PerspectivesAre you interested in submitting a Perspective Article? Be sure to read The Science Advisory Board's Editorial Guides for Perspective Articles. Click here. Special Topic Focus: Plant Research Excerpts from “Biotechnology Provides New Tools for Plant Breeding” by Trevor V. Suslow, Bruce R. Thomas and Kent J. Bradford Introduction Biotechnology in its broadest sense refers to the use of living organisms or their components to provide useful products. This definition can include activities as diverse as making wine, beer, or bread; composting organic materials; releasing parasitic wasps to control insect pests; breeding plants or animals; and producing crops and livestock. In fact, agriculture itself can be considered to be the original biological technology. New biotechnology techniques now allow the identification, isolation, and alteration of genes and their reintroduction into living organisms to produce transgenic varieties. Traditional agricultural biotechnology is being supplemented with these new biotechnology methods to enhance the production of food, fiber, and other agricultural products. Transgenic crop varieties have been rapidly adopted by farmers in the United States, Canada, Argentina, and other countries. The introduction of transgenic crops has not been without controversy. Fears have been raised that these new techniques present unknown dangers and unacceptable risks compared to traditional methods of crop improvement. To evaluate these concerns, it is necessary to understand how modern crop varieties have been developed using plant breeding and biotechnology. This article describes the genetic basis of crop improvement and how traditional breeding technology compares to and integrates with the new biotechnology methods. Genetic Basis of Crop Improvement Agricultural technology is based on the domestication of wild plants to create the crops that we have come to depend upon. Humans invented agriculture approximately 10,000 years ago when they began to harvest and cultivate specific plants to produce food. The improved plant traits selected by early agriculturists were transmitted genetically to succeeding generations of plants. For example, domestication of corn by prehistoric agriculturalists has modified the plant to such an extent that it hardly resembles the wild teosinte plants from which it was originally selected. The majority of the altered traits associated with domestication of crop plants (e.g., seed retention on the plant, easier harvesting, greater size of the harvested seeds or fruits, changes in plant form, reduction or loss of bitter and toxic substances, etc.) were already accomplished by the time of historic agricultural civilizations such as the Egyptian, Chinese, or Mayan. In addition, these primitive crop cultivars, also known as land races, were adapted to local growing conditions and preferences, and therefore remained genetically diverse for traits such as product qualities, stress tolerance, disease resistance, and yield stability. The domesticated plants on which our agricultural technology is based have resulted from the genetic modification of wild plants through thousands of years of gradual selection. An understanding of genetic principles and their application to plant breeding technology has greatly accelerated the rate of improvement of our crop plants. Modern crop cultivars are more genetically uniform than the land race cultivars described above. Breeders maintain a continual search for novel genetic combinations from which to select plants with superior traits, such as crop quality, yield, regional performance, and tolerance to pests and diseases. Breeders often make sexual crosses between diverse genotypes to produce new combinations of genetic traits, which then result in diverse phenotypes, or observable morphological or quality traits in the progeny plants. A primary source of genetic variation is the wide array of germplasm within each crop species and a few closely related wild species that are capable of interbreeding. For many crops, breeders have relied heavily on the introduction of genes from closely related wild plants to increase genetic variation in the crops. Hybridization between a crop plant and a related wild species (a wide cross) enables valuable genes from the wild species to be used for genetic improvement of the crop plant. For example, virtually all modern crop varieties incorporate resistances to fungal, bacterial, and viral diseases that have been introduced through wide crosses between domesticated varieties and related wild species (introgression). Genetic variation can also be increased by inducing mutations, or changes in the DNA sequences of the plants. Since the 1950s, over 2,252 crop varieties have been developed by inducing mutations to randomly alter genetic traits and then selecting among the progeny for improved types. Biotechnology Provides New Tools for Plant Breeding Biotechnology in its broad sense has been used for many years in crop improvement, and new techniques now provide additional tools for genetic enhancement of crop plants. Together, the older and the newer biotechnology techniques constitute a spectrum of tools for altering the properties of crop plants to better suit our needs. Grafting and tissue culture techniques Grafting of tissues from two different varieties of a plant species has long been used in woody tree and vine crops such as citrus, peaches, walnuts, grapes and many shade trees. Surgically cutting and grafting a scion or bud onto a rootstock from a different genetic variety is commonly used to enhance the disease resistance, productivity, and growth habit of these perennial crops. Superior varieties, often developed through chance or induced mutations as well as through sexual crosses, can be rapidly propagated by grafting buds onto the rootstocks of other varieties. Tissue culture has been used in crop improvement since the 1940s. In the simplest cases, this refers to culturing embryos or small plants in the laboratory on specific nutrient media until they can be moved into soil. The tiny growing tips of plants can also be grown in culture to produce entire plants. In addition, plants have the unique property of being