Plant breeding 147483 223313591 2008-07-03T14:54:25Z 4.88.161.5 /* See also */ '''Plant breeding''' is the art and science of changing the genetics of plants for the benefit of humankind<ref>Breeding Field Crops. 1995. Sleper and Poehlman. Page 3</ref>. Plant breeding can be accomplished through many different techniques ranging from simply selecting plants with desirable characteristics for propagation, to more complex molecular techniques (see [[cultigen]] and [[cultivar]]). Plant breeding has been practised for thousands of years, since near the beginning of human civilization. It is now practised worldwide by individuals such as gardeners and farmers, or by professional plant breeders employed by organizations such as [[government]] institutions, universities, crop-specific industry associations or research centres. International development agencies believe that breeding new crops is important for ensuring [[food security]] by developing new varieties that are higher-yielding, resistant to pests and diseases, drought-resistant or regionally adapted to different environments and growing conditions. ==Domestication== [[Image:Feature2originmap600.png|right|thumb|400px|This map shows the sites of domestication for a number of crops. Places where crops were initially domesticated are called centres of origin]] {{main|Domestication}} Plant breeding in certain situations may lead the [[domestication]] of wild plants. [[Domestication]] of plants is an [[artificial selection]] process conducted by [[human]]s to produce plants that have more desirable traits than wild plants, and which renders them dependent on artificial (usually enhanced) environments for their continued existence. The practice is estimated to date back 9,000-11,000 years. Many crops in present day cultivation are the result of domestication in ancient times, about 5,000 years ago in the [[Old World]] and 3,000 years ago in the [[New World]]. In the [[Neolithic]] period, domestication took a minimum of 1,000 years and a maximum of 7,000 years. Today, all of our principal [[food]] crops come from domesticated varieties. A plant whose origin or selection is due primarily to intentional human activity is called a [[cultigen]], and a cultivated crop [[species]] that has evolved from [[wild]] populations due to selective pressures from traditional [[farmer]]s is called a [[landrace]]. Landraces, which can be the result of natural forces or domestication, are plants (or [[animal]]s) that are ideally suited to a particular region or environment. An example are the landraces of [[rice]], ''Oryza sativa'' [[subspecies]] ''indica'', which was developed in [[South Asia]], and ''Oryza sativa'' subspecies ''japonica'', which was developed in [[China]]. For more on the mechanisms of domestication, see [[Hybrid (biology)]]. ==Classical plant breeding== Classical plant breeding uses deliberate interbreeding ('''crossing''') of closely or distantly related individuals to produce new crop varieties or lines with desirable properties. Plants are crossbred to introduce [[Trait (biological)|trait]]s/[[gene]]s from one variety or line into a new genetic background. For example, a [[mildew]]-resistant [[pea]] may be crossed with a high-yielding but susceptible pea, the goal of the cross being to introduce mildew resistance without losing the high-yield characteristics. Progeny from the cross would then be crossed with the high-yielding parent to ensure that the progeny were most like the high-yielding parent, ('''[[backcrossing]]'''). The progeny from that cross would then be tested for yield and mildew resistance and high-yielding resistant plants would be further developed. Plants may also be crossed with themselves to produce '''inbred''' varieties for breeding. Classical breeding relies largely on [[genetic recombination|homologous recombination]] between chromosomes to generate [[genetic diversity]]. The classical plant breeder may also makes use of a number of ''in vitro'' techniques such as protoplast fusion, embryo rescue or mutagenesis (see below) to generate diversity and produce hybrid plants that would not exist in [[nature]]. [[Image:Wheat selection k10183-1.jpg|right|thumb|270px|The Yecoro [[wheat]] (right) [[cultivar]] is sensitive to salinity, plants resulting from a hybrid cross with cultivar W4910 (left) show greater tolerance to high salinity]] Traits that breeders have tried to incorporate into crop plants in