The Use of Transgenic Rootstock for Improving Nontransgenic Scion Cultivars: Opportunities and Challenges

The use of transgenic rootstocks to improve the performance of nontransgenic scion cultivars provides a potential to address the biosafety concerns about transgene flow and transgene containment in fruit crops. Mobility of transgene products depends on transgene(s). Immobile transgene products (e.g., large-molecule proteins) have little chance of being translocated across the graft union and thus they do not pose any concern about transgene containment in nontransgenic scion parts. Mobile transgene products [e.g., phytohormones, small-molecule proteins, and small interference RNAs (siRNAs)] can be translocated across the graft union. In our recent studies, mobile siRNAs of Prunus necrotic ringspot virus (PNRSV) in transgenic cherry rootstock were translocated across the graft union and entered nontransgenic sweet cherry ‘Emperor Francis’ at a frequency of approximately 0.5% of the total transgenic siRNAs produced in rootstock. The translocated siRNAs enabled PNRSV resistance in the nontransgenic ‘Emperor Francis’. Alternatively, constitutive expression of a blueberry (Vaccinium corymbosum)-derived FLOWERING LOCUS T gene (VcFT) promoted flowering of transgenic tobacco plants; however, early flowering was not observed in nontransgenic tobacco scions grafted on this VcFT-overexpression rootstock, suggesting an inefficient conveyance of the transgenic trait. In both cases, the use of transgenic rootstocks is able to eliminate the concerns about transgene flow from the nontransgenic scion parts. In terms of the concerns about transgene containment in fruits harvested from nontransgenic scion cultivars grafted on transgenic rootstocks, it relies on both the nature of transgene products and their mobility.