Genetic transformation is the heritable change in a cell or organism brought about by the uptake and establishment of introduced DNA. Genetic engineering allows the use of several desirable genes in a single event and reduces the time to introgress novel genes into elite background. Biotechnology has provided several unique opportunities that include: access to novel molecules, ability to change the level of gene expression, capability to change the expression pattern of genes, and develop transgenic plants with novel genes. As a result of advances in genetic transformation there has been a rapid progress in using genetic engineering for crop improvement. The potential of this technology has now been widely recognized. The basic requirements for genetic transformation are: a target genome, a candidate gene, a vector to carry the gene, tissue culture and regeneration system, modification of the foreign DNA to increase the level of gene expression, method to deliver the plasmid DNA into the cell, protocols to identify the transformed cell, and characterization of the putative transgenic plants at the molecular and genetic levels.
A Genetically Modified or transgenic plant is a plant that has a novel combination of genetic material obtained through the use of modern biotechnology. A transgenic crop plant contains a gene or genes which have been artificially introduced instead of the plant acquiring them through pollination. These genes are introduced with a view to expressing a novel trait which is not normally found normally in the given species.
Currently there are several approaches to the generation of stably transformed plants, and the approach adopted varies according to the aims of the research programme. The steps involved in generation transgenic plants include: DNA delivery, selection of transgenic tissues; and the recovery of transgenic whole plants.
Methods of Gene Delivery:
Agrobacterium mediated delivery
Agrobacterium tumafciens is a Gram-negative bacterium that uses horizontal gene transfer to cause tumors in plants. Agrobacterium is well known for its ability to transfer DNA between itself and plants, and for this reason it has become an important tool for plant improvement by genetic engineering. When the bacterial DNA is integrated into a plant chromosome, it becomes part of the plants chromosomes and the integrated DNA sequence stably expressed and inherited. This natural phenomenon is exploited by scientists to introduce the genes of interest into plant genome.
Fig 1: Agrobacterium
Particle Bombardment Method
The alternative to Agrobacterium-mediated transformation technique is particle bombardment. The aims are to penetrate cells or tissues with accelerated metal microspheres, which are coated with DNA. Such microspheres, of tungsten or gold (used because of their high densities), and of diameters of 1-4mm, have been shown to penetrate cells when accelerated at high velocities, and the DNA can be released and expressed transiently or can be stably integrated into chromosomes. The DNA-coated microprojectiles are placed on a macroprojectile, which is accelerated towards the tissue following an explosive discharge. The accelerated macroprojectile hits a stopping plate and microprojectiles are propelled onwards and penetrate the tissue depositing the DNA as they go. Micro-projectile bombardment has been successfully used to transform tissues that show good morphogenetic responses in a wide range of species including tobacco, soybean, rice, maize, barley and wheat.
Fig 2: Plant Transformation
Benefits of Transgenic or GM Plants:
Producer and Environmental Benefits
- Enhanced resistance to insects and significant reduction in use of chemical pesticides (e.g. Bt cotton and Bt corn);
- Increased resistance to viral, bacterial and fungal diseases, with improved safety for human consumption (e.g. cucumbers and papaya);
- Tolerance to herbicide (e.g. Roundup Ready soybeans)
- Increased tolerance to environmental stresses such as heat, cold, water logging, drought, and salinity;
- Increase in the ability of plants to remove toxic metals from soils (bioremediation);
- Production of more biodegradable industrial products.
- The development of novel oils, starches, and industrial applications, including raw materials for biodegradable plastics (e.g. soybeans with higher levels of oleic acid);
- The enhancement of vitamins and minerals in food grains (e.g. golden rice)
- The elimination of certain allergens and anti-nutritional compounds from foods;
- The production of pharmaceutical products, including edible vaccines, recombinant antibodies (antibodies) and anti-coagulant compounds (e.g. in bananas);
- Improved transport and self life of certain products (e.g. controlled ripening in melons, peas, peppers and tomatoes).
Fig 1: www.ppws.vt.edu
Fig 2: http://www.ag.ndsu.edu/pubs/plantsci/crops/a1219-2.gif