the last 100 years include: # Increased [[quality]] and [[yield]] of the crop # Increased [[Physiological tolerance|tolerance]] of environmental pressures ([[salinity]], extreme [[temperature]], [[drought]]) # Resistance to [[virus]]es, [[fungus|fungi]] and [[bacterium|bacteria]] # Increased tolerance to [[insect]] pests # Increased tolerance of [[herbicide]]s ===Before World War II=== Intraspecific [[Hybrid_%28biology%29#Interspecific_hybrids|hybrid]]ization within a plant species was demonstrated by [[Charles Darwin]] and [[Gregor Mendel]], and was further developed by [[geneticist]]s and plant breeders. In the early 20th century, plant breeders realized that Mendel's findings on the non-random nature of [[Mendelian inheritance|inheritance]] could be applied to [[seedling]] populations produced through deliberate [[pollination]]s to predict the frequencies of different types. In 1908, [[George Harrison Shull]] described [[heterosis]], also known as hybrid vigor. Heterosis describes the tendency of the progeny of a specific cross to outperform both parents. The detection of the usefulness of heterosis for plant breeding has led to the development of inbred lines that reveal a heterotic yield advantage when they are crossed. [[Maize]] was the first species where heterosis was widely used to produce hybrids. By the 1920s, [[statistics|statistical]] methods were developed to analyze gene action and distinguish heritable variation from variation caused by environment. In 1933, another important breeding technique, [[cytoplasmic male sterility]] (CMS), developed in maize, was described by [[Marcus Morton Rhoades]]. CMS is a maternally inherited trait that makes the plant produce sterile [[pollen]]. This enables the production of hybrids without the need for labour intensive [[detasseling]]. These early breeding techniques resulted in large yield increase in the [[United States]] in the early 20th century. Similar yield increases were not produced elsewhere until after [[World War II]], the [[Green Revolution]] increased crop production in the developing world in the 1960s. ===After World War II=== Following [[World War II]] a number of techniques were developed that allowed plant breeders to hybridize distantly related species, and artificially induce genetic diversity. When distantly related species are crossed, plant breeders make use of a number of [[plant tissue culture]] techniques to produce progeny from otherwise fruitless mating. Interspecific and intergeneric hybrids are produced from a cross of related species or genera that do not normally [[reproduction|sexually reproduce]] with each other. These crosses are referred to as ''Wide crosses''. For example, the [[cereal]] [[triticale]] is a [[wheat]] and [[rye]] hybrid. The cells in the plants derived from the first generation created from the cross contained an uneven number of chromosomes and as result was sterile. The [[cell division]] inhibitor [[colchicine]] was used to double the number of [[chromosome]]s in the [[Cell (biology)|cell]] and thus allow the production of a fertile line. Failure to produce a hybrid may be due to pre- or post-[[fertilization]] incompatibility. If fertilization is possible between two species or genera, the hybrid [[embryo]] may abort before maturation. If this does occur the embryo resulting from an interspecific or intergeneric cross can sometimes be rescued and cultured to produce a whole plant. Such a method is referred to as ''Embryo Rescue''. This technique has been used to produce [[New Rice for Africa|new rice for Africa]], an interspecific cross of Asian rice ''(Oryza sativa)'' and African rice ''(Oryza glaberrima)''. Hybrids may also be produced by a technique called [[protoplast]] fusion. In this case protoplasts are fused, usually in an electric field. Viable recombinants can be regenerated in culture. Chemical [[mutagen]]s like [[ethyl methanesulfonate|EMS]] and [[dimethyl sulfate|DMS]], [[radiation]] and [[transposon]]s are used to generate [[mutant]]s with desirable traits to be bred with other [[cultivar]]s. Classical plant breeders also generate genetic diversity within a species by exploiting a process called [[somaclonal variation]], which occurs in plants produced from tissue culture, particularly plants derived from [[plant tissue culture|callus]]. Induced [[polyploidy]], and the addition or removal of chromosomes using a technique called [[chromosome engineering]] may