Plant breeding

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.

Plant breeding is an artificial version of natural evolution. Rather than natural selection of heritable traits, plant breeding involves artificial selection of desired plant characteristics.

Different plant breeding approaches are not used in isolation. Traditional breeding programs have generated germplasm resouce such as high-performing varieties which are absolutely essential for economic success of transgenic traits such as Bacillus thuringiensis (Bt)-based insect tolerance created by genetic engineering methods. Conventional breeding methods are invaluable for enabling transgenic traits to be transferred to useful cultivars after they have been introduced into crops.

On the other hand, although molecular genetics has generated marker-assisted breeding techniques which speed up slow classical breeding, marker-assisted breeding cannot by itself introduce 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 .

Range of approaches used

The starting points for plant breeding is a population of parental organisms that exhibit heritable differences from one another, and the use of induced mutation and other techniques to increase the range of genetic variability. Like natural evolution, plant breeding exploits genetic recombination to generate novelty, and this includes processes such as meiotic recombination and independent assortment of alleles.

In addition to taking advantage of natural gene flow and horizontal gene transfer events to allow new traits to be added to existing crops, it also uses artificial means for gene transfer such as embryo rescue and biolistics to overcome natural barriers to gene flow between different gene-pools.

Male cytoplasmic sterility and other approaches is exploited to create artificial hybrids, and the greater vigor from hybrid vigor (heterosis) is taken advantage of in several important crops (e.g. maize, rice, canola), whether it is a natural hybrid system of a synthetic one. One of the earliest artificial hybrids is Triticale, first created in the late 19th century.

Hypbrid vigor is related partly to presence of heterozygosity, that is two alternative version of the same gene. Another another factor having a similar effect is polyploidy, or presence of multiple sets of chromosomes. All crop genomes are polyploid, bread wheat for instance is hexaploid, and maize an ancient tetraploid. These genetic features of crop plants add extra subtlety to crop breeding and ensure that germplasm collections have great practical value .

Techniques for increasing the available heritable variation in the intial population include introduction of new germplasm 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, and creation of mutations by irradiation or chemical treatment.

Genetic engineering to generate transgenic plants, and gene silencing (called RNA interference or cisgenics) are other methods now used to obtain useful variants.

Barriers to gene-flow between different plant species are overcome in a number of ways. Colchicine treatment to create artificial polyploids can overcome some of the sterility problems from inter-species cross pollination. Cochicine interferes with early steps in meiosis and results in formation of diploid rather than haploid gametes. Crosses made using protoplast fusion (somatic hybridization) can also be used, and embryo rescue methods can also circumvent gene-flow barriers.

Many artificial selection methods have been developed to allow crop imprvement to be sucessful. This include molecular-marker assisted breeding, and use of statistical principles to design field tests of crop candidate performance with sufficient power to detect improvement.

Genome science (chromosome sequence decoding and computer assisted dissection of gene functions and structure) is also being bought into play to assist plant breeders identify traits and select improved progeny. 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.

Identification of the particular gene and DNA sequence determining a phenotype of relevance to agriculture (such as mildew resistance) opens up numerous ways of creating new useful genetic variation by direct manipulation of DNA, and for devising convenient test for tracking traits in breeding experiments, thus speeding up plant breeding in the greenhouse and field trial stages.

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.

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

• Centro Internacional de Mejoramiento de Maiz y Trigo (maize and wheat; CIMMYT) Genebank, (Mexico.
• International Center for Agricultural Research in the Dry Areas (ICARDA) Genebank, (Syria).
• International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) Genebank, (India).
• International Rice Research Institute IRRI Genebank, (Philipines).
• Internacional de Agricultura Tropical (International Center for Tropical Agriculture; CIAT) Genebank, (Columbia)

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.

See also Horizontal gene transfer in plants.

Twenty first century plant breeding

See main article on Biotechnology and Plant breeding.

See also Cisgenic plants.

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

This is not a new issue though. Recent studies 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.. There is also increasing emphasis on breeding crops for nutritional improvement .

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 .

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 . 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.

General Bibliography

• Borojevic, S. 1990. Principles and Methods of Plant Breeding. Elserier, Amsterdam. ISBN 0-444-98832-7
• Chrispeels, M.J.,and Sadava, D.E. 2003 Editors. Plants, Genes, and Crop Biotechnology. 2nd Edition. Jones and Bartlett/American Society of Plant Biologists ISBN 0-7637-1586-7
• Duvick DN. (2001) Biotechnology in the 1930s: the development of hybrid maize. Nat Rev Genet. 2001 Jan;2(1):69-74.

• Gepts, P. (2002). A Comparison between Crop Domestication, Classical Plant Breeding, and Genetic Engineering. Crop Science 42:1780–1790
• Origins of Agriculture and Crop Domestication - The Harlan Symposium
• Pfeiffer, T. W. 2003. From classical plant breeding to modern crop improvement. Chapter 14 In Chrispeels, M.J.,and Sadava, D.E. 2003 Editors. Plants, Genes, and Crop Biotechnology. 2nd Edition. Jones and Bartlett/American Society of Plant Biologists ISBN 0-7637-1586-7
• Fedoroff, N. V. and Brown, N. M. 2004 Mendel in the Kitchen: A Scientist's View of Genetically Modified Food. National Academy Press. ISBN 0-3090-9205-1
• McCouch, S. 2004. Diversifying Selection in Plant Breeding. PLoS Biol 2(10): e347.
• news@nature.com. 1999 Are non-GM crops safe?
• Sun, C. et al. 1998. 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

• Making genetically engineered plants
• Adoption of Genetically Engineered Crops in the U.S.(1996-2006) ERS USDA
• ISAAA Briefs 34-2005: Global Status of Commercialized Biotech/GM Crops: 2005
• Biotech Crops Reduce Pesticide Use, Greenhouse Gas Emissions Planting of these crops generates additional US$27.5 billion in global farm income 2005
• 2006 Update of Impacts on US Agriculture of Biotechnology-Derived Crops Planted in 2005]