"Resistance to specific virus diseases may be added as heritable characteristics to susceptible cultivars by transformation of the cultivars with specific virus-derived genes. In practice, however, resistance to the virus varies among transformed plants and transformation often changes cultivar yield and quality characteristics that are agronomically important…

Although it is true that some viral genes will confer transgenic resistance, and this resistance may be added to a cultivar as a single characteristic, it is not true that resistance is achieved in every transgenic plant line. In fact, expression of resistance may be rare among lines transformed with a viral gene, and degree of resistance conferred by a gene can vary continuously from no resistance to complete protection from disease… The gene itself can affect the plant growth and type….

Off-type plants can often be identified among new transformants in tissue culture media by their lack of vigor or by conformational aberrations….

More sublte vigor and growth aberration defects that are not obvious at an earlier stage are often exhibited after plantlets are transferred from soil flats to the field…

In our experience with potatoes…[although] growth aberration is usually associated with poor vigor, it does sometimes occur in vigorous lines, but such lines have usually produced low yield and off-type tubers….

The agronomic characteristics of transgenic lines may differ somewhat from the parental line…

On occasion there are abnormalities associated with agrobacterium mediated transformation and abnormalities may be much more common in plants produced via particle gun transformation…

Gene stability through successive plant generations is a major concern…

Crop improvements by genetic engineering go far beyond virus resistance. At present, the most popular traits are insect tolerance and herbicide resistance. Genes for fungi resistance and for crop quality improvements are under investigation. All these traits can be combined with virus resistance…

Generating plants with multitraits by transformation with multigene vectors or retransformation can cause unexpected problems. Possibilities of interaction between genes introduced need to be considered. Repeats of some genetic elements can trigger gene silencing. Place of introgression into chromosome could increase in importance. Plants with multiple copies of new inserts could be especially vulnerable and perform in unexpected or unwanted manner….

Complementation, the process by which a functional gene of one virus corrects for defectiveness in the same function of another, coinfecting virus, is a well known phenomenon. Thus, expression of a transgene could induce susceptibility in the transgenic plant to new viruses if the expressed gene provided an essential function which the new virus could not itself provide…

Synergism is the interaction between an infecting virus and the product of a transgene that results in more severe symptoms than should be expected by infection of the parental cultivar. This occurrence would be indicated by severe symptoms that were not associated with alterations in the infecting virus…

If a virus infecting a transgenic plant were encapsulated with coat protein produced by the transgene, the infecting virus could acquire the transmission characteristics and seriological properties of the transgene virus.

A new hybrid virus could be created by genomic recombination between the genome of a virus infecting a transgenic plant and the transgene or its mRNA. Sedimentation characteristics of the new hybrid would probably be different from the original virus, and its occurrence might be indicated by production of symptoms unusual for any of the viruses that infect the crop species….

Alterations in the homologous virus may be difficult to detect since the alteration may have but minor effects on many properties of the virus. Isolating the altered virions would be difficult unless the alteration provided a competitive advantage in the transgenic host or caused a change in host range that would facilitate its isolation…

There is a potential risk that a transgene could be introduced into wild species or other crop plants by outcrossing…

There is a theoretical possibility that a crop species that normally will not compete as a weed could acquire this ability by virtue of its transformation with a viral gene…"

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