Differential Responses to Virus Challenge of Laboratory and Wild Accessions of Australian Species of <i>Nicotiana</i>, and Comparative Analysis of <i>RDR1</i> Gene Sequences

<div><p><i>Nicotiana benthamiana</i> is a model plant utilised internationally in plant virology because of its apparent hyper-susceptibility to virus infection. Previously, others showed that all laboratory accessions of <i>N</i>. <i>benthamiana</i> have a very narrow genetic basis, probably originating from a single source. It is unknown if responses to virus infection exhibited by the laboratory accession are typical of the species as a whole. To test this, 23 accessions of <i>N</i>. <i>benthamiana</i> were collected from wild populations and challenged with one to four viruses. Additionally, accessions of 21 other <i>Nicotiana</i> species and subspecies from Australia, one from Peru and one from Namibia were tested for susceptibility to the viruses, and for the presence of a mutated RNA-dependent RNA polymerase I allele (<i>Nb-RDR1m</i>) described previously from a laboratory accession of <i>N</i>. <i>benthamiana</i>. All Australian <i>Nicotiana</i> accessions tested were susceptible to virus infections, although there was symptom variability within and between species. The most striking difference was that plants of a laboratory accession of <i>N</i>. <i>benthamiana</i> (RA-4) exhibited hypersensitivity to Yellow tailflower mild mottle tobamovirus infection and died, whereas plants of wild <i>N</i>. <i>benthamiana</i> accessions responded with non-necrotic symptoms. Plants of certain <i>N</i>. <i>occidentalis</i> accessions also exhibited initial hypersensitivity to Yellow tailflower mild mottle virus resembling that of <i>N</i>. <i>benthamiana</i> RA-4 plants, but later recovered. The mutant <i>Nb-RDR1m</i> allele was identified from <i>N</i>. <i>benthamiana</i> RA-4 but not from any of 51 other <i>Nicotiana</i> accessions, including wild accessions of <i>N</i>. <i>benthamiana</i>, demonstrating that the accession of <i>N</i>. <i>benthamiana</i> used widely in laboratories is unusual.</p>