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 ter minus of the TEV coat protein were initially susceptible to TEV infect ion, and typical systemic symptoms developed. However, 3 to 5 weeks af ter a TEV infection was established, transgenic plants ''recovered'' f rom the TEV infection, and new stem and leaf tissue emerged symptom an d virus free. A TEV-resistant state was induced in the recovered tissu e. The resistance was virus specific. Recovered plant tissue could not be infected with TEV, but was susceptible to the closely related viru s, potato virus Y. The resistance phenotype was functional at the sing le-cell level because protoplasts from recovered transgenic tissue did not support TEV replication. Surprisingly, steady state transgene mRN A levels in recovered tissue were 12- to 22-fold less than transgene m RNA levels in uninoculated transgenic tissue of the same developmental stage. However, nuclear run-off studies suggested that transgene tran scription rates in recovered and uninoculated plants were similar. We propose that the resistant state and reduced steady state levels of tr ansgene transcript accumulation are mediated at the cellular level by a cytoplasmic activity that targets specific RNA sequences for inactiv ation.