| Generation of recombinant viruses
Some viral promoters, e.g. CaMV35S, have been used to drive transgenes. It has been demonstrated that it can be inactivated in transgenics if Cauliflower Mosaic Virus (CaMV) infects. Use of such promoters may require appropriate investigations, particularly in crops susceptible to the viral source. Alternatively, several other promoters allowing expression of gene in the whole plant or in a particular organ or state can be utilized.
It has been noted that infection with multiple viruses results in homologous and nonhomologous recombination between viruses, resulting in new viral strains. Similar to natural situation, recombination with viral genes cannot be excluded altogether. However, it has been found that most recombinant viruses are compromised in fitness. Although squash and papaya transgenics with virus resistance genes have been grown for some time, no novel viruses have been reported yet. The likelihood of detecting such an event would be high if new virus causes adverse effect. Recently, use of small sequences by way of RNAi technology for viral resistance has been proposed and it is also likely to reduce the chances of recombination.
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