining or modifying the genomes; and how to select, capture, propagate and deliver the desirable genetic variation in forms suited to the agricultural systems and their societies. Much remains to be learned and understood.
Plant breeding is the science ultimately concerned with the holistic and systematic creation of cultivars, cultivated varieties of plant species better suited to the needs and pleasures of human societies. In many ways, plant breeding is analogous to a large river system such as the Mississippi: it has a primary source (the gene pool of the plant species), a main river (the elite gene pool of plant breeding methods), tributaries (new technology and scientific disciplines), the ability to adapt to the prevailing conditions and forces of nature, and it is replenished by recycled water (germplasm, all genotypes of a species). Plant breeders devise and deploy methods that, in accordance with their resources, the nature of the plant and production environment, and the prevailing goals of society, integrate information and material from the tributaries to produce better cultivars. The scientific tributaries have included the biological (e.g., genetics, botany, biochemistry, plant pathology, entomology), physical (e.g., mathematics, chemistry, computer), and analytical (e.g., experimental design and statistics) sciences. Some important technological tributaries include methods for storing seed or other propagules, the computer for data analysis and management, and tools or machines for conducting the many experiments and evaluating the progeny needed to create a superior cultivar. New tributaries include genomics, molecular biology, and genetic engineering. While the tributaries have varied with the nature of the crop species and the resources and goals of the societies they support, the primary roles of the plant breeder—integration, evaluation, and selection—have been constant.
Components and Challenges of Plant Breeding Programs
Plant breeding programs consist of several steps that are usually conducted as reiterative procedures:
- hire talented and cooperative scientists (e.g., plant breeder, scientists in other disciplines, and technical staff)
- understand the ecology of the plant, the target environment, the system of crop production, and the consumers
- define the target environment for crop production (e.g., Where and how are the crops grown? What is the prevailing ecology therein?)
- assemble and maintain the necessary physical resources
- identify clear goals for selection regarding the type of cultivar, the traits, and their expression
- select or create testing environments representative of the target environment
- survey and choose germplasm to serve as parents and sources of genes (the crop and other species, cultivars, accessions [individual samples of seed] from germplasm reserves, and genes)
- identify and create genetic variation among the parents and their progeny by evaluating the parents, mating the parents, and evaluating their progeny and occasionally by modifying the parents' genome or introducing genes through genetic engineering and transformation
- evaluate and select the progeny that optimize production in the target environment.
When practiced on a continuous basis, these steps have achieved impressive results for several species and target environments.
Plant breeding programs negotiate numerous challenges along the path of improvement. The reproductive biology and growth habit of the plant are primary factors that dictate breeding methods, their implementation, progress from selection, and the type of cultivar (e.g., hybrid , pure line, clonal, population , or other). Some important considerations include the mode of reproduction (sexual, vegetative, or both), flower structure (perfect or imperfect), prevailing type of pollination (e.g., autogamous [self] or allogamous [other], wind, or insect), and methods to induce flowering, make controlled matings between the selected parents, and produce an adequate supply of progeny for evaluation and distribution. Considerations of the growth habit would include the length of the juvenile period (especially with trees) and if the species has an annual or perennial habit in the target environment.
The organization of the plant's genome also affects breeding strategy and the rate of progress. The plant genome is partitioned into the nucleus, mitochondrion (mt), and plastid (pt; e.g., chloroplast). The mt and pt genomes contain relatively few genes (hundreds) and in most angiosperm species are transmitted to the progeny exclusively through the cytoplasm of the female gametes (the egg cell in the embryo sac). The maternal inheritance of those genomes may dictate which parents are used as males and females. Plant nuclear genomes contain tens of thousands of genes as parts of several independent chromosomes, are inherited biparentally through the male (sperm nuclei in the generative cell of the pollen grain) and female gametes, and often contain more than two complete sets of chromosomes (polypoidy). For example, maize (Zea mays L.) and rice (Oryza sativa L.) are diploid because the nuclei of their somatic cells contain two complete sets of chromosomes, one each from the maternal and paternal parents. In contrast, cultivated alflalfa (Medicago sativa L.) and bread wheat (Triticum aestivum L.) are autotetraploid and allohexaploid because their somatic cells contain four (from the same species) and six (from three different progenitor species) complete sets of chromosomes, respectively. Polyploidy challenges breeders because it leads to more complex inheritance patterns and may hinder identification of desirable progeny in segregating populations.
