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==Genetic modification and its post-1975 consequences for plant breeding==
==Genetic modification and its post-1975 consequences for plant breeding==
:''See main article on [[Transgenic plants]].''


[[Biotechnology and Plant breeding]]
:''See main article on [[Biotechnology and Plant breeding]] .''
 
:''See also [[Transgenic plants]].''
[[Genetic modification]] of plants is achieved by adding a specific gene or genes to a plant, or by knocking out the expressing of a gene with [[RNAi]], to produce a desirable [[phenotype]]. The plants resulting from adding a gene are often referred to as [[transgenic plants]]. Plants in which RNAi is used to silence genes are now starting to be 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 knocked-out 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 of 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 ''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]]).
 
Marker assisted breeding refers to direct detection of small DNA subregions, such as [[restriction fragment length polymorphism]]s ([[RFLP]]s) or  [[micro-satellites]], with specific molecular tests such as the [[polymerase chain reaction]]. An alternative term is DNA-fingerprinting. While not actually a genetic engineering techniques themselves, they are now part of mainstream plant biotechnology, were invented using [[genetic engineering]] methods, and are heavily dependant on [[molecular biology]] insights.
 
DNA markers are useful for backcrossing major genes (such as those conferring pest-tolerance) into proven high performing cultivars <ref>[http://www.ars.usda.gov/SP2UserFiles/ad_hoc/66452500Publications/Holland/Holland4thInt'lCropSciCong04.pdf Implementation of molecular markers for quantitative traits in breeding programs - challenges and opportunities James B. Holland 2004. "New directions for a diverse planet". Proceedings of the 4th International Crop Science Congress.]</ref> . They can aid selection for traits that are not easily assayed in individual plants. Introduction of unwanted genes, genetically linked to the desired trait ([[linkage drag]] <ref>[Young ND, Tanksley SD (1989) RFLP analysis of the size of chromosomal segments retained around the Tm-2 locus of tomato during backcross breeding. Theoretical and Applied Genetics 77, 353-359.]</ref>) can be minimized, and the time needed to obtain a plant with a high percentage ( 98 to 99 percent) of the original desirable genetic background can be substantially reduced. <ref>[http://crop.scijournals.org/cgi/content/abstract/39/5/1295 Frisch M, Bohn M, Melchinger AE (1998) Comparison of selection strategies for marker-assisted backcrossing of a gene. Crop Science 39, 1295-1301] </ref>. Such additional genes are a significant issue when classical breeding methods used to transfer major traits.
 
A good example illustrating the several advantages of marker assisted backcrossing was reported by Chinese scientists in 2000 working with rice, and improving bacterial blight resistance with the Xa21 gene. For this fine achievement Chen, Lin, Xu and Zhang used [[RFLP]] DNA markers to assist their breeding <ref>[http://crop.scijournals.org/cgi/content/abstract/40/1/239 Chen S, Lin XH, Xu CG, Zhang Q (2000) Improvement of bacterial blight resistance 'Minghui 63', an elite restorer line of hybrid rice, by molecular marker-assisted selection. Crop Science 40, 239-244.]</ref>.


==Twenty first century plant breeding==
==Twenty first century plant breeding==

Revision as of 00:32, 29 November 2006

See Agriculture for related topics and context.

Plant breeding is the purposeful manipulation of plant species in order to create desired genotypes and phenotypes for specific purposes, such as food production, forestry, or ornamental flowers. This manipulation relies on a wide range of often complementary techniques and approaches.Plant breeding often, but not always, leads to plant domestication, and complements other approaches (such as introduction of new crops, changed rotations and tillage practices, irrigation, and integrated pest management) for improving crop productivitry and land stewardship.

Different Plant breeding approaches are not used in isolation. Traditional breeding programs have generated germplasm collections which are abosulutely essential for practical application of transgenic traits such as Bt-based insect tolerance created by genetic engineering methods, and conventional breeding methods are invaluable for enabling transgenic traits to be deployed after they have been introduced into crops. Molecular genetics has generated marker-assisted breeding techniques which speed up slow classical breeding, but marker-assisted breeding cannot by itself transfer valuable traits like mildew resistance if they are found outside the usual crop gene pool.

Plant breeding has been practiced for thousands of years, since near the beginning of human civilization. It is now practiced worldwide by government institutions and commercial enterprises. International development agencies believe that breeding new crops is important for ensuring food security and developing practices of sustainable agriculture through the development of crops suitable for their environment [1] [2].

