Biosafety considerations of RNAi-mediated virus resistance in fruit-tree cultivars and in rootstock.

A major application of RNA interference(RNAi) is envisaged for the production of virusresistanttransgenic plants. For fruit trees, this remainsthe most, if not the only, viable option for the controlof plant viral disease outbreaks in cultivated orchards,due to the difficulties associated with the use oftraditional and conventional disease-control measures.The use of RNAi might provide an additional benefitfor woody crops if silenced rootstock can efficientlytransmit the silencing signal to non-transformedscions, as has already been demonstrated in herbaceousplants. This would provide a great opportunityto produce non-transgenic fruit from transgenic rootstock.In this review, we scrutinise some of theconcerns that might arise with the use of RNAi forengineering virus-resistant plants, and we speculatethat this virus resistance has fewer biosafety concerns.This is mainly because RNAi-eliciting constructs onlyexpress small RNA molecules rather than proteins,and because this technology can be applied using plantrootstock that can confer virus resistance to the scion,leaving the scion untransformed. We discuss the mainbiosafety concerns related to the release of new typesof virus-resistant plants and the risk assessmentapproaches in the application of existing regulatorysystems (in particular, those of the European Union,the USA, and Canada) for the evaluation and approvalof RNAi-mediated virus-resistant plants, either astransgenic varieties or as plant virus resistanceinduced by transgenic rootstock.