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Genetic engineering or recombinant DNA technology involves the whole specter of techniques for finding a specific gene in the genome of a species, isolating, cloning, determining the sequence of nucleotides, changing it, and installing it in genome of the same or different species. By selecting the proper regulatory regions (e.g. promoter), gene in the target organism can be activated according to desire of researchers in the specific conditions or in specific tissues. That technology is being applied successfully for almost 30 years in scientific research and has achieved useful properties of different types of organisms.

No matter whether the final aim of the application of genetic engineering is scientific research or achieving useful properties of commercially valuable organisms, the procedures are the same. Also, regardless of whether carried on prokaryotic organisms, plants or animals, the basic techniques are very similar and almost always involve insertion of the desired DNA fragments into bacterial plasmid, using restriction endonucleases and DNA ligase. Fragment (gene) that needs to be incorporated into the genome of the target organism is prepared, analyzed and multiplied using plasmids in Escherichia coli. Depending on the organism (bacteria, plant or animals), the procedures of inserting the desired gene into the genome may vary. In plants, for this purpose, the bacterium Agrobacterium tumefaciens is most commonly used, or the desired gene gets inserted into the cell by device called "gene gun". Once the desired gene is incorporated into the cell genome of certain plant, the whole plants regenerate from these cells, and those plants are commonly referred to as transgenic plants.

The Properties of Agrobacterium Tumefaciens and Interaction with Plants
A. tumefaciens lives in the soil almost everywhere in the world. Genetic Material of this bacteria is made of one large circular double-stranded DNA molecule (Bacterial chromosome) and one relatively large plasmid (plasmid Ti). The bacteria has developed unusual behavior during evolution - when the plant close to the ground is injured, bacteria move towards that place by chemotaxis, enter the intercellular spaces, and attach to the healthy plant cells, inserting into the plant genome a specific part of their plasmids (that part of the plasmid is called T-DNA). T-DNA contains two sets of genes: genes forgrowth regulators (plant hormones) and the genes for the substances that bacteria use as food (all the genes contain eukaryotic promoters so they are active in transcription in plant cells). Cells in which the genome of the bacterial T-DNA is incorporated are transformed cells. Growth regulators stimulate uncontrolled proliferation of transformed plant cells, and from such cells in a relatively short period of time a cluster of identical cells is developed (the tumor). Because of the different sets of genes in the T-DNA, tumor cells produce and secret into the intracellular space substances that bacteria use as food.

How is A. tumefaciens used in genetic engineering?
After the plasmid Ti is isolated from bacteria, the T-DNA genes are all cropped leaving only peripheral T-DNA sequences about 25 nucleotide pairs long (these edges of the T-DNA are essential for the transfer of T-DNA from the bacterium into the plant genome). Instead of the original gene in T-DNA, using restriction endonucleases and DNA ligaze, the desired gene, which was previously prepared and amplified by another plasmid in the bacterium E. coli, is incorporated. Such a recombinant plasmid is inserted into the bacterium A. tumefaciens and the bacteria are grown in a liquid medium. Then the cells of the plant we want to transform are added in the bacterial suspension. Since bacteria do not distinguish the original T-DNA from T-DNA in which desired genes are incorporated, it will incorporate desired genes into the genome of plant cells by the same mechanism by which it naturally does. From plant cells which have incorporated the desired gene, the plants are regenerated using appropriate nutrient medium. The new plants are then planted in the ground.

Recombinant DNA technology and genetic engineering provide almost incredible opportunities for improving the properties of organisms used for various purposes. Some properties that are the target for hundreds of years, it is possible to get in a very elegant way using the application of these technologies. Genetic engineering is currently the only technology used in breeding plants, and the most important thing is that by its application we know exactly what kind of changes in the genetic material caused the new properties.