Induction of a Highly Specific Antiviral State in Transgenic Plants: Implications for Regulation of Gene Expression and Virus Resistance.

Transgenic tobacco plants expressing either a full-length form of the tobacco etch virus (TEV) coat protein or a form truncated at the N terminus of the TEV coat protein were initially susceptible to TEV infection, and typical systemic symptoms developed. However, 3 to 5 weeks after a TEV infection was established, transgenic plants "recovered" from the TEV infection, and new stem and leaf tissue emerged symptom and virus free. A TEV-resistant state was induced in the recovered tissue. The resistance was virus specific. Recovered plant tissue could not be infected with TEV, but was susceptible to the closely related virus, potato virus Y. The resistance phenotype was functional at the single-cell level because protoplasts from recovered transgenic tissue did not support TEV replication. Surprisingly, steady state transgene mRNA levels in recovered tissue were 12-to 22-fold less than transgene mRNA levels in uninoculated transgenic tissue of the same developmental stage. However, nuclear run-off studies suggested that transgene transcription rates in recovered and uninoculated plants were similar. We propose that the resistant state and reduced steady state levels of transgene transcript accumulation are mediated at the cellular level by a cytoplasmic activity that targets specific RNA sequences for inactivation.