Introduction of proteins that have not previously been present in food may result in allergenicity . In all cases, it is emphasized that there are already good regulatory mechanisms for testing food and drugs for safety. Will often require crossing into desireable parents. Plant varieties are already required to pass variety registration which guarantees agronomic merit. Another possibility is the introduction of disease susceptibility into previously resistant crops.
Species develop because of reproductive barriers.
Put another, way, you can't have two coexisting species in same geographical area if they are not sexually isolated. Study on crossing of genes for herbicide resistance from oilseed Brassica sp. This study showed that the probability of crosses occurring were essentially zero. This resulted from characteristic differences in flowering times, hybrid sterility and pollen competition between species, where a more closely related species will fertilize the ovum. No specific evidence that bacteria or viruses transmit plant genes from one species to another.