Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2004
Publication Date: 4/20/2004
Citation: Yang, S., Carter, S.A., Cole, A.B., Cheng, N., Nelson, R. 2004. A natural variant of a host rna-dependent rna polymerase is associated with increased susceptibility to viruses by Nicotiana benthamiana. Proceedings of the National Academy of Sciences. 101(16):6297-6302. Interpretive Summary: In nature, some plants are easily infected by pathogens, while others aren't. For example, one of the tobacco species, called Nicotiana benthamiana, can be attacked by most of, if not all, plant viral pathogens. The scientists are wondering for many years why it happens only in this species, rather than others and hypothesize that the genetic variation contributes to the weakness of this species in defending against diseases. However, until recent studies, the molecular mechanism underlying the extreme susceptibility of the plant to viral pathogens is still unclear. In this study, the scientists have found that one gene is naturally mutated in this tobacco species. This gene can produce a key enzyme in plants that is required for synthesis of double strands of nucleotide chain, one of critical steps in triggering plant defense systems. Having the abnormal enzyme, the plants lose the ability to defeat the viral infection, whereas when the normal protein introduced into, the plant shows enhanced resistance. The scientists have also found that only this enzyme, not another similar one in the plant, is response to disease resistance. Why does the mutation happen in this species and how is it related to the evolution of this family of tobacco? It will be the direction for the future investigation. Nevertheless, this study provides a unique insight into the mechanism of disease resistance that has a huge impact on agricultural practices.
Technical Abstract: Nicotiana benthamiana often displays more intense symptoms after infection by RNA viruses than do other Nicotiana species. Here, we examined the role of RNA-dependent RNA polymerases (RdRPs) in N. benthamiana antiviral defense. cDNAs representing only two genes encoding RdRPs were identified in N. benthamiana. One RdRP was similar in sequence to SDE1/SGS2 required for maintenance of transgene silencing, whereas the second, named NbRdRP1m, was >90% identical in sequence to the salicylic acid (SA)-inducible RdRP from Nicotiana tabacum required for defense against viruses. NbRdRP1m expression was induced by SA treatment or challenge with Tobacco mosaic virus, but the gene and transcript sequences differed from those of other SA-inducible RdRPs in that they contained a 72-nt insert with tandem in-frame stop codons in the 5' portion of the ORF. N. benthamiana plants transformed with an SA-inducible RdRP gene from Medicago truncatula were more resistant to infection by Tobacco mosaic virus, Turnip vein-clearing virus, and Sunn hemp mosaic virus (members of Tobamovirus genus), but not to Cucumber mosaic virus and Potato virus X (members of different genera than the tobamoviruses). Our results indicate that N. benthamiana lacks an active SA- and virus-inducible RdRP and thus is hypersusceptible to viruses normally limited in their accumulation by this RdRP. These findings are significant for those studying virus-induced gene silencing, the hypersensitive response and systemic acquired resistance.