Genetic Engineering

Genetic engineering is a term used for the directed manipulation of genes, i. e. the transfer of genes between organisms or changes in the sequence of a gene. Closely related to this field are methods which use genes or specific sequences for the identification of traits and other analytical purposes. In plant breeding, the most important and already widely used method of this kind is Restriction Fragment Length Polymorphism (RFLP). The basics of this technique are described below. The principles of plant genetic engineering will be described in the next chapter. For those who are not familiar with the principles of genetics, the glossary at the end of this report explains the most important terms.

Fragment Length Polymorphism (RFLP)

The techniques of traditional breeding are very time-consuming. By making crosses, also a large number of undesired genes is introduced into the genome of the plant. The undesired genes have to be "sorted out" by back-crossing. The use of Restriction Fragment Length Polymorphism greatly facilitates conventional plant breeding, because one can progress through a breeding program much faster, with smaller populations and without relying entirely on testing for the desired phenotype.

RFLP makes use of restriction endonucleases. These are enzymes which recognize and cut specific nucleotide sequences in DNA. For example, the sequence GAATTC is cut by the endonuclease EcoRl. After treatment of a plant genome which restriction endonucleases, the plant DNA is cut into pieces of different length, depending on the number of recognition sites on the DNA. These fragments can be separated according to their size by using gel electrophoresis and are made visible as bands on the gel by hybridizing the plant DNA fragments with radiolabeled or fluorescent DNA probes. As two genomes are not identical even within a given species due to mutations, the number of restriction sites and therefore the length and numbers of DNA fragments differ, resulting in a different banding pattern on the electrophoresis gel. This variability has been termed restriction fragment length polymorphism (RFLP). The closer two organisms are related, the more the pattern of bands overlap. If a restriction site lies close to or even within an important gene, the existence of a particular band correlates with the particular trait of a plant, e.g. disease resistance. By looking at the banding pattern, breeders can identify individuals which have inherited resistance genes, and resistant plants can be selected for further breeding. The use of this technique will not only accelerate progress in plant breeding considerably, but will also facilitate the identification of resistance genes, thereby opening new possibilities in plant breeding