Gene Silencing without DNA: RNA-Mediated Cross-Protection between Viruses

Abstract

Previously, it was shown that the upper leaves of plants infected with nepoviruses and caulimoviruses are symptom free and contain reduced levels of virus. These leaves are said to be recovered. Recovery is associated with RNA-mediated cross-protection against secondary virus infection. Here, by analyzing plants infected with viruses that are quite distinct from the nepovirus or caulimovirus groups, we demonstrate that this RNA-mediated defense is a general response to virus infection. Upon infection with a tobravirus, plants exhibited RNA-mediated cross-protection and recovery, as occurs in nepovirus-infected plants. However, upon infection with a potexvirus, plants exhibited RNA-mediated cross-protection without recovery. In both instances, a transient gene expression assay showed that RNA-mediated cross-protection was functionally equivalent to post-transcriptional gene silencing. Combined, these data provide direct evidence that post-transcriptional gene silencing of nuclear genes is a manifestation of a natural defense mechanism that is induced by a wide range of viruses. INTRODUCTIONSeveral lines of evidence suggest a link between post-transcriptional gene silencing (PTGS) in transgenic plants and viruses (Baulcombe, 1996a;Pruss et al., 1997). For example, transgene-induced PTGS causes resistance against viruses that have nucleotide sequences similar to that of the transgene (Smith et al., 1994;Mueller et al., 1995;English et al., 1996; Goodwin et al., 1996). This type of transgenic virus resistance is referred to as RNA homology-dependent resistance. Viruses can also induce PTGS of homologous transgenes (Angell and Baulcombe, 1997;Al-Kaff et al., 1998;Atkinson et al., 1998;Kjemtrup et al., 1998;Ruiz et al., 1998). In some transgenic plants, the virus can be both an inducer and a target of gene silencing. The lower leaves of these plants display the normal viral symptoms. However, upper leaves emerging after systemic infection are symptom and virus free. These upper leaves are resistant to secondary infection by the inducing virus and are said to be "recovered" (Lindbo et al., 1993; Guo and Garcia, 1997).There is also a link between PTGS and viruses in nontransgenic plants. For example, PTGS is induced by recombinant virus vectors carrying inserts that are homologous to endogenous genes. This virus-induced gene silencing may be mediated by tobacco mosaic virus (TMV; a tobamovirus) (Kumagai et al., 1995) and potato virus X (PVX; a potexvirus) (Ruiz et al., 1998) vectors with RNA genomes and by tomato golden mosaic virus (a geminivirus) (Kjemtrup et al., 1998) with a DNA genome. A PTGS-like mechanism is also induced by nepoviruses and caulimoviruses (Covey et al., 1997;Ratcliff et al., 1997) that do not have homology to endogenous genes. In these examples, the infected plants exhibit a response very similar to the virus-induced recovery on transgenic plants in that the upper leaves are symptom free and contain reduced levels of virus. In nepovirus-infected plants, the recovered leaves exhibit homology-depend...

Effects of viral silencing suppressors on tobacco ringspot virus infection in two Nicotiana species

“…to 'recovery' of newly grown tissues from the disease. The recovered tissues are resistant to secondary infections of viruses carrying homologous sequences, thus proving that the resistance is sequence specific and is mediated by RNA silencing (Ratcliff et al, 1997(Ratcliff et al, , 1999Al-Kaff et al, 1998). The recovery phenomenon occurs frequently in infections caused by caulimo-, tobra-and nepoviruses, suggesting that these viruses do not suppress silencing very efficiently.…”

A cucumber mosaic virus mutant lacking the 2b counter-defence protein gene provides protection against wild-type strains

“…Cross-protection was The mechanism, or mechanisms, behind cross-protection has remained obscure but a number of explanations have been proposed. Currently, the leading hypothesis used to explain cross-protection is that the protective strain induces RNA silencing against its own RNA and homologous sequences, such as those occurring in closely related strains of the same virus (Ratcliff et al, 1999;Hull, 2002;Gal-On & Shiboleth, 2006). Thus, it is hypothesized that the protective strain is acting as an elicitor of a natural antiviral response, RNA silencing, which underlies other natural resistance phenomena, such as recovery and 'green island' formation, as well as many instances of pathogenderived resistance in transgenic plants (Ratcliff et al, 1997; Moore et al, 2001;Voinnet, 2001;Goldbach et al, 2003).…”

Arabidopsis RTM2 Gene Is Necessary for Specific Restriction of Tobacco Etch Virus and Encodes an Unusual Small Heat Shock–like Protein

“…Virus-induced silencing is adaptive, meaning that a naive host can recognize viral nucleic acids and customize a sequence-specific response (Baulcombe, 1999;Grant, 1999). Thus, virus-induced silencing can limit virus accumulation, promote recovery (in some cases) from a systemic infection, and confer resistance to secondary infections with the same or homologous viruses (Ratcliff et al, 1997;Al-Kaff et al, 1998;Ratcliff et al, 1999). In contrast to resistance triggered by NBS-LRR-type R genes, resistance through silencing appears not to depend on a gene-for-gene recognition event.…”