RNA AND GENE-SILENCING
The genetic information of all organisms is contained in nucleotide sequences (see text box ‘Plant viruses’, page 7). With the exception of viruses, DNA can be seen as the universal carrier of genetic information. In higher organisms (plants, animals) DNA consists of two strands (see Figure 4). Every cell in a given organism contains the same DNA. However, not every cell needs the same information. For example, a liver cell needs to make different proteins than an eye cell. In order for specific information to be used in the production of a protein, two steps are necessary (Figure 4). First a piece of code (gene) that is necessary for protein production is transcribed into RNA. This single strand moves from the nucleus to the cytoplasm of the cell, where the code is translated. For this reason this RNA is often referred to as ‘messenger RNA’. The translation of the messenger RNA coincides with the production of the protein for which the original piece of DNA encodes. RNA plays a crucial role in this process. If the RNA is broken down before it can be translated then the message contained in the DNA will never lead to the production of a protein. This phenomenon is called ‘gene silencing’, literally shutting down a gene’s function by deactivating the corresponding RNA. The process is naturally used by plants to defend against viruses. The immune system of a plant is based on the presenceof double stranded RNA. As previously mentioned a plant’s messenger RNA consists of a single strand. When viral RNA multiplies in the plant cell, double stranded RNA molecules are formed. These are recognized as foreign structures by the plant cell and to defend itself it breaks down the double stranded RNA. This process activates a mechanism that destroys all similar RNA molecules, even if they are single stranded.32 This RNA silencing process can be considered a kind of immune system that works at the genetic level.32 Not all plants are equally efficient with regards to carrying out this system and viruses also make attempts to defend themselves against such attacks. That is why papayais naturally susceptible to PRSV.