The tricky step in genetic engineering of plants is to insert the desired segment of DNA into the genome of the plant. This dynamism occurs in nature even before the scientists thought of it. The unpleasing lumpy tumor called 'gall' on the plant stem is the result of 'crown gall disease'. It is caused by a gram negative, rod shaped soil bacterium known as Agrobacterium tumefaciens. In nature it infects dicotyledons (known to infect more than 60 different plant families) and also damage economically important crops such as roses, walnuts, and tomatoes.
When a plant is injured for example by eating off by the insects Agrobacterium tumefaciens opportunistically infects the plant. It delivers a package of its own genetic material into the plant by horizontal gene transfer. The package consists of a stretch of DNA called T-DNA that is cautiously excised from a special plasmid known as Ti plasmid . The T-DNA comprises of oncogenes coding for the enzymes which forms tumors and genes coding the production of opines (The components formed by the condensation between sugars and amino acids which are manufactured and excreted by crown gall cells and used by Agrobacterium tumefaciens as a source of nitrogen and carbon).
Once the DNA is inserted into the plant host it gets incorporated into the host cell's DNA and generates plant growth hormones and specialised proteins which serve as bacterial nutritional source. These enforce simultaneous division of the plant cell and also bacterial growth by forming a positive feedback mechanism. The growth hormones promote plant cells to multiply at a very rapid rate along with the invasive bacterial DNA which is also copied at each time the cell divides so that high quantities of nutrients for bacteria and plant growth hormones are produced. This results in uncontrolled cell division forming lumpy clusters of cells, the gall which for the invader serves as a nutrient resource.
Agrobacterium tumefaciens is an indespensible tool in the gene delivery because it is capable of carrying the gene of interest within the Ti plasmid and insert it into the target plants's DNA with high efficiency. Due to this capacity of Agrobacterium it is used to create transgenic plants. The desired genes of interest incorporated into host plants may be for:
- Increasing nutritional value: amino acids, proteins, lipids, vitamins
- Therapeutic components: vaccines, antibodies, therapeutic proteins
- Economic value: resistance to pathogens, herbicides, tolerance to stress
- Altered proteins: recombinant protein production
- New traits: flower colour, ripening of fruit
- Bioremediation: tolerance to salt, pollution control
- Regulation of metabolic pathways: production of sugars, essential oil, capture of nutrients
- Agronomic factors: Disease resistance (bacterial and fungal), seed dormacy, resistance to nematodes etc.
Since the Beginnings of plant genetic engineering in 1970's it has been the core for genetic engineers and advances in technology has enlarged its kingdom to even the most resistant crop species. Researchers can insert any gene of interest they want to into the plasmid in the place of tumor inducing gene and then successionally into the host plants genome. In the early 1980's the age of plant transformation was set going with the Agrobacterium tumefaciens mediated gene transfer progress were limited to the, solonaceae tobacco in specific. This situation instantly changed throughout the nineteen eighties and nineties and is now able to generate a wide variety of plants including many agriculturally important crops such as rice,wheat, cotton, soybean, peanuts, corn, sunflower, Arabidopsis thaliana and so on. Utilizing Agrobacteriums Ti plasmid some examples of genes that are inserted into plants genome include:
- Resistance to herbicides : plants are transformed that can degrade herbicides like roundups.
- Anti-softening genes in tomatoes: providing them with increased shelf life.
- Insecticidal genes: rendering plants toxic to insects.
- Shining in the dark: tobacco plants have been engineered with luciferase gene from fireflyes.
Agrobacterium tumefaciens not only affects plants in the wild it has also been shown to infect other species including fungus and even animals. Taking the profit of the bacteria's capability advanced technological studies are being carried out to knock genes into diverse species. Despite the many recent developments in plant biotechnology and development of modernised alternative gene transfer techniques such as direct DNA delivery techniques, laser, tissue electrophoration, silicon carbide whiskers etc Agrobacterium tumefaciens still prevail as a oldest and widely used method for plant transformation.
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