callus

Callus (cell biology)

From Wikipedia, the free encyclopedia

Plant callus (plural calluses or calli) is a mass of undifferentiated cells derived from plant tissue (explants) for use in biological research and biotechnology. In plant biology, callus cells are those cells that cover a plant wound.^[1] To induce callus formation, plant tissues are surface sterilized and then plated onto in vitro tissue culture medium. Plant hormones, such as auxins, cytokinins, and gibberellins, are supplemented into the medium to initiate callus formation or somatic embryogenesis. Callus tissue initiation has been described for a number of plant taxonomic divisions:

Callus Nicotiana tabacum

• Marchantiophyta^[2]
• Anthocerotophyta^[3]
• Bryophyta^[4]
• Lycopodiophyta^[5]
• Pteridophyta^[6]
• Cycadophyta^[7]
• Ginkophyta^[8]
• Pinophyta^[9]
• Gnetophyta^[10]
• Magnoliophyta^[11][12]

Contents   [hide]  1 Callus Induction and Tissue Culture 2 Morphology 3 Uses 4 History 5 See also 6 References

[edit]Callus Induction and Tissue Culture

Callus cells forming during a process called "induction" in Pteris vittata

A callus cell culture is usually sustained on gel medium. Callus induction medium consists of agar and a mixture of macronutrients and micronutrients for the given cell type. There are several types of basal salt mixtures used in plant tissue culture, but most notably modified Murashige and Skoog medium,^[13] White's medium,^[14] and woody plant medium.^[15] Vitamins are also provided to enhance growth such as Gamborg B5 vitamins.^[16] For plant cells, enrichment with nitrogen, phosphorus, andpotassium is especially important.

Callus induced from Pteris vitattagametophytes

[edit]Morphology

Plant callus is usually derived from somatic tissues. The tissues used to initiate callus formation depends on plant species and which tissues are available for explant culture. The cells that give rise to callus and somatic embryos usually undergo rapid division or are partially undifferentiated such asmeristematic tissue. In alfalfa, Medicago truncatula, however callus and somatic embryos are derived from mesophyll cells that undergo dedifferentiation.^[17] Plant hormones are used to initiate callus growth. Specific auxin to cytokinin ratios in plant tissue culture medium give rise to an unorganized growing and dividing mass of callus cells. Callus cultures are often broadly classified as being either compact or friable. Friable calluses fall apart easily, and can be used to generate cell suspension cultures. Callus can directly undergo direct organogenesis and/or embryogenesis where the cells will form an entirely new plant. Callus can brown and die during culture, but the causes for callus browning are not well understood. In Jatropha curcas callus cells, small organized callus cells became disorganized and varied in size after browning occurred.^[18] Browning has also been associated with oxidation and phenolic compounds in both explant tissues and explant secretions.^[19]

[edit]Uses

Callus cells are not necessarily genetically homogeneous because a callus is often made from structural tissue, not individual cells. Nevertheless, callus cells are often considered similar enough for standard scientific analysis to be performed as if on a single subject. For example, an experiment may have half a callus undergo a treatment as the experimental group, while the other half undergoes a similar but non-active treatment as the control group.

Plant calli can differentiate into a whole plant, a process called regeneration, through addition of plant hormones in culture medium. This ability is known as totipotency. Regeneration of a whole plant from a single cell allows researchers to recover whole plants that have a copy of the transgene in every cell. Regeneration of a whole plant that has some genetically transformed cells and some untransformed cells is called achimera. In general, chimeras are not useful for genetic research or agricultural applications.

Genes can be inserted into callus cells using biolistic bombardment, also known as a gene gun, or Agrobacterium tumefaciens. Cells that receive the gene of interest can then be recovered into whole plants using a combination of plant hormones. The whole plants that are recovered can be used for experiment to determine gene function(s), or to enhance crop plant traits for modern agriculture.

