Strong host resistance targeted against a viral suppressor of the plant gene silencing defence mechanism

Abstract

The 2b protein encoded by cucumber mosaic cucumovirus (Cmv2b) acts as an important virulence determinant by suppressing post-transcriptional gene silencing (PTGS), a natural plant defence mechanism against viruses. We report here that the tomato aspermy cucumovirus 2b protein (Tav2b), when expressed from the unrelated tobacco mosaic tobamovirus (TMV) RNA genome, activates strong host resistance responses to TMV in tobacco which are typical of the gene-for-gene disease resistance mechanism. Domain swapping between Cmv2b, which does not elicit these responses, and Tav2b, revealed functional domains in Tav2b critical for triggering virus resistance and hypersensitive cell death. Furthermore, substitution of two amino acids from Tav2b by those found at the same positions in Cmv2b, Lys21→Val and Arg28→Ser, abolished the ability to induce hypersensitive cell death and virus resistance. However, in Nicotiana benthamiana, a species related to tobacco, Tav2b functions as a virulence determinant and suppresses PTGS. Thus, a viral suppressor of the host gene silencing defence mechanism is the target of another independent host resistance mechanism. Our results provide new insights into the complex molecular strategies employed by viruses and their hosts for defence, counter-defence and counter counter-defence.

Viral Virulence Protein Suppresses RNA Silencing–Mediated Defense but Upregulates the Role of MicroRNA in Host Gene Expression[W]

“…Total plant RNA extraction and RNA gel blotting analysis were performed as described previously (Li et al, 1999). Hybridization probes were labeled with 32 P using the Amersham Megaprime DNA labeling kit (Piscataway, NJ).…”

Biological and Molecular Characterization of Cucumber mosaic virus Subgroup II Isolate Causing Severe Mosaic in Cucumber

“…N-terminal region of 1a protein contains putative methyltransferase domain [39] and C-terminal region shows sequence similarity to viral helicases [16]. RNA 2 encodes for another protein 2b, which is expressed from the subgenomic RNA 4A and acts as suppressor of gene silencing [4,5,27]; plays a role in long distance movement of the virus [9,23,43,54] and also behave as pathogenicity determinant [10,41,43]. A recent finding suggested that this suppressor protein is indirectly involved in aphid transmission [56].…”

Identification of amino acids of the beet necrotic yellow vein virus p25 protein required for induction of the resistance response in leaves of Beta vulgaris plants

“…In one case, an RNA sequence was also shown to act as an Avr factor (Szittya & Burgyan, 2001). Some of these Avr determinants have been reported to function as pathogenic determinants; for instance, turnip crinkle virus CP (Ren et al, 2000;Qu et al, 2003), cucumovirus 2b protein (Li et al, 1999), cauliflower mosaic virus transactivator/viroplasmin protein (Kobayashi & Hohn, 2004) and potyvirus P3 protein (Johansen et al, 2001;Jenner et al, 2003).…”