also be used. When a desirable trait has been bred into a species, a number of crosses to the favored parent are made to make the new plant as similar to the favored parent as possible. Returning to the example of the mildew resistant pea being crossed with a high-yielding but susceptible pea, to make the mildew resistant progeny of the cross most like the high-yielding parent, the progeny will be crossed back to that parent for several generations (See [[backcrossing]] ). This process removes most of the genetic contribution of the mildew resistant parent. Classical breeding is therefore a cyclical process. It should be noted that with classical breeding techniques, the breeder does not know exactly what genes have been introduced to the new cultivars. Some [[scientist]]s therefore argue that plants produced by classical breeding methods should undergo the same safety testing regime as [[genetic engineering|genetically modified]] plants. There have been instances where plants bred using classical techniques have been unsuitable for human consumption, for example the [[poison]] [[solanine]] was unintentionally increased to unacceptable levels in certain varieties of [[potato]] though plant breeding. New potato varieties are often screened for solanine levels before reaching the marketplace. ==Modern plant breeding== Modern plant breeding uses techniques of molecular biology to select, or in the case of genetic modification, to insert, desirable traits into plants. [[Image:Adelport 270.jpg|left|350px|thumb|Modern facilities in molecular biology has converted classical plant breeding to molecular plant breeding [[Photo: Rosyara]].]] ===Steps of Plant Breeding=== Major activities of plant breeding are following; #Creation variation #Selection #Evaluation #Release #Multiplication #Distribution of the new variety ===Marker assisted selection=== :''See main article on [[Marker assisted selection]].'' Sometimes many different genes can influence a desirable trait in plant breeding. The use of tools such as [[molecular marker]]s or [[DNA fingerprinting]] can map thousands of genes. This allows plant breeders to screen large populations of plants for those that possess the trait of interest. The screening is based on the presence or absence of a certain gene as determined by laboratory procedures, rather than on the visual identification of the expressed trait in the plant. ===Reverse Breeding and Doubled Haploids (DH)=== :''See also main article on [[Doubled haploidy]].'' A method for efficiently producing [[homozygous]] plants from a [[heterozygous]] starting plant, which has all desirable traits. This starting plant is induced to produce [[Doubled haploidy|doubled haploid]] from haploid cells, and later on creating homozygous/[[Doubled haploidy|doubled haploid]] plants from those cells. While in natural offspring [[recombination]] occurs and traits can be unlinked from each other, in doubled haploid cells and in the resulting DH plants [[recombination]] is no longer an issue. There, a recombination between two corresponding [[chromosomes]] does not lead to un-linkage of [[allele]]s or traits, since it just leads to recombination with its identical copy. Thus, traits on one [[chromosome]] stay linked. Selecting those offspring having the desired set of chromosomes and crossing them will result in a final [[F1 hybrid]] plant, having exactly the same set of chromosomes, genes and traits as the starting hybrid plant. The homozygous parental lines can reconstitute the original heterozygous plant by crossing, if desired even in a large quantity. An individual heterozygous plant can be converted into a heterozygous variety ([[F1 hybrid]]) without the necessity of [[vegetative]] propagation but as the result of the cross of two homozygous/doubled haploid lines derived from the originally selected plant. [http://www.wipo.int/pctdb/en/wo.jsp?wo=2003017753&IA=WO2003017753&DISPLAY=STATUS patent] ===Genetic modification=== :''See main article on [[Transgenic plants]].'' [[Genetic modification]] of plants is achieved by adding a specific gene or genes to a plant, or by knocking out a gene with [[RNAi]], to produce a desirable [[phenotype]]. The plants resulting from adding a gene are often referred to as [[transgenic plants]]. If for genetic modification genes of the species or of a crossable plant are used under control of their native promoter, then they are called [[Cisgenic