The regulatory implications of using transgrafted plants are currently unknown. A plant's vascular system can selectively transport across graft junctions endogenous elements such as full-length RNAs, sRNAs, proteins, hormones, metabolites and vitamins, and even elicit epigenetic effects, heritably changing the way genes are expressed without changing the actual DNA sequence. However, not all of these elements are transported freely, and they either require specific molecular signals or cellular transporters to aid in their movement through a plant's vascular system.

These transfers are understood to a degree (Haroldsen et al. 2012), but what is less clear is how the movement of these elements from transgenic rootstocks to scions might affect the regulatory approval process for a transgrafted plant — a product developed using transgenic tools and yet not containing transgenic DNA in the scion product. It cannot be said with certainty if transgenic RNAs, sRNAs or proteins produced in rootstocks may make their way to the nontransgenic scion. Furthermore, some of these elements may have short half-lives, making it difficult to determine by testing whether the final crop was produced using a transgraft.

There is no precedent within the regulatory framework coordinated by the U.S. Department of Agriculture, U.S. Food and Drug Administration and U.S. Environmental Protection Agency regarding how a transgrafted, genetically engineered rootstock and wild-type scion might be regulated. U.S. regulation identifies genetically engineered crops through a product-based policy; that is, if the final product contains transgenic material, then it is considered genetically engineered. However, even if scions are shown to be free of transgenic DNA, since transgrafted crop products are new to consumption, it is likely that safety assessments will be required prior to their market release. They would potentially, however, be classified as a conventional and not genetically modified food in the United States.

Conversely, in the European Union, if biotechnology tools are used in the process of developing a crop, then they fall under EU legislation for genetically engineered crops. In this case, regardless of whether the final transgrafted crop product contains transgenic material (DNA, RNA or proteins) or not, it would be classified as genetically modified. For example, German authorities decided in 2010 that any grapes or wine produced from transgenic rootstocks must be labeled as genetically engineered (Heselmans 2011).

This international policy difference will put EU regulators in a difficult situation in the future, when importing crops harvested from transgrafted plants produced in the United States. How will they identify a nontransgenic crop product that has been developed using transgenic tools? How can they be certain that crops imported from countries such as the United States are not genetically engineered (by EU definitions) when testing may not differentiate between conventionally grown crops and those from transgrafted plants? While the European Union may elect to implement process monitoring of new technologies to ensure proper labeling, documenting every step of the production process and tracking the final products of transgrafted crops, it would be difficult to guarantee the genetically engineered status of imports from outside the European Union.

To address this difficulty, and assuming tests can be developed that easily and robustly detect the presence or absence of transgene elements in the final crop, a threshold limitation could be established. The EU threshold for allowable levels of “adventitious mixing” of genetically engineered seed into conventional seed is 0.9%. The expectation for transgrafted crops, in particular first-generation transgrafts ( see page 67 ), is that any transgenic DNA, mRNA, siRNA or protein would fall below the 0.9% level. While current EU legislation would likely need to be revised, it is possible that EU regulators would allow transgrafted fruit or nut products to enter the European Union, so long as transgenic material was below the 0.9% threshold. While strictly conjecture, at the least, this sort of threshold limitation should be included in discussions of alternatives to current regulatory requirements.

In the United States, transgrafting applications will likely be dealt with on a case-by-case basis as they are introduced into the regulatory process (C. Wood, USDA Biotechnology Regulatory Services, personal communication, September 2010). This would be in line with suggestions by the Dutch scientific advisory committee on genetically modified organisms (COGEM 2006). In anticipation of regulatory scrutiny, it will be important for scientists to gather experimental information determining to what degree transgenic elements move across the graft junction in different plant species and different types of coding and noncoding genetic constructs.

Unlike plant model systems, such as Arabidopsis sp., analyses of genetic material from fruit and nut crops tend to be compounded by high levels of phenolic compounds, polysaccharides and other secondary metabolites. Nevertheless, laboratory experiments have been carried out in walnut, grape and tomato regarding the mobility of transgenic elements, and the results are in the process of being published. This information should assist regulatory bodies in determining what portion of the scion, if any, should be regulated.

To illustrate these issues, imagine that a transgrafted orange is developed with transgenic siRNA in the rootstock that wards off nematodes. Tests on the scion leaf material do not reveal the presence of siRNA, but when the fruit is tested transgenic siRNA is detected. However, tests also show that after the oranges are harvested, the transgenic siRNA decreases over a short time to nondetectable levels. In the United States, after regulatory approval the oranges would not be required to be labeled as genetically engineered. If these sames oranges were exported to the European Union, siRNA would be undetectable in tests regardless of the trans-graft, but under EU legislation they would be classified and labeled as genetically engineered. So unless the U.S. seller directly informs the importer that the oranges were grown with a transgraft, they would have no way of knowing since the siRNA is undetectable after picking. This example highlights the difficulties arising from policy differences, which could hamper the future commercialization of transgrafting technologies currently in the developmental pipeline.

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