Virus resistance in transgenic plants expressing translatable and untranslatable forms of the tobacco etch virus coat protein gene sequenc

Tobacco etch virus (TEV) is a member of the Potyvirus genus, one of the largest and most economically important groups of plant viruses. In an attempt to generate TEV-resistant transgenic plants, I have applied the concept of pathogen derived resistance (PDR). The concept of PDR proposes that pathogen resistance genes can be derived from a pathogen's own genetic material. To investigate this hypothesis, transgenic Nicotiana tabacum plants expressing one of several different forms of the TEV coat protein (CP) gene sequence were generated. Transgenic plants were screened both for transgene expression and TEV-resistance. The particular TEV-resistance phenotype observed varied with the particular form of the TEV CP gene expressed by the plant.;Transgenic plant lines expressing untranslatable sense-stranded transcripts often showed complete resistance to TEV. Protoplasts from these TEV-resistant plant lines would not support TEV replication. In this group of transgenic plants steady state transgene RNA levels did not correlate with virus resistance.;Plant lines accumulating either full length or truncated forms of CP displayed different resistance phenotypes, depending upon the particular form of CP which accumulated in the plant. Some of the resistance phenotypes observed in these TEV-challenged transgenic plant lines were: (1) a delay in the appearance of TEV-induced symptoms; (2) an attenuation of TEV-induced symptoms; or (3) the ability to recover from (or outgrow) TEV infection. Whole plant inoculation experiments were used to demonstrate that TEV-recovered transgenic plant tissue could not be infected with TEV. Protoplast experiments demonstrated that TEV-recovered transgenic plant tissue would not support TEV replication. TEV-recovered transgenic plant tissue was, however, susceptible to the related potyvirus potato virus Y. Interestingly, transgene RNA and protein levels were reduced 10-20 fold in TEV-recovered tissue. It is proposed that the TEV-resistant state and reduction in transgene RNA levels are mediated by a cytoplasmically-based activity which targets both the transgene RNA and TEV genomic RNA for degradation. Such an activity could be functioning in seemingly unrelated biological phenomenon such as cross protection and some examples of sense- (or co-) suppression of genes.