Callus tissue is of particular use in micropropagation where it can be used to grow genetically identical copies of plants with desirable characteristics.

transgenic animal production

Production

Further information: Genetic engineering, Genetic modification, Horizontal gene transfer, Molecular cloning, Recombinant DNA andTransformation (genetics)

Genetic modification involves the insertion or deletion of genes. When genes are inserted, they usually come from a different species, which is a form of horizontal gene transfer. In nature this can occur when exogenous DNA penetrates the cell membrane for any reason. To do this artificially may require attaching the genes to a virus or just physically inserting the extra DNA into the nucleus of the intended host with a very small syringe, or with very small particles fired from a gene gun.^[1] However, other methods exploit natural forms of gene transfer, such as the ability of Agrobacterium to transfer genetic material to plants,^[2] or the ability of lentiviruses to transfer genes to animal cells.^[3]

[edit]History

This section requires expansion.

The general principle of producing a GMO is to add new genetic material into an organism's genome. This is called genetic engineering and was made possible through the discovery of DNA and the creation of the first recombinant bacteria in 1973; an existing bacterium E. coli expressing an exogenic Salmonella gene.^[4] This led to concerns in the scientific community about potential risks from genetic engineering, which were first discussed in depth at the Asilomar Conference in 1975. One of the main recommendations from this meeting was that government oversight of recombinant DNA research should be established until the technology was deemed safe.^[5][6] Herbert Boyer then founded the first company to use recombinant DNA technology, Genentech, and in 1978 the company announced creation of an E. coli strain producing the human proteininsulin.^[7]

In 1986, field tests of bacteria genetically engineered to protect plants from frost damage (ice-minus bacteria) at a small biotechnology company called Advanced Genetic Sciences of Oakland, California, were repeatedly delayed by opponents of biotechnology. In the same year, a proposed field test of a microbe genetically engineered for a pest resistance protein by Monsanto Company was dropped.

In the late 1980s and early 1990s, guidance on assessing the safety of genetically engineered plants and food emerged from organizations including the FAO and WHO.^{[8][9][10][11]}

Small scale experimental plantings of genetically modified (GM) plants began in Canada and the U.S. in the late 1980s. The first approvals for large scale, commercial cultivation came in the mid 1990s. Since that time, adoption of GM plants by farmers has increased annually.

[edit]Uses

GMOs are used in biological and medical research, production of pharmaceutical drugs, experimental medicine (e.g. gene therapy), and agriculture (e.g. golden rice). The term "genetically modified organism" does not always imply, but can include, targeted insertions of genes from one species into another. For example, a gene from a jellyfish, encoding a fluorescent protein called GFP, can be physically linked and thus co-expressed with mammalian genes to identify the location of the protein encoded by the GFP-tagged gene in the mammalian cell. Such methods are useful tools for biologists in many areas of research, including those who study the mechanisms of human and other diseases or fundamental biological processes in eukaryotic or prokaryotic cells.

To date the most controversial but also the most widely adopted application of GMO technology is patent-protected food crops which are resistant to commercial herbicides or are able to produce pesticidal proteins from within the plant, or stacked trait seeds, which do both. The largest share of the GMO crops planted globally are owned by the US firm Monsanto.^[12] In 2007, Monsanto's trait technologies were planted on 246 million acres (1,000,000 km²) throughout the world, a growth of 13 percent from 2006. However, patents on the first Monsanto products to enter the marketplace will begin to expire in 2014, democratizing Monsanto products. In addition, a 2007 report from the European Joint Research Commission predicts that by 2015, more than 40 per cent of new GM plants entering the global marketplace will have been developed in Asia.^[13]

In the corn market, Monsanto's triple-stack corn—which combines Roundup Ready 2 weed control technology with YieldGard Corn Borer and YieldGard Rootworm insect control—is the market leader in the United States. U.S. corn farmers planted more than 32 million acres (130,000 km²) of triple-stack corn in 2008,^[14] and it is estimated the product could be planted on 56 million acres (230,000 km²) in 2014–2015. In the cotton market, Bollgard II with Roundup Ready Flex was planted on approximately 5 million acres (20,000 km²) of U.S. cotton in 2008.^[15]

