Biotechnology and plant breeding

See main article on Plant breeding.

See also Transgenic plants, Biotechnology, Genetic engineering.

The discovery around 1975 of methods to directly change DNA (usually called genetic engineering), and of ways to decode DNA sequences (usually referred to as DNA sequencing) ushered in a revolution in the biological sciences which is continuing, seemingly unabated, into the current millennium.

Rapid scientific progress fostered by these discoveries quickly extended to plant genetics and plant breeding. After 1975 many new plant breeding techniques were invented, older methods were refined by the new experimental options, and further powerful methods (such as the polymerase chain reaction(PCR) and automated DNA sequencing were opened up by the exanding science of molecular biology .

The new methods did not displace Classical plant breeding, and direct manipulation of DNA is complementary to the strengths of classical breeding. Additionally, genetic engineering allowed the creation of several new methods such as DNA fingerprinting (DNA markers) that accellerated the relatively slow process of classical breeding. DNA sequencing and gene sequence analysis applied to whole genomes, known as genomics, eventually became both possible and practical, and, in turn generated massive quanties of useful new knowledge that assisted scientific plant breeding.

Genetic engineering is used to generate transgenic plants, for the new field of deliberate RNA silencing (cisgenics), and (with genomics underpins molecular-marker assisted breeding, which is valuable for speeding up classical breeding.

Genetic modification using direct DNA manipulation

Genetic modification of plants by DNA manipulation, usually just called Genetic Modification or "GM", 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 via direct DNA manipulation 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 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 viruses 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 pests and herbicides. 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 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).

Marker assisted breeding refers to direct detection of small DNA subregions, such as restriction fragment length polymorphisms (RFLPs) 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 . 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 ) 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. . 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 .

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.

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 , 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.

Plant genomics

Template:Stub In the twenty first century Genome science involving (chromosome sequence decoding and computer assisted dissection of gene functions and stucture) is increasingly used to assist plant breeders. One important approach is to compare gene arrangements 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.

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
• 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
• 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

[[Category:Biotechnology]