able to regenerate entire plants from a single cell (totipotency). Under carefully controlled conditions, tissues can be taken from a plant, separated into individual cells that are grown in the laboratory as callus, then induced to develop back into whole plants (regenerate). These techniques have been used in a number of ways in crop propagation and improvement. Micropropagation is the production of multiple copies of a single plant using tissue culture techniques. The tissue used often is the tiny growing point from the tip of a stem (meristem), where new leaves and stems are produced. Each growing tip on a plant can be excised and grown into a complete new plant. In many cases, viruses that infect a plant are not present in the growing tip, so viruses that have infected the parent plant are eliminated from plants produced by micropropagation. For example, potatoes are normally propagated by planting the buds, or “eyes,” present on the tubers, and garlic is propagated by planting cloves from last year’s crop. This method of propagation allows viruses to be transmitted to the new crop each year, resulting in diseased plants and reduced yields. Micropropagation of potato and garlic plants in the laboratory can eliminate virus diseases and ensure that each new crop is planted with virus-free materials. This has greatly increased yields of both potatoes and garlic. Micropropagation has been the starting point for each new cycle of strawberry plants grown in California for over 20 years both in commercial fields and home gardens. Micropropagation is also used in asparagus and other crops to multiply individual superior plants to create parent plants used in production of hybrid seeds. Many ornamental plants (e.g. orchids and gerbera) are now almost exclusively propagated in this way. Embryo culture has been used to rescue hybrid plants from wide crosses. Many types of wide crosses fail to produce mature viable seeds; in these cases the immature embryo tissue is removed from the developing seeds and cultured in the laboratory to produce the hybrid plants. Embryo culture enables the breeder to successfully make wide crosses with a greater number of related species of wild plants and have access to a much wider range of genes that can be used for genetic improvement of crop plants. Wide crosses and embryo culture have been valuable tools especially for transfer of disease resistance genes from wild plants into crop plants. Virtually every crop species has been genetically enhanced using these methods. Marker-assisted breeding and genomics Marker-assisted (or molecular-assisted) breeding provides a dramatic improvement in the efficiency with which breeders can select plants with desirable combinations of genes. A marker is a "genetic tags" that identifies a particular location within a plant's DNA sequences. Markers can be used in transferring a single gene into a new cultivar or in testing plants for the inheritance of many genes at once. Markers can be based upon either DNA or proteins. Both DNA- and protein-based markers have been widely used in plant breeding, but DNA-based markers are expected to predominate in the future. Greater numbers of DNA-based markers can be identified to cover all regions of an organism's DNA, and they are not based upon the developmental stage of the plant, as many protein markers are. DNA-based markers can be visualized from seed or seedlings in rapid screening tests, performed by automated robotic systems in advanced applications, and plants lacking the desired trait can be discarded before moving to more expensive or lengthy greenhouse and field trials. When transferring a specific desirable gene from a wild plant into a crop plant via wide crosses, the simultaneous transfer of undesired genes from the wild plant is often a problem. In repeated crosses to the cultivated type, the percentage of wild type genes in each generation would be 50 percent in the first generation, 25 percent in the second, 12.5 percent in the third, 6.25 percent in the fourth, etc. Even after many generations, wild type genes located close to the desired gene on the chromosome may still be present (i.e., are closely linked to the desired gene). Marker-assisted breeding is helpful to identify plants that have inherited the desirable gene together with as few of the undesired genes as possible. One can think of the desired gene as the "needle in the [genetic] haystack." Traditional methods require sorting through the entire haystack to find the needle, while markers provide a "metal detector" to easily find the specific part of the haystack where the needle is located. DNA markers are useful to test plants for inheritance of many different genes simultaneously. A simple way to appreciate this powerful tool for selection of improved crop plants is to visualize the UPC barcodes found on most products in the supermarket. The band pattern of the UPC code uniquely identifies the product and can be linked to a database of product traits such as cost, supplier, inventory, etc. In a similar manner, the pattern of DNA markers allows identification of many desirable or undesirable genes that the plant may contain. As the number of traits (genes) that are desired increases, the number of plants that must be screened to identify plants that have the rare superior combinations of genes increases even more rapidly. By providing quick and efficient tests for many different genes, DNA markers have become valuable new tools for breeding crop varieties having optimal combinations of desirable genes. DNA markers have been used for transferring pest resistance genes to cultivated varieties, assisting selection of complex multigene traits (such as flavor), aiding evaluation of regionally and seasonally optimized varieties, allowing genetic purity testing, and enabling proprietary variety protection and patent enforcement. Recombinant DNA techniques Identification and multiplication of specific DNA molecules became possible in 1973, based on discoveries by researchers at the University of California and Stanford University. They developed methods for isolating specific segments of DNA molecules, inserting them into other DNA molecules (or vectors), and using bacteria as "biological copy machines" to produce large quantities of these DNA molecules. These recombinant DNA techniques enable researchers to isolate genes from any organism, amplify (or clone) them in large quantities, and study their characteristics and functions. Transgenic plants Transfer of cloned genes into plants (transformation) is somewhat more difficult than transfer into bacterial and animal cells, but it can be accomplished by several methods. The most commonly used transformation method employs the bacterium Agrobacterium tumefaciens to transfer the desired DNA into the plant. When it encounters a plant, this bacterium naturally transfers part of its DNA into the plant’s chromosomes. These genes cause the plant to make a structure to house the bacterium (a gall) and produce unique compounds that the bacterium consumes. In the 1980s, scientists discovered how to use Agrobacterium to transfer cloned genes into plant chromosomes while preventing the formation of galls. Desired genes can be spliced into the bacterial DNA, and then Agrobacterium, like a video editor, will transfer them into the plant’s chromosomes. A second approach for production of transgenic plants, known as biolistics, coats the desired DNA onto microscopic metal beads and then fires these at high speed into pieces of plant tissue. The beads deliver the DNA into the plant cells, some of which will incorporate the DNA into their chromosomes. The transgenic cells resulting from these processes can be selected and regenerated into whole plants by tissue culture. These plants will then express the characteristics encoded by the cloned genes. Transgenic plant methods can be used to increase expression of an existing characteristic or to decrease expression of an undesirable characteristic in a crop plant. A critical part of the transformation process is the selection of transgenic individuals, as only a small fraction of cells or seeds is likely to have incorporated the desired transgene. A way to do this is to include a second transgene that can act as a selectable marker. One type of marker gene encodes resistance to an antibiotic. Cells that have incorporated the antibiotic resistance gene can then be selected by their ability to grow in the presence of the antibiotic. Those cells have a high probability of also containing the desired transgene. Some people have raised concerns about whether the widespread use of such genes will increase the development of antibiotic resistance in human pathogens and make current medicines obsolete. However, the likelihood of this is extremely remote. The antibiotics used in plant transformation were chosen because they are seldom used in human medicine. Nonetheless, alternative types of marker genes have been developed and will likely replace the antibiotic resistance method in the future. Transgenic plants are sometimes referred to as genetically modified organisms (GMOs). Despite the widespread use of this term, using the term "GMO" in this way is misleading because all of our domesticated plants and animals have been genetically modified using a variety of traditional and modern technologies, as described above. These plants can be described more precisely as "transgenic plants" or plants containing cloned genes. The ability to transfer cloned genes allows plant breeders to use genes from essentially any source as tools for crop improvement. For example, to enable rice grains to accumulate beta-carotene (which is converted into vitamin A when consumed by animals) and create the so-called “Golden rice,” scientists used genes from daffodil, pea, a bacterium, and a virus. Transgenic plant methods enable these four well-characterized genes to be inserted into a transgenic plant, producing a highly specific change in only the trait of interest. In contrast, many unknown genes are introduced when a breeder uses wide crosses to transfer a desired gene from a wild plant into a crop plant. For example, when the Mi gene for nematode resistance was transferred into tomato via wide crosses, the total amount of DNA exchanged was over 70 times larger than the Mi gene alone. Using the cloned Mi gene, only the nematode resistance trait would be introduced into other plants without transferring any additional DNA (and potentially undesirable traits) from the wild species. Woody tree and vine crops are exceedingly difficult to improve by traditional breeding technology because it takes a number of years for a seedling to begin flowering, and the unique traits of specific varieties are hard to regain after sexual crosses. For example, some wine grape varieties have been propagated for hundreds of years by cuttings, since plants produced from seed will segregate for genetic traits, often losing the distinctive characteristics desired in the variety. On the other hand, if only one or a few genes could be transferred into a traditional variety, for example, to confer resistance to a disease or insect, the valuable genetic combination represented by the variety would be preserved. ### Trevor V. Suslow Vegetable Research and Information Center Department of Vegetable Crops, University of California, Davis, CA http://vric.ucdavis.edu/ Bruce R. Thomas and Kent J. Bradford Seed Biotechnology Center University of California, Davis, CA http://sbc.ucdavis.edu © 2001 by the Regents of the University of California, Division of Agriculture and Natural Resources (ANR). All rights reserved. Excerpted by The Science Advisory Board with kind permission of the ANR Communication Services. A full-text version of this article (Publication 8042) is available at DANR Communication Services website at http://anrcatalog.ucdavis.edu. Sources of Additional Information ABC Series, Agricultural Biotechnology in California http://sbc.ucdavis.edu/outreach/abc/abc_series.htm Biotechnology Resource Series, Seed Biotechnology Center, UC Davis http://sbc.ucdavis.edu/outreach/resource/resource_series.htm UCBiotech, University of California, Berkeley http://ucbiotech.org/index.html Burke J, McGloughlin M. 2000. Biotechnology - Present Position and Future Developments. Published by Teagasc, Dublin. ### << Previous Next >> [ View All Perspectives ] |
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