plants]]. Genetic modification can produce a plant with the desired trait or traits faster than classical breeding because the majority of the plant's genome is not altered. To genetically modify a plant, a genetic construct must be designed so that the gene to be added or removed will be expressed by the plant. To do this, a [[promoter]] to drive [[Transcription (genetics)|transcription]] and a termination sequence to stop transcription of the new gene, and the gene or genes of interest must be introduced to the plant. A marker for the selection of transformed plants is also included. In the [[laboratory]], [[antibiotic resistance]] is a commonly used marker: plants that have been successfully transformed will grow on media containing antibiotics; plants that have not been transformed will die. In some instances markers for selection are removed by [[backcrossing]] with the parent plant prior to commercial release. The construct can be inserted in the plant genome by [[genetic recombination]] using the bacteria ''[[Agrobacterium tumefaciens]]'' or ''[[Agrobacterium rhizogenes|A. rhizogenes]]'', or by direct methods like the [[gene gun]] or [[microinjection]]. Using plant [[virus]]es to insert genetic constructs into plants is also a possibility, but the technique is limited by the host range of the virus. For example, [[Cauliflower mosaic virus]] (CaMV) only infects [[cauliflower]] and related species. Another limitation of viral vectors is that the virus is not usually passed on the progeny, so every plant has to be inoculated. The majority of commercially released transgenic plants, are currently limited to plants that have introduced resistance to [[insect]] [[pest (animal)|pest]]s and [[herbicide]]s. Insect resistance is achieved through incorporation of a gene from ''[[Bacillus thuringiensis]]'' (Bt) that encodes a [[protein]] that is toxic to some insects. For example, the [[Helicoverpa zea|cotton bollworm]], a common cotton pest, feeds on Bt cotton it will ingest the toxin and die. Herbicides usually work by binding to certain plant enzymes and inhibiting their action. The enzymes that the herbicide inhibits are known as the herbicides ''target site''. Herbicide resistance can be engineered into crops by expressing a version of ''target site'' protein that is not inhibited by the herbicide. This is the method used to produce glyphosate resistant crop plants (See [[Glyphosate#Glyphosate resistance|Glyphosate]]) Genetic modification of plants that can produce [[pharmaceutical]]s (and industrial chemicals), sometimes called ''[[pharmacrop]]s'', is a rather radical new area of plant breeding. ==Issues and concerns== Modern plant breeding, whether classical or through genetic engineering, comes with issues of concern, particularly with regard to food crops. The question of whether breeding can have a negative effect on [[nutrition]]al value is central in this respect. Although relatively little direct research in this area has been done, there are scientific indications that, by favoring certain aspects of a plant's development, other aspects may be retarded. A study published in the ''[[Journal of the American College of Nutrition]]'' in 2004, entitled ''Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999'', compared nutritional analysis of [[vegetable]]s done in 1950 and in 1999, and found substantial decreases in six of 13 [[nutrient]]s measured, including 6% of [[protein]] and 38% of [[riboflavin]]. Reductions in [[calcium]], [[phosphorus]], [[iron]] and [[ascorbic acid]] were also found. The study, conducted at the Biochemical Institute, [[University of Texas at Austin]], concluded in summary: ''"We suggest that any real declines are generally most easily explained by changes in cultivated varieties between 1950 and 1999, in which there may be trade-offs between [[yield]] and nutrient content.<ref>Davis, D.R., Epp, M.D., and Riordan, H.D. 2004. [http://www.jacn.org/cgi/content/abstract/23/6/669?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=donald+davis&andorexacttitle=and&andorexacttitleabs=and&andorexactfulltext=and&searchid=1110477116345_697&stored_search=&FIRSTINDEX=0&sortspec=relevance&journalcode=jamcnutr Changes in USDA Food Composition Data for 43 Garden Crops], 1950 to 1999. ''Journal of the American College of Nutrition'' 23(6):669-682</ref>"'' The debate surrounding genetically modified food peaked in 1999 in terms of media coverage and risk perception,<ref>Costa-Font, J. and E. Mossialos. 