The ecology of the target environment and the plant affect the evaluation and selection of parents and progeny in myriad ways (e.g., climate, soil, organic diversity, and the subsequent stress on crop production). The relative merit of the germplasm (e.g., parent, progeny, or cultivar) may vary greatly and depend upon certain elements of the environment (i.e., geno-type and environment interaction, GxE). For example, a disease-resistant cultivar may have superior productivity when evaluated in a disease-laden environment but the same cultivar may be inferior when tested in a disease-free environment. GxE is a major challenge for every plant-breeding program because so many factors could influence the plant's growth and productivity during its life cycle. GxE is managed by testing germplasm in samples of relatively few environments and treatments intended to resemble the prevailing conditions of the target environment. Inadequate testing may result in a poor choice of genotypes, less genetic progress, and, sometimes, truly inferior cultivars.
Accomplishments of Plant Breeding Programs: Some Examples
Plant breeders have achieved some significant genetic modifications of plant species. Crop domestication , although unrecorded for most plants, provided the critical foundation for subsequent cycles of distribution, adaptation, mating, and selection. Some products of those cycles include rice and wheat of short stature and increased yield, beets (Beta vulgaris L.) with increased sucrose concentration, Brassica napus L. with edible oil, the forms of Brassica oleracea L. (e.g., cauliflower, cabbage, kale, broccoli, kohlrabi, and brussels sprouts), and high-yielding maize (corn). The achievements with rice and wheat (the Green Revolution) significantly enhanced food production for billions of persons and were partially recognized in 1970 when Norman Borlaug received a Nobel Prize for his role in developing and promoting new cultivars of wheat.
In the United States maize is an example of a crop that has been quickly and significantly modified through plant breeding. Maize is a tropical grass domesticated by central American natives, possibly from the wild relative teosinte (Zea mexicana ). It was cultivated throughout the Americas before the colonization by Europeans, who adopted and expanded maize production. In the 1920s breeding methods changed dramatically: inbred lines (parents) were developed through generations of self-pollination and selection; the inbred lines were mated in specific combinations; many combinations were tested; a few combinations exhibited exceptional vigor and productivity; and the seed produced from selected matings was grown as the crop (i.e., the F1 or hybrid generation of the mating between the inbred lines). Previously, breeding methods in the United States emphasized selection of seed and individual plants produced through random, uncontrolled matings within locally adapted varieties and such open-pollinated seed was grown as the crop. In the 1930s farmers quickly substituted hybrid cultivars for the open-pollinated cultivars because the hybrids had higher and more consistent yield. Concomitantly, management practices changed. The average grain yield of maize increased from 30 to 120 bushels per acre from the 1930s to the 1990s. About 50 percent of the increase is due to genetic changes mediated by breeding for higher yield of grain, resistance to biotic and abiotic stress, and the ability to respond to more intensive management (e.g., increased application of fertilizer and seeding rates).
Plant Breeding Programs and Germplasm Reserves. The target environment and societies' goals sometimes change in dynamic and unpredictable ways that render existing cultivars obsolete. Cultivars with improved adaptation may be bred if genetic variation (i.e., genes and combinations thereof) exists for the traits of interest. To manage this uncertainty, germplasm reserves or gene banks have been established worldwide with the primary goals of collecting and maintaining the broadest possible array of genetic variation for economically important plant species. In the United States a network of Plant Introduction Stations are financed by the federal and state governments to provide this service. The reserves are important because:
- the gene pool of existing cultivars represents only a subsample of the genetic variation for a given species
- favorable genes are certainly contained in other gene pools
- human activity has reduced the native gene pools of most crop species and their wild relatives in agricultural and natural settings
- the reserves have provided useful genes
- methods for investigating gene pools have been crude but there are good prospects for improvement
- our ability to engineer genes and complete organisms to meet the demands of crop production is woefully inadequate and will need all the help provided by nature.
see also Breeder; Cultivar; genetic Engineering; Green Revolution; hybrids and Hybridization; Polyploidy; Propagation; Reproduction, Sexual; Seeds.
Crabb, Richard. The Hybrid Corn-Makers. Chicago, IL: West Chicago Publishing Company, 1993.
Fehr, Walter R. Principles of Cultivar Development, Vol. 1: Theory and Technique. New York: Macmillan, 1991.
Sauer, Jonathan D. Historical Geography of Crop Plants: A Select Roster. Boca Raton, FL: CRC Press, 1993.
Smartt, J. and N. W. Simmonds . Evolution of Crop Plants. Essex, England: Longman Scientific & Technical, 1995.
Wallace, Henry Agard, and William L. Brown. Corn and Its Early Fathers. Ames, IA:Iowa State University Press, 1988.
"Breeding." Plant Sciences. . Encyclopedia.com. (May 24, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/breeding
"Breeding." Plant Sciences. . Retrieved May 24, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/breeding
"breeding." A Dictionary of Biology. . Encyclopedia.com. (May 24, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/breeding
"breeding." A Dictionary of Biology. . Retrieved May 24, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/breeding