Full-length cDNAs of the wild-type (wt) Tobacco mosaic virus (TMV) and of the coat protein gene-deleted (ΔCP) derivative of wt-TMV, under control of the 35S promoter and downstream ribozyme sequence to produce accurate viral transcripts, were transformed to tobacco plants to analyze plant--virus interactions through different stages of plant development. Surprisingly, young wt-TMV transgenics accumulated only very low levels of viral RNA, remained free of symptoms, and were moderately resistant against exogenous inoculations. This early resistance caused significant stress to the plants, as indicated by reduced growth. Approximately 7 to 8 weeks after germination, the resistance was broken and plants developed typical wt-TMV symptoms, with high accumulation of the viral RNAs and proteins. The ΔCP-TMV plants likewise were initially resistant to the endogenous inoculum and were stunted, although to a lesser extent than the wt-TMV plants. The resistance was broken at the same time as in the wt-TMV plants, but the mutant replicated to much lower levels and produced much milder symptoms than the wt virus. TMV-specific small interfering RNAs as well as increased transgene methylation were detected in the plants only after the resistance break, indicating that the resistance in the young plants was not due to RNA silencing.
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