Plant Defense and Counter Defense by Viruses

Abstract

RNA silencing is a robust sequence-specific RNA degradation process triggered by the formation of double-stranded RNA (dsRNA). RNA silencing was first discovered in trans- genic plants, where it was termed co-suppression or post-transcriptional gene silencing (PTGS). In plants, it serves as an antiviral defense, and small RNA pathways serve as a defense against viruses and other invading nucleic acids. This chapter focuses on the inter- actions between host small RNA pathways and viral suppressors of silencing. Invading viruses carry genetic material that controls the host cell’s machinery and tricks it into pro- ducing proteins and new viruses. Through RNA silencing, plant cells recognize this viral genetic material, remember and copy it so that other cells in the organism can be warned to destroy the virus. All cells in microbes, fungi, plants and mammals employ RNA silencing. However, viruses are known to fight back using RNA silencing suppressors, proteins that inhibit this defense mechanism. RNA silencing suppressors have been reported recently in other forms of pathogens like bacteria and oomycetes, which suggest that these pathogens have this inherent capability of counter defense across various kingdoms. In this chapter, we discuss some of these phenomenal counter defense mechanisms by the viruses.