2007. [http://www.aseanbiotechnology.info/Abstract/21020573.pdf]''Food Quality and Preference'' 18(2007):173-182</ref> but it continues today. The debate encompasses the [[Ecological impact of transgenic plants|ecological impact of genetically modified plants]], the safety of [[genetically modified food]] and concepts used for safety evaluation like [[substantial equivalence]]. Such concerns are not new to plant breeding. Most countries have regulatory processes in place to help ensure that new crop varieties entering the marketplace are both safe and meet farmers' needs. Examples include variety registration, seed schemes, regulatory authorizations for GM plants, etc. [[Plant breeders' rights]] is also a major and controversial issue. Today, production of new varieties is dominated by commercial plant breeders, who seek to protect their work and collect royalties through national and international agreements based in [[intellectual property rights]]. The range of related issues is complex. In the simplest terms, critics of the increasingly restrictive regulations argue that, through a combination of technical and economic pressures, commercial breeders are reducing [[biodiversity]] and significantly constraining individuals (such as farmers) from developing and trading seed on a regional level{{fact | date = November 2007}}. Efforts to strengthen breeders' rights, for example, by lengthening periods of variety protection{{fact | date = November 2007}}, are ongoing. When a new plant breeds or cultivars are breed, they must be maintained and propagated. Some plants are propagated by asexual means while others are propagated by seeds. Seed propagated cultivars require specific control over seed source and production procedures to maintain the integrity of the plant breeds results. Isolation is necessary to prevent cross contamination with related plants or the mixing of seeds after harvesting. Isolation is normally accomplished by planting distance but in certain crops, plants are enclosed in greenhouses or cages (most comely used when producing F1 hybrids.) ==See also== *[[Smart breeding]] *[[Marker assisted selection]] *[[Physiological and molecular wheat breeding]] ==List of plant breeders== *[[Gregor Mendel]] *[[Luther Burbank]] *[[Roger Doucet]] *[[Carol Deppe]] *[[Colin Wyatt]] *[[Glenn Drowns]] *[[Alan Kapuler]] ==Notes== {{reflist}} ==References== *Borojevic, S. 1990. ''Principles and Methods of Plant Breeding''. Elserier, Amsterdam. ISBN 0-444-98832-7 *McCouch, S. 2004. [http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0020347 Diversifying Selection in Plant Breeding]. ''PLoS Biol'' 2(10): e347. *Briggs, F.N. and Knowles, P.F. 1967. ''Introduction to Plant Breeding''. Reinhold Publishing Corporation, New York. *Gepts, P. (2002). [http://crop.scijournals.org/cgi/content/full/42/6/1780 A Comparison between Crop Domestication, Classical Plant Breeding, and Genetic Engineering]. ''Crop Science'' 42:1780–1790 *''[http://www.ipgri.cgiar.org/publications/pubfile.asp?ID_PUB=47 The Origins of Agriculture and Crop Domestication - The Harlan Symposium]'' *news@nature.com. 1999 ''[http://www.nature.com/news/1999/990923/pf/990923-3_pf.html Are non-GM crops safe?] * Schouten, Henk J., Frans A. Krens & Evert Jacobsen Do cisgenic plants warrant less stringent oversight? Nature Biotechnology 24, 753 (2006). * Schouten, Henk J., Frans A Krens & Evert Jacobsen Cisgenic plants are similar to traditionally bred plants: EMBO reports 7, 750 - 753 (2006). * Sun, C. et ''al''. 1998. [http://www.carleton.ca/~bgordon/Rice/papers/SUN98.htm From indica and japonica splitting in common wild rice DNA to the origin and evolution of Asian cultivated rice]. ''Agricultural Archaeology'' 1998:21-29 ==External links== * [http://pubs.cas.psu.edu/freepubs/pdfs/uk102.pdf Making genetically engineered plants] * [http://www.nytimes.com/2007/08/28/science/28crop.html?8dpc=&pagewanted=all NYTimes - Useful Mutants, Bred With Radiation] * [http://www.infography.com/content/098241002810.html Infography about the History of Plant Breeding] [[Category:Agronomy]] [[Category:Molecular biology]] [[Category:Plant reproduction]] [[Category:Pollination management]] [[Category:Breeding]] [[cs:Rostlinná biotechnologie]] [[de:Pflanzenzüchtung]] [[fr:Culture sélective des plantes]] [[id:Pemuliaan tanaman]] [[nl:Plantenveredeling]] [[pl:Hodowla roślin]] [[sv:Växtförädling]]