Transformation of Nicotiana benthamiana with the potato mop-top virus coat protein gene produces a novel resistance phenotype mediated by the coat protein

Transformed lines of Nicotiana benthamiana expressing a translatable transgene (CPtrans) encoding the coat protein (CP) gene of potato mop-top virus (PMTV) differed considerably in steady-state levels of transgene RNA transcript and CP that were positively correlated. A mean of 88% of transgenic T-1 plants from seven transgenic lines were resistant to manual PMTV inoculation. Resistance was identified by failure to develop symptoms in plants and inability to recover infectious virus with a sensitive infectivity assay: many of the 12% of transgenic plants that gave a positive infectivity assay did not produce symptoms and virus was not detected in repeat assays on the same plants. Resistant plants did not express a "recovery" phenotype and resistance was not overcome by use of RNA inoculum, Strong resistance was expressed in all CPtrans lines irrespective of the amount of transcript RNA or CP detected, and there was no evidence that resistance was mediated by a "transgene silencing" mechanism. Lines transformed with PMTV CP gene sequence in a nontranslatable form (CPnontrans) also contained differing steady-state levels of RNA transcript, but were not resistant to PMTV, although in some lines symptoms were slightly milder and mere slower to develop than those in nontransgenic control plants. These results suggest that CPtrans is unique because, although it depends on CP translation to be effective, it mediates very strong resistance in 10 independent transformed lines (tested here and in other work).