Mitochondrial alternative oxidase (AOX) plays a role in protecting plant cells against reactive oxygen species. The SA-inducible RNA-directed RNA polymerase 1 (RDR1), contributes to viral RNA degradation via RNA interference. Previous data suggested that these enzymes comprise separately regulated, redundant elements in SA-induced resistance to viruses. To test this hypothesis, I constructed transgenic tobacco<i> (Nicotiana tabacum) </i>and <i>N. benthamiana </i>plants compromised simultaneously in AOX function and RDR1 activity. Transgenic tobacco and <i>N. benthamiana </i>plants were characterised by measuring alternative respiratory pathway (AP) capacity and RDR enzyme activity. The resistance/susceptibility status of the transgenic plants was assessed by analysing <i>Tobacco mosaic virus</i> (TMV) accumulation in the chemically treated, directly-inoculated tissues. Antimycin A (AA)-induced resistance to TMV was inhibited in transgenic <i>N. benthamiana </i>with increased AP capacity, and SA- and AA-induced resistance was enhanced in transgenic <i>N. benthamiana </i>with decreased AP capacity. However, SA-induced resistance to TMV in directly-inoculated leaves was still unaffected in transgenic tobacco and <i>N. benthamiana </i>compromised in AOX function and RDR1 activity. This suggests that SA-induced resistance to viruses involves additional, unknown mechanisms. Surprisingly, SA can enhance RDR activity in transgenic 35S-<i>MtRDR1 N. benthamiana </i>but not wild-type and vector-control plants (natural mutants of RDR1). This SA-enhanced RDR activity resulted from increased MtRDR1 protein level, indicating the post-transcriptional regulation of MtRDR1 enzyme activity. SA-induced resistance to systemic movement was enhanced in transgenic 35S-<i>MtRDR1</i> <i>N. benthamiana </i>plants, suggesting that SA-induced increase in RDR1 activity plays a role in induced resistance to systemic movement of viruses. Basal resistance to viruses was studied in transgenic tobacco (<i>nn </i>or NN genotype) and <i>N. benthamiana </i>plants with modified AP capacity or RDR activity. Modification of AP capacity had no effect on TMV accumulation in HR lesions from transgenic tobacco plants overexpressing the <i>Aoxla</i> construct (<i>NN</i> background). Notably, transgenic <i>N. benthamiana </i>plants with increased AP capacity were more susceptibility to <i>Potato virus X </i>(PVX) than non-transgenic plants. This was seen in the transgenic plant with increased AP capacity that PVX accumulated to higher level in both directly-inoculated and systemic leaf tissues. It was also nearly discovered that transgenic 35S-<i>MtRDR1 N. benthamiana</i> plants were more resistant to <i>Potato virus Y </i>ordinary strain. The results suggest that altering AP capacity has effect on basal resistance to some viruses and confirms that RDR1 plays a role on basal resistance.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:599252 |
| Date | January 2009 |
| Creators | Fu, S.-F. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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