Blue roses might be achieved (without gene splicing) in a variety of ways. Some more likely than others.
Myricetin, a precursor of delphinidin, is found in some roses. This was reported by Gupta, Pahnajamani and Seshadri in 1957 (Journal of Scientific & Industrial Research). I’m still looking for a copy of the paper, hoping they gave a list of the 20 HTs they found that produce myricetin. Breeding from these roses is not a guaranteed route to delphinidin, but roses with myricetin are more likely to produce a delphinidin colored rose than varieties that lack the precursor.
Another route seems more promising. Some roses already carry blue pigments, the rosacyanins. These pigments have some important advantages. “The accumulation of rosacyanin As may be a better way to engineer blue roses than that of delphinidin, as rosacyanin As are consistently blue even when co-pigments and metal ions are absent and the vacuolar pH is low.”
Tetrahedron 62(41): 9661-9670 (9 October 2006)
Two novel blue pigments with ellagitannin moiety, rosacyanins A1 and A2, isolated from the petals of Rosa hybrida
This was previously reported to this forum by Don, on Mon, Oct 27, 2008
The variety studied was ‘Mme. Violet’. It would be interesting to learn which of its parents, ‘Lady X’ or ‘Sterling Silver’, contributed the pigments.
Breeding for an increase of specific substances has been done in the past. The sugar beet was bred from common beets by selection for increased sugar content. And in a 50 year long breeding experiment on corn, selective breeding for lines with increased content of oil, protein, and amylopecton (a tapioca-like starch) was successful. Once the desired improvements were accomplished, the strains were used for further breeding.
It is likely the same thing could be accomplished for rosacyanins by breeding from ‘Mme Violet’, selecting for the greatest concentration of rosacyanins relative to the concentrations of other pigments (e.g., cyanin).
Crossing ‘Mme Violet’ to other mauve roses is not likely to bring the desired improvement unless they also produce rosacyanins.