Range of approaches used

The diverse techniques used in plant breeding include, introduction of new gemplasm from distant geographical regions or from seed-bank collections, cross-pollination, either within the species, or between related species and genera - including wide-crosses using wild relatives of domesticated plants to introduce pest resistant traits needed in domesticated varieties, creation of artificial hybrids and exploitation of hybrid vigor (heterosis), creation of mutants by irradiation or chemical treatment, embryo-rescue, colchicine treatment to create artificial polyploids, protoplast-fusion, genetic engineering to generate transgenic plants, RNA silencing (cisgenics), artificial selection of progeny and molecular-marker assisted breeding[3], and use of statistical principles to design field tests of new variety performance with sufficient power to detect improvement.

Genome science (chromosome sequence decoding and computer assisted dissection of gene functions and stucture) is also being bought into play to assist plant breeders. One approach is to compare gene arrangement in different species (comparative genomics) to take advantage of the greater ease of gene sequencing and faster progress with smaller more compact genomes such as those of Arabidopsis thaliana, or of rice, to provide clues for gene function and location in crop species with larger genomes.

Domestication

Domestication of plants is an artificial selection process conducted by humans to produce plants that have fewer undesirable traits of 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 cultivated crop species that has evolved from wild populations due to selective pressures from traditional farmers is called a landrace. Landraces, which can be the result of natural forces or domestication, are plants (or animals) 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.

Germplasm collections

It was in the 1930s that Russian scientist Nikolai Vavilov first called attention to the value of wild crop relatives as a source of genes for improving agriculture, and in travels over five continents amassed the largest collection of (at that time) of species and strains of cultivated plants in the world. [4] [5]

Vavilov's intent was to promote crop improvement but since his time other considerations have added to expansion of seed-banks. One major concern is the limited genetic diversity of crop plants, and the vulnrabilities to crop diseases that it introduces into the food supply. This genetic vulnrability was highlighted in 1970 by a severe outbreak of Southern corn leaf blight in the United States.

The FAO estimate that globally, distinct seed samples in plant seed collections total over 6 million samples, held in in 1300 genebanks worldwide (B. Koo and others in Saving Seeds 2004, citing FAO 1998). About 10 percent of these are held by the substantial international network of crop germplasm collections managed by the Consultative Group on International Agricultural Research (CGIAR).

CGIAR is a strategic alliance of countries, international and regional organizations, and private foundations supporting 15 international agricultural centers that was created in 1971. CGIAR genbanks include

Recent achievements coming from this germplasm resource and the associated CGIAR network of scientists include:

  • Quality Protein Maize (QPM) varieties have been released in 25 countries, and are grown on more than 600,000 hectares
  • New Rices for Africa (NERICAs) from Africa Rice Center (WARDA) that are transforming agriculture in the West Africa region. In 2003 it is estimated that NERICAs were planted on 23,000 hectares, and their use is spreading across Africa, for instance to Uganda and Guinea.
  • Release across Latin America of many new bean varieties and improved forages that are grown on over 100 million hectares in that region.

Classical plant breeding

See Classical plant breeding.

Genetic modification and its post-1975 consequences for plant breeding

See main article on Biotechnology and Plant breeding .
See also Transgenic plants.

Twenty first century plant breeding

Template:Stub The scope of plant breeding continues to expand in the twenty first century. Genomics, marker-assisted breeding, and RNA interferance (RNAi, siRNA, cisgenics) are increasingly effective in accellerating commercial breeding, identifying the functions of physiologically relevant genes, and in allowing traits to be modified. Recent work with identifying wheat genes that infuence protein content illustrates how RNAi and marker assisted breeding come together in providing faster methods for crop improvement, although it needs to be borne in mind that improved protein quality and crop yield represent a trade-off.[6]

Modern plant breeding allows plants to be modified to express proteins such as a therapeutic monoclonal antibody used in the treatment of arthritis, or for treatment of diarrhea [7], which can save thousands, if not millions of childrens lives in the developing world. The term plant-made pharmaceuticals, refers to these therapeutic agents (pharmaceutical proteins) produced in live plants. The production of plant-based pharmaceuticals is an emerging area of modern crop biotechnology.

Issues and concerns

Modern plant breeding, whether classical or through genetic engineering, comes with issues of concern, particularly with regard to food crops.

Surveys of changes in American foods 1950-1999 have suggested there may be decreases in nutitional quality of many garden crops over this time period, possibly because of breeding for higher yield [8]

This is not a new issue though. Recent studies [9] [10] have revealed that at the begining of agriculture, a gene was lost from wheat that mobilizes nutrients from leaves, causing better yields at the expense of protein content. It has long been known that among the many varieties of wheat used in modern times, there is an inverse relationship between yield and protein content.[11]. There is also increasing emphasis on breeding crops for nutritional improvement [12].

