Ok... think it about this way. Selective breeding is completely possible under natural circumstances, because we are purely manipulating natural evolutionary processes. The only reason domestication dont naturally occur is becuase generally the desirable traits are often lethal to wild organisms. We are the evolutionary pressure. If we die, so would the majority of our crops and livestock.

Genetic engineering completely bypasses that process, and can even introduce genes from other species that would be so improable to breed, you might as say its impossible. You'll never be able to breed glow in the dark corn because no parent you can breed with will have the genes, so you would be left with two natural choices... a random mutation, or a random horizontal gene transfer.