Retention of the virus-derived sequences in the nuclear genome of grapevine as a potential pathway to virus resistance

<p>Abstract</p> <p>Background</p> <p>Previous studies have revealed a wide-spread occurence of the partial and complete genomes of the reverse-transcribing pararetroviruses in the nuclear genomes of herbaceous plants. Although the absence of the virus-encoded integrases attests to the random and incidental incorporation of the viral sequences, their presence could have functional implications for the virus-host interactions.</p> <p>Hypothesis</p> <p>Analyses of two nuclear genomes of grapevine revealed multiple events of horizontal gene transfer from pararetroviruses. The ~200–800 bp inserts that corresponded to partial ORFs encoding reverse transcriptase apparently derived from unknown or extinct caulimoviruses and tungroviruses, were found in 11 grapevine chromosomes. In contrast to the previous reports, no reliable cases of the inserts derived from the positive-strand RNA viruses were found. Because grapevine is known to be infected by the diverse positive-strand RNA viruses, but not pararetroviruses, we hypothesize that pararetroviral inserts have conferred host resistance to these viruses. Furthermore, we propose that such resistance involves RNA interference-related mechanisms acting via small RNA-mediated methylation of pararetroviral DNAs and/or via degradation of the viral mRNAs.</p> <p>Conclusion</p> <p>The pararetroviral sequences in plant genomes may be maintained due to the benefits of virus resistance to this class of viruses conferred by their presence. Such resistance could be particularly significant for the woody plants that must withstand years- to centuries-long virus assault. Experimental research into the RNA interference pathways involving the integrated pararetroviral inserts is required to test this hypothesis.</p> <p>Reviewers</p> <p>This article was reviewed by Arcady R. Mushegian, I. King Jordan, and Eugene V. Koonin.</p

P35 and P52, two aphid polypeptides potentially involved in the capture of BWYV, that increase the efficiency of its vector transmission

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Effects of Point Mutations in the Readthrough Domain of the Beet Western Yellows Virus Minor Capsid Protein on Virus Accumulation In Planta and on Transmission by Aphids

Point mutations were introduced into or near five conserved sequence motifs of the readthrough domain of the beet western yellows virus minor capsid protein P74. The mutant virus was tested for its ability to accumulate efficiently in agroinfected plants and to be transmitted by its aphid vector, Myzus persicae. The stability of the mutants in the agroinfected and aphid-infected plants was followed by sequence analysis of the progeny virus. Only the mutation Y201D was found to strongly inhibit virus accumulation in planta following agroinfection, but high accumulation levels were restored by reversion or pseudoreversion at this site. Four of the five mutants were poorly aphid transmissible, but in three cases successful transmission was restored by pseudoreversion or second-site mutations. The same second-site mutations in the nonconserved motif PVT(32-34) were shown to compensate for two distinct primary mutations (R24A and E59A/D60A), one on each side of the PVT sequence. In the latter case, a second-site mutation in the PVT motif restored the ability of the virus to move from the hemocoel through the accessory salivary gland following microinjection of mutant virus into the aphid hemocoel but did not permit virus movement across the epithelium separating the intestine from the hemocoel. Successful movement of the mutant virus across both barriers was accompanied by conversion of A59 to E or T, indicating that distinct features of the readthrough domain in this region operate at different stages of the transmission process

RESISTANCE TO VIRUSES, PHYTOPLASMAS AND THEIR VECTORS IN THE GRAPEVINE IN EUROPE: A REVIEW

Control of grapevine virus and phytoplasma diseases is currently based on prophylactic measures and cultural practices. Certification programs aim to avoid the introduction of diseased grapevines into healthy vineyards, and cultural practices aim to reduce the populations of virus vectors to limit virus spread. These approaches however are of limited effectiveness. Additionally, there is no host resistance to viruses in grapevine identified so far. Genetic engineering provides new approaches to develop pathogen-resistant grapevines. Transgenic grapevines expressing virus-derived genes that can confer resistance have been obtained in several laboratories. The viral coat protein gene has been the most commonly used to engineer resistance. More recently, new approaches based on gene silencing, specifically triggering plant defence mechanisms, have opened new ways to engineer pathogen resistance in grapevines. The possibilities, benefits and advantages, but also the risks involved with the introduction of transgenic grapevines in the field, as well as their acceptability, are discussed in this review