RNA silencing is a process in which there is a degradation in the plant cells (and other organisms ) of selected RNA molecules . This mechanism plays a key role in two extremely important for plant processes : the post-transcriptional regulation of gene expression and in defense against viruses . The sequence-specific degradation of the RNA molecules involved short RNA fragments with a length of 21-25 nucleotides and ribonuclease plant . RNA silencing trigger signal throughout the plant , and thereby inducing the natural defense system against viral infection is widespread in the plant in response to virus infection . It is not know what is and how it spreads the signal in plants . Some plant viruses encode suppressors of RNA silencing stages , thereby preventing the normal course of plant defense response to virus attack . The system described resembles in some ways the immune system of mammals .