The debate surrounding plant breeding genetic modification of plants is huge, encompassing the ecological impact of genetically modified plants and the safety of genetically modified food. It extends also to the issue of Food security because of the strong link between increases in crop output and matching of food supply to growing food demand caused by population growth and economic growth. Agencies such as the International Food Policy Research Institute (IFPRI) have highlighted the mis-match between amount of agricultural R&D and food security in the developing world [13].

Plant breeders' rights is also a major and controversial issue. Efforts to strengthen breeders' rights, for example, by lengthening periods of variety protection, are ongoing. 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 crop-breeding 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.

But seed breeding is a specialised economic activity that most farmers do not have the time to pursue, and better seed provides a simple means of technology transfer that provides an economic benefit to the farmer. Expansion of a commercialized seed industry is historically associated with substantial economic gains in that sector as illustrated by hybrid maize in the USA, and more recently, the Indian cotton seed industry [14] [15] [16]. Critics of excessive precautionary regulation argue that costly regulatory burdens and delayes imposed on new seed-breeding technologies restrict investment in much modern agricultural technology to organisations having substantial financial assets, which limits the effectiveness of public research efforts in developing countries.

Citations

  1. ↑ Ngambeki, D.S. (2005) Science and technology platform for African Development: towards a green revolution in Africa, The New Partnership for Africa's Development
  2. ↑ Consultative Group on International Agricultural Research. 2002. Agriculture and the environment, partnership for a sustainable future
  3. ↑ Coordinated Agricultural project , UC Davis.
  4. ↑ [Tanksley SD, McCouch SR.(1997). Seed banks and molecular maps: unlocking genetic potential from the wild. Science. 1997 Aug 22;277(5329):1063-6. citing N. I. Vavilov, in The New Systematics, J. Huxley, Ed. (Clarendon, Oxford, 1940), pp. 549–566.]
  5. ↑ B Koo, International Food Policy Research Institute, (IFPRI), Washington D C, USA; P G Pardey, University of Minnesota, USA; B D Wright, University of California, Berkeley, USA, and others. (2004). Saving Seeds: The Economics of Conserving Crop Genetic Resources Ex Situ in the Future Harvest Centres of CGIAR, page 1 citing Resnick S and Vavilov Y (1997) The Russian Scientist Nicolay Vavilov. Preface to the English translation of Five Continents by N. I Vavilov, International Plant Genetic Resources Institute, Rome.
  6. ↑ Scientific American November 24, 2006 Crossing Wild and Conventional Wheat Boosts Protein, Avoids Genetic Modification
  7. ↑ May 1, 2006 – A Breakthrough For Second Leading Killer of Children Under Five – A Medical Food for Acute Diarrhea. The results of a recent study show that adding Lactiva and Lysomin to oral rehydration solution helps to reduce the duration and recurrence of acute diarrhea in children
  8. ↑ Davis, D.R., Epp, M.D., and Riordan, H.D. (2004). Changes in USDA Food Composition Data for 43 Garden Crops 1950 to 1999. Journal of the American College of Nutrition 23(6):669-682
  9. ↑ Wheat gene may boost foods' nutrient content
  10. ↑ Uauy C, Assaf Distelfeld, A, Fahima, T, AnnBlechl, A, Dubcovsky, J (2006) A NAC Gene Regulating Senescence Improves Grain Protein, Zinc, and Iron Content in Wheat Science 24 November 2006: Vol. 314. no. 5803, pp. 1298 - 1301 DOI: 10.1126/science.1133649
  11. ↑ SIMMONDS NW (1995) THE RELATION BETWEEN YIELD AND PROTEIN IN CEREAL GRAIN JOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE 67 (3): 309-315 MAR 1995
  12. ↑ Philip G. Pardey, Julian M. Alston, and Roley R. Piggott, eds. (2006) Agricultural R&D in the Developing World Too Little, Too Late/ DOI: http://dx.doi.org/10.2499/089629756XAGRD
  13. ↑ Philip G. Pardey, Julian M. Alston, and Roley R. Piggott, eds. (2006) Agricultural R&D in the Developing World Too Little, Too Late/ DOI: http://dx.doi.org/10.2499/089629756XAGRD
  14. ↑ C Kameswara Rao 2006) PERFORMANCE OF Bt COTTON IN INDIA: THE 2005-06 SEASON, Foundation for Biotechnology Awareness and Education, Bangalore, India
  15. ↑ Milind Murugkar, Bharat Ramaswami, Mahesh Shelar, January 2006, Liberalization, Biotechnology and the Private Seed Sector: The Case of India’s Cotton Seed Market Discussion Paper 06-05, Indian Statistical Institute, Delhi
  16. ↑ Duvick DN. (2001) Biotechnology in the 1930s: the development of hybrid maize. Nat Rev Genet. 2001 Jan;2(1):69-74.

General Bibliography

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