Engineering resistance against viral diseases in plants

Plum pox potyvirus resistance associated to transgene silencing that can be stabilized after different number of plant generations

Nicotiana benthamiana plants were transformed with a fragment of the plum pox potyvirus (PPV) genome that encodes the nuclear inclusion a (NIa) and b (NIb) proteins and the N-terminus of the capsid protein (NIa-NIb-CP). Lines transformed with this PPV genomic fragment harboring mutations in the GDD replicase-motif were also obtained. Plants of NIaDeltaV lines that carry a GDD to VDD mutation in the PPV transgene, were immune to PPV infection. The resistance was highly specific, since it was only partially overcome by a PPV strain different to that from which the transgene was derived, and no resistance was observed after inoculation with a second potyvirus. PPV was not able to replicate in protoplasts isolated from NIaDeltaV transgenic plants, indicating that the resistance was functional at the single cell level. Only a fraction of plants from lines transformed with the NIa-NIb-CP fragment harboring a GDD to ADD mutation (NIaDeltaA lines), were resistant to PPV infection. This same phenotype was observed in plants expressing the wild-type construction (NIaDelta), although the progeny of some non-infected plants seemed to be completely resistant to PPV, independently of the allelic status of the parental plant. In all cases, the resistance phenotype correlated positively with low levels of transgene mRNA accumulation, suggesting that it was mainly due to a gene silencing mechanism. Our results show that, although the transgene was not silenced in all R1 plants from some individual lines, a stable silenced status could be reached in the following generations.