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

Abstract

Arabidopsis plants have a system to specifically restrict the long-distance movement of tobacco etch potyvirus (TEV) without involving either hypersensitive cell death or systemic acquired resistance. At least two dominant genes, RTM1 and RTM2 , are necessary for this restriction. Through a series of coinfection experiments with heterologous viruses, the RTM1/RTM2-mediated restriction was shown to be highly specific for TEV. The RTM2 gene was isolated by a mapbased cloning strategy. Isolation of RTM2 was confirmed by transgenic complementation and sequence analysis of wild-type and mutant alleles. The RTM2 gene product is a multidomain protein containing an N-terminal region with high similarity to plant small heat shock proteins (HSPs). Phylogenetic analysis revealed that the RTM2 small HSP-like domain is evolutionarily distinct from each of the five known classes of plant small HSPs. Unlike most other plant genes encoding small HSPs, expression of the RTM2 gene was not induced by high temperature and did not contribute to thermotolerance of seedlings. The RTM2 gene product was also shown to contain a large C-terminal region with multiple repeating sequences. INTRODUCTIONSystemic infection of plants by viruses generally requires genome replication in individual cells, cell-to-cell movement through plasmodesmata, and long-distance movement through the phloem Baker et al., 1997). A block at any of these steps, either by active defense responses or by incompatible combinations of viral and host factors, can lead to resistance. Several types of active resistance responses to viruses are known (Baker et al., 1997;Carrington and Whitham, 1998). Dominant or semidominant resistance ( R ) genes that trigger hypersensitive cell death or extreme resistance at initial infection sites, occurring most often in a strain-specific or "gene-for-gene" manner, are well known. The tobacco N gene and the potato Rx gene, which condition resistance against tobacco mosaic virus and potato virus X strains, respectively, belong to the nucleotide binding site, leucine-rich repeat (NBS-LRR) class of R genes (Whitham et al., 1994;Bendahmane et al., 1999). Whereas N confers a hypersensitive response concomitant with localized cell-to-cell movement in inoculated leaves, Rx confers an extreme resistance in initially infected cells (Bendahmane et al., 1995).A second type of active resistance involves gene-silencing-like mechanisms. 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. Passive resistance has been sugge...

REPRESSOR OF SILENCING5 Encodes a Member of the Small Heat Shock Protein Family and Is Required for DNA Demethylation in Arabidopsis

“…An analysis of the Arabidopsis genome has identified 25 new members with one or more ACDs that are unrelated to the 11 subfamilies of the classical sHSPs; these are also referred to as ACD proteins, although their biological roles are largely unknown (Scharf et al, 2001). One of these ACD proteins, RESTRICTED TEV MOVE-MENT2, is required for restricting the long-distance movement of Tobacco etch virus in Arabidopsis (Whitham et al, 2000).…”

“…A seedling thermotolerance assay was performed as described previously with some modifications (Whitham et al, 2000). Briefly, wild-type C24, ros5-1, and ros5-2 seeds were sterilized and plated on MS medium containing 2% (w/v) Suc and 1% (w/v) agar.…”

Arabidopsis thaliana has the enzymatic machinery for replicating representative viroid species of the family Pospiviroidae

“…Research on plant viruses has also benefited from the use of such a versatile system. In particular, studies with viruses that naturally or experimentally infect Arabidopsis have led to the identification of host factors involved in their amplification (3,4) and movement (5,6) and in posttranscriptional gene silencing-mediated phenomena that include disease induction (7) and plant defense responses (8,9). However, no viroid has been reported to infect Arabidopsis so far.…”

ARGONAUTE2 Mediates RNA-Silencing Antiviral Defenses againstPotato virus Xin Arabidopsis

“…As obligate parasites, viruses are unique in that their genetic material is transcribed, translated, and replicated entirely inside the host cell. As such, the inability of a virus to infect a given plant may be due to an incompatibility between viral proteins and the host translational machinery (Robaglia and Caranta, 2006), inhibition of viral replication (Ishibashi et al, 2007(Ishibashi et al, , 2009 and movement Whitham et al, 2000;Cosson et al, 2010), or by detection and elimination by disease resistance (R) proteins (Moffett, 2009).…”