| RNA silencing is a plant defence mechanism involved in degradation of exogenous nucleic acid via sequence-specific interations. To counteract this, many kinds of viruses have envolved suppressor proteins that help them infect host successfully. Interaction within them have dynamic synergistic evolvement phenomenon, which was affected by some environment factors. Induced by transgenes of viral nucleic acid RNA silencing could degradate transgenes mRNA together with homology to viral RNA. This defence mechanism was called RNA mediated virus resistance. In this research project, gradient was set to examine temperature effect on interactions between plants and viruses. Meanwhile we also investigated the probability of RNA-mediated resistance and its transmission using different resistant transgenic plants transformed with nontranslated 25KD movement protein of PVX. The main results are as the following:1. RNA silencing-mediated antiviral defense was temperature dependent, and 30℃was the turning point at which nicotinan glutinose could recover from virus disease such as PVX and PVYN. After dealing with this temperature, recovery phenomenon occurred in N.glutinose infected by viruses and the upper leaves were healthy while the lower leaves remained viral disease. N.glutinose infected had persistent symptom if the temperature lower to 30℃, while up to 30℃, infected plants cameback more rapidly.2. High temperature activated RNA silencing-mediated antiviral defense in plants, which resulted in viral symptoms attenuated even disappear. At low temperature, virus-mediated RNA silencing was inhibited. Consistently, the levels of virus-derived siRNAs were low or too little to be detected by Northern blot. In contrast, with rising temperature up to 30℃or even higher, RNA silencing was activated and the amount of virus-derived siRNAs dramatically increased in lower leaves which remained symptom while in upper leaves which were healthy, there were no siRNA exist.3. The transgenic plants with 25KD movement protein of PVX were inoculated with PVX. Both The results of symptom observation and ELISA detection showed that 33 out of 72 transgenic plants were highly resistant to PVX infection, the proportions of disease resistance were 45.8%.4. The total RNA was extracted from transgenic plants with susceptible or resistant responses to PVX. Northern blot showed the mRNA accumulation of leaves varied among transgenic lines. The resistant plants had a low-level accumulation of mRNA transcribed from PVX-p25, whereas, the RNA in susceptible plants was accumulated to higher levels. The results revealed an inverse correlation between transgenic mRNA accumulation and virus resistance.5. By single graft experiments, we discovered that RNA-mediated resistance could not be transmitted from highly resistant rootstocks to susceptible scions in spite of the presence of the foreign transgene in the scions. Compered with nontransgene scions, transgene susceptible scions could delay symptom appearance about 2 weeks.6. Double graft experiments indicated that PVX could not move through a stem of high PVX resistance to lead to severe symptoms on the upper scion. And the middle stem didn't show any PVX symptom either, the ELISA analysis was negative.
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