Defective interfering DNAs of plant viruses

Abstract

Several plant DNA viruses produce significant quantities of deleted versions of their DNA in the infected plants, which is generally correlated with a slowing down of replication process of the viral DNA. These deleted versions of the viral DNA are called Defective-Interfering (DI) DNA because of their inhibitory effect on the helper virus. The sizes of the DI-DNAs for different plant viruses can vary from one tenth of the size of the viral genome to one half and are mostly encapsidated. Sequence analysis suggests that DI-DNAs are formed by deletion, duplication, inversion, rearrangement and sometimes by insertion of non-viral DNA sequences involving the viral genome and its satellites. The role of the host plant in the formation of DI-DNAs is also important as DI-DNAs are readily formed in experimental hosts rather than their natural hosts. Symptom modulation by DI-DNAs is believed to be by competition for essential viral and host factors, thereby reducing the levels of the helper virus. Evidence is also emerging of its role in activating PTGS in the plant against viral transcripts, which is also likely to contribute towards symptom amelioration. The possibility of DI-DNAs playing a role towards the integration of pieces of viral DNA into plant genomes also exists. Most importantly, DI-DNAs have the potential to act as a tool in developing novel control strategies against viruses of crop plants and acting as gene expression/silencing vectors.