There are a few plants that have been subject to some rather intense breeding programs. Corn
is likely the most breed grain
– and the rose
the most bred flowering plant. Through all of this selective breeding, the rose has undergone several changes: different sized blooms (often larger), different colored petals, and climatic resistance are a few of the traits that have been bred for.
With all of this selective breeding for various visual and health traits, the rose has lost is scent – it's just not as intense as it once was (over 18,000 varieties of roses, most of them have bland, little, or no scent at all).
David Weiss, a geneticist at the Hebrew University in Jerusalem, looked at two varieties of roses - the Fragrant Cloud (large red bloom with a strong rose smell) and the Golden Gate (smaller yellow bloom with no scent at all). Comparing the genes of these to roses, 1,288 genes were unique to the Fragrant Cloud, and 746 were unique to the Golden Gate. Certainly, not all of these genes encode for smell - some encode for size, or color, or shape. To figure out what genes were smell producing, the bud in a mature flower was examined and the genes that were active there were examined (the genes active on a petal don't produce a leaf). This narrowed it down to 77 genes in the Fragrant Cloud that were more active than in the Golden Gate, and 40 of these genes were active in the bud of mature flowers. Looking at the 40 genes now, 7 of these were found to be encoding for enzymes that were involved in the synthesis of complex organic molecules. Two of these enzymes were more common in petals than in other parts of the plant and one of these makes a volatile compound that is found in plant scents. In the Fragrant Cloud, this compound is 30x more common in the air around the bud than in the Golden Gate. The quest however is not complete - this compound only makes up 10% of the volatile compounds given off by the Fragrant Cloud.
So, assuming that at some point in the future, the scent of the rose will be identified and pined down it would be possible to genetically engineer the smell of a rose. This is not the only genetic engineering project - another one hopes to create a blue rose. There are questions, however, about how consumers will respond to genetically engineered flowers - there is very little danger (compared to the little danger of genetically engineered foods that we eat - tomatoes that stay ripe longer and such). Some rose growers and breeders are worried about the possibility of having the patient craft of developing a particular flower turned into an exercise of programming the genes right - a loss of the romanticism that exists today with both the creation of the flower and the flower itself.