| Summary: | Salicylic acid (SA) induces systemic acquired resistance (SAR) to a wide range of pathogenic microbes. The defensive signal transduction pathway branches downstream of SA in tobacco (<I>Nicotiana tabacum</I>) and <I>Arabidopsis thaliana</I>. One branch leads to resistance to fungi and bacteria. The other, antimycin A (AA)-induced, branch, the virus-specific signalling pathway (VSSP), leads to resistance to viruses. In this study I showed, using cDNA-AFLP, that certain genes induced or repressed in tobacco leaf tissue expressing SAR were also induced or repressed by the VSSP (induced using AA). The regulation of three of these genes was verified using RT-PCR. A preliminary investigation of the temporal and tissue-specific regulation one of these genes (<I>B2</I>) was carried out. <I>B2 </I>was constitutively expressed most strongly in the leaf lamina, less strongly in the vein, weakly in the stem and not at all in the roots of healthy tobacco plants. Following induction of SAR or the VSSP <I>B2</I> transcript was found to increase steadily for up to 24 hours. <I>B2 </I>accumulation increased from the basal level only in the leaf lamina. <I>B2 </I>transcript was induced in both the pathogen inoculated and the distant (non-inoculated) leaves of tobacco resisting <I>Tobacco mosaic virus </I>(TMV)-infection. These experiments strengthened the correlation between <I>B2 </I>induction and resistance to viruses. Post-transcriptional gene silencing (PTGS) is another inducible anti-viral defence mechanism in plants. PTGS requires the activity of an RNA-dependent RNA polymerase (RdRp). The recent discovery of a SA-regulated, plant-encoded RdRp (<I>NtRdRp1</I>) in tobacco suggested that there is a link between PTGS and SA-dependent resistance. I found that although <I>NtRdRp1 </I>is indeed induced by SA, it is not induced by AA, and thus this gene is unlikely to be regulated by the VSSP. It has been proposed that the VSSP requires the activity of alternative oxidase (AOX), the terminal oxidase of the plant mitochondrial alternative respiratory pathway (AP). I tested this theory in transgenic tobacco lines with increased or decreased AP capacity, relative to untransformed tobacco. I found that TMV accumulation in inoculated tissue was similar in untreated transformed and untransformed plants.
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