Nicotiana benthamiana plants transformed with the 54-kDa region of the pepper mild mottle tobamovirus replicase gene exhibit two types of resistance responses against viral infection.

Nicotiana benthamiana plants transformed with the 54-kDa region of the pepper mild mottle tobamovirus (PMMV) replicase gene were generated and six independently transformed plant lines were analyzed for resistance to PMMV. Two different resistance responses were obtained. Some of the transgenic plants from only two lines showed a preestablished, complete, and highly resistant phenotype since no viral symptoms were observed, although a low level of virus replication occurred. The remaining plants from these two lines and all of the plants from the other four lines tested showed a delayed, induced, and also highly resistant phenotype since they were susceptible early, but were able to recover from the systemic PMMV infection. Recovered, symptomless leaves were resistant to the PMMV strain from which the 54-kDa gene was derived and to a closely related strain but not to tobacco mosaic virus. Such a delayed resistance phenotype has not been previously described for any plant expressing viral replicase sequences. The transgenic plants within the lines displaying complete or delayed resistance phenotypes were analyzed for transgene expression before and after PMMV inoculation and the two types of resistance responses were shown to be independent of the transgene transcript level.