An interspecies hybrid RNA virus is significantly more virulent than either parental virus.

Abstract

Cucumber mosaic cucumovirus (CMV) infects a very wide range of plant species (>1000 species). We recently demonstrated that a previously undescribed gene (2b) encoded by RNA 2 of the tripartite RNA genome of CMV is required for systemic virus spread and disease induction in its hosts. Herein we report that when this CMV gene is replaced by its homologue from tomato aspermy cucumovirus (TAV), the resultant hybrid virus is significantly more virulent, induces earlier onset of systemic symptoms, and accumulates to a higher level in seven host species from three families than either of the parents. Our results indicate that CMV and the TAV 2b protein interact synergistically despite the fact that no synergism occurs in double infections with the two parental viruses. To our knowledge, this is the first example of an interspecific hybrid made from plant or animal RNA viruses that is more efficient in systemic infection ofa number of hosts than the naturally occurring parents. As CMV and the hybrid virus accumulated to a similar level in the infected tobacco protoplasts, the observed synergistic responses most likely resulted from an increased efficacy of the hybrid virus in systemic spread in host plants provided by the TAV 2b protein.The relevance of our finding to the application of pathogenderived resistance is discussed.

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].…”

Alfalfa mosaic virus and ilarviruses: involvement of coat protein in multiple steps of the replication cycle.

“…A subgenomic messenger corresponding to ORF 2b (RNA 4A) has been identified in infected plants and this RNA is translated in vitro into the expected 22 kDa protein (Xin et al, 1998). The genome of cucumoviruses contains a 2b gene at a position similar to that of the ilarvirus 2b gene (Ding et al, 1996). However, the ilarvirus 2b gene is almost twice as large as the cucumovirus 2b gene and there is no sequence similarity between the two types of gene whereas there is significant similarity between the four ilarvirus 2b genes.…”

“…However, the ilarvirus 2b gene is almost twice as large as the cucumovirus 2b gene and there is no sequence similarity between the two types of gene whereas there is significant similarity between the four ilarvirus 2b genes. The cucumoviral 2b gene plays a role in systemic spread of the virus and virulence determination, possibly by suppressing a host RNA silencing mechanism (Ding et al, 1996 ;Brigneti et al, 1998). A possible role of the 2b gene in ilarvirus replication has not yet been analysed.…”

Symptom induction and RNA silencing suppression by the cucumber mosaic virus 2b protein

“…1 The 2b protein is a determinant of virus symptoms and movement, and consequently plays an important part in the virulence of CMV. 2,3 For example, by transferring the 2b gene of a severe cucumovirus into a mild one, the symptoms induced by the mild strain were significantly enhanced. 3 The 2b protein is also a suppressor of two key plant antiviral defense systems: salicylic acid-mediated resistance, 4 and RNA silencing.…”