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Genetically modified crops are plants that have been created by adding new genes using methods of gene technology.
Most plant crops grown today have been created by breeding. Breeding achieves introduction of new traits into existing crop lines by crossing them with other lines of the same crop that have a desired trait.
During initial stages of breeding, all genes of parental lines are recombined in a random order. Because resulting plants are quite a mix, both genetically and phenotypically, they are backcrossed to one of the parents, and then backcrossed again several times until the original genotype of the parent is restored to large degree.
Now, think of the time it takes to do all the backcrossing. On average it takes about ten generations, or five years of back crossing, to develop a new variety of wheat by conventional breeding methods. Breeding a new variety of apples takes about fifteen years.
Another challenge plant breeders face is that they can only transfer genes between closely related species. Even if there was, say, a very useful draught tolerance gene in oranges, it could never be transferred to apples by crossing.
These two limitations of breeding can be bypassed by introducing genes into plant genome directly, either with Agrobacterium or with a gene gun.
A gene that is being introduced can be isolated from a related plant species, isolated from a completely different organism, or it can be designed on a computer and synthesized in a test tube.
Agrobacterium tumefaciens is a soil microorganism that can naturally transfer some of its DNA into plant cells.
In wild Agrobacterium strains this DNA, called transfer DNA, contains several genes that hijack plant biochemical pathways and make plant cells synthesize chemical compounds that Agrobacterium uses for food.
Using methods of molecular cloning, all bacterial genes on transfer DNA can be removed and replaced with the genes of interest. Agrobacterium then delivers these genes into plant nucleus where they integrate into plant genome.
Another technology widely used for plant transformation is a gene gun. Instead of inserting genes of interest into Agrobacterium, the DNA is loaded onto microscopic gold particles and it is literally shot into plant cells. Some of the gold particles land in cell nuclei and DNA from them gets inserted into plant chromosomes.
Both Agrobacterim- and gene gun-mediated gene delivery is done in plant tissue culture – little clamps of plant cells that later on differentiate and grow into actual plants.
So, in the form of breeding, genetic modification of plants has been practiced for thousands of years. Modern methods of gene technology make this process faster, more precise, and open to using genes from unrelated species.