According to the International Service for the Acquisition of Agri-Biotech Applications (ISAAA), of the approximately 14 million farmers who grew biotech crops in 2009, some 90% were resource-poor farmers in developing countries. These include some 7 million farmers in the cotton-growing areas of China, an estimated 5.6 million small farmers in India (Bacillus thuringiensis cotton), 250,000 in the Philippines, South Africa (biotech cotton, maize and soybeans often grown by subsistence women farmers) and the other twelve developing countries which grew biotech crops in 2009.^[16] 10 million more small and resource-poor farmers may have been secondary beneficiaries of Bt cotton in China.

The global commercial value of biotech crops grown in 2008 was estimated to be US$130 billion.^[16]

In the United States, the United States Department of Agriculture (USDA) reports on the total area of GMO varieties planted.^[17] According toNational Agricultural Statistics Service, the states published in these tables represent 81–86 percent of all corn planted area, 88–90 percent of all soybean planted area, and 81–93 percent of all upland cotton planted area (depending on the year).

USDA does not collect data for global area. Estimates are produced by the International Service for the Acquisition of Agri-biotech Applications (ISAAA) and can be found in the report, "Global Status of Commercialized Transgenic Crops: 2007".^[18]

Transgenic animals are also becoming useful commercially. On February 6, 2009 the U.S. Food and Drug Administration approved the first human biological drug produced from such an animal, a goat. The drug, ATryn, is an anticoagulant which reduces the probability of blood clotsduring surgery or childbirth. It is extracted from the goat's milk.^[19]

[edit]Detection

Testing on GMOs in food and feed is routinely done by molecular techniques like DNA microarrays or qPCR. The test can be based on screening elements (like p35S, tNos, pat, or bar) or event-specific markers for the official GMOs (like Mon810, Bt11, or GT73). The array-based method combines multiplex PCR and array technology to screen samples for different potential GMOs,^[20] combining different approaches (screening elements, plant-specific markers, and event-specific markers). The qPCR is used to detect specific GMO events by usage of specificprimers for screening elements or event-specific markers.

To avoid any kind of false positive or false negative testing outcome, comprehensive controls for every step of the process is mandatory. ACaMV check is important to avoid false positive outcomes based on virus contamination of the sample.

[edit]Transgenic microbes

Bacteria were the first organisms to be modified in the laboratory, due to their simple genetics.^[21] These organisms are now used for several purposes, and are particularly important in producing large amounts of pure human proteins for use in medicine.^[22]

Genetically modified bacteria are used to produce the protein insulin to treat diabetes.^[23] Similar bacteria have been used to produce clotting factors to treat haemophilia,^[24] and human growth hormone to treat various forms of dwarfism.^[25][26]

[edit]Transgenic animals

Some chimeras, like the blotched mouse shown, are created through genetic modification techniques like gene targeting.

Transgenic animals are used as experimental models to perform phenotypic and for testing in biomedical research.^[27]

Genetically Modified (Genetically Engineered) animals are becoming more vital to the discovery and development of cures and treatments for many serious diseases. By altering the DNA or transferring DNA to an animal, we can develop certain proteins that may be used in medical treatment. Stable expressions of human proteins have been developed in many animals, including sheep, pigs, and rats.

Some examples are: Human-alpha-1-antitrypsin,^[28] which has been developed in sheep and is used in treating humans with this deficency and transgenic pigs with human-histo-compatibility have been studied in the hopes that the organs will be suitable for transplant with less chances of rejection. Transgenic livestock have been used as bioreactors since the 1990s. Many medicines, includinginsulin and many immunizations are developed in transgenic animals.^[29] In March 2011, the bioactive recombinant Human Lysozyne was expressed in the milk of cloned transgenic cattle. This field is growing rapidly and new pharming uses are being discovered and developed. The extent that trangenic animals will be useful in the medical field as well as other fields is very promising based on results thus far.^[30]