ABSTRACT
Plants have evolved effi cient defense mechanisms against biotic and abiotic stresses (Dangl et al. 2001, Takken et al. 2010, Walling 2000) regulated by different cross-communicating signaling pathways (Baker et al. 1997, Champigny et al.2009, Dong 2001, Gilliland et al. 2006, Kunkel et al. 2002, McCarty et al. 2000). Plants can mobilize panoply of responses to cope with virus invasion. Innate responses include triggering resistance gene products, local cell death and systemic acquired resistance (Whitham et al. 2006). RNA silencing is a universal defense mechanism. The fi rst discovered natural function of RNA silencing was anti-viral response in plants (Carrington et al. 2001) wherein replication of RNA and DNA viruses is associated with the accumulation of virus-derived small RNAs that help cleave viral messengers in a sequence specifi c manner (Hamilton et al. 1999). This mode of RNA silencing was referred as posttranscriptional gene silencing ( PTGS). The molecular basis of RNA silencing is being quickly clarifi ed (Pratt et al. 2009). RNA silencing has helped explain the phenomena of recovery from virus infection (Lindbo et al. 1993) and of cross protection (Ratcliff et al. 1999). To evade gene silencing, viruses utilize various proteins, including proteases, coat proteins, replicative enhancers, and, most commonly, transport proteins and pathogenicity factors. These viral suppressors prevent the accumulation of short RNAs and abolish both local and systemic silencing (Dunoyer et al. 2004, Roth et al. 2004).
