 RNA interference is a vital part of the immune response to viruses and other foreign genetic material , especially in plants where it may also prevent the self @-@ propagation of transposons . Plants such as Arabidopsis thaliana express multiple dicer homologs that are specialized to react differently when the plant is exposed to different viruses . Even before the RNAi pathway was fully understood , it was known that induced gene silencing in plants could spread throughout the plant in a systemic effect and could be transferred from stock to scion plants via grafting . This phenomenon has since been recognized as a feature of the plant adaptive immune system and allows the entire plant to respond to a virus after an initial localized encounter . In response , many plant viruses have evolved elaborate mechanisms to suppress the RNAi response . These include viral proteins that bind short double @-@ stranded RNA fragments with single @-@ stranded overhang ends , such as those produced by dicer . Some plant genomes also express endogenous siRNAs in response to infection by specific types of bacteria . These effects may be part of a generalized response to pathogens that downregulates any metabolic process in the host that aids the infection process . 
