Salicylic acid induced resistance to plant viruses

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 (Nicotiana tabacum) and N. benthamiana plants compromised simultaneously in AOX function and RDR1 activity. Transgenic tobacco and N. benthamiana 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 Tobacco mosaic virus (TMV) accumulation in the chemically treated, directly-inoculated tissues. Antimycin A (AA)-induced resistance to TMV was inhibited in transgenic N. benthamiana with increased AP capacity, and SA- and AA-induced resistance was enhanced in transgenic N. benthamiana with decreased AP capacity. However, SA-induced resistance to TMV in directly-inoculated leaves was still unaffected in transgenic tobacco and N. benthamiana 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-MtRDR1 N. benthamiana 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-MtRDR1 N. benthamiana 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 (nn or NN genotype) and N. benthamiana 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 Aoxla construct (NN background). Notably, transgenic N. benthamiana plants with increased AP capacity were more susceptibility to Potato virus X (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-MtRDR1 N. benthamiana plants were more resistant to Potato virus Y 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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo