Analysis of the in planta antiviral activity of elderberry ribosome-inactivating proteins.

Although the type-2 ribosome-inactivating proteins (SNA-I, SNA-V, SNLRP) from elderberry (Sambucus nigra L.) are all devoid of rRNA N-glycosylase activity towards plant ribosomes, some of them clearly show polynucleotide-adenosine glycosylase activity towards tobacco mosaic virus RNA. This particular substrate specificity was exploited to further unravel the mechanism underlying the in planta antiviral activity of ribosome-inactivating proteins. Transgenic tobacco (Nicotiana tabacum L. cv Samsun NN) plants expressing the elderberry ribosome-inactivating proteins were generated and challenged with tobacco mosaic virus in order to analyze their antiviral properties. Although some transgenic plants clearly showed antiviral activity, no clear correlation was observed between in planta antiviral activity of transgenic tobacco lines expressing the different ribosome-inactivating proteins and the in vitro polynucleotide-adenosine glycosylase activity of the respective proteins towards tobacco mosaic virus genomic RNA. However, our results suggest that the in planta antiviral activity of some ribosome-inactivating proteins may rely on a direct mechanism on the virus. In addition, it is evident that the working mechanism proposed for pokeweed antiviral protein cannot be extrapolated to elderberry ribosome-inactivating proteins because the expression of SNA-V is not accompanied by induction of pathogenesis-related proteins.