What Chinese roses lack in resistance to cold and disease, they make up for with repeated blooming and a distinctive fragrance. It isn't surprising, then, that plant breeders, fascinated with this ancient rose family, brought it to Europe in the nineteenth century to generate the now popular hybrid tea roses, among many others. But how did the distinctive scent of Chinese roses evolve? Gabriel Scalliet and colleagues now provide the answer (G. Scalliet et al. Proc. Natl Acad. Sci. USA 105, 5927–5932; 2008).
Unlike their European counterparts, the main scent component of Chinese roses and their descendants is dimethoxytoluene. In the final steps of its formation, two methyl groups are sequentially added to a precursor molecule in reactions catalysed by enzymes called OOMT1 and OOMT2. These enzymes are almost identical in amino-acid sequence, yet they target different substrates.
Scalliet and colleagues pinpoint the crucial amino-acid residues of the enzymes that confer substrate specificity: a tyrosine in OOMT1 and a phenylalanine in OOMT2. Swapping these single residues between the two enzymes switched their substrate specificity, most probably by changing the steric and hydrophobic properties of their substrate-binding sites, where these residues reside. Moreover, OOMT2 could methylate the target of OOMT1 in vitro, albeit with lower efficiency.
Two nearly identical enzymes with nearly identical activities, that can both perform similar reactions, although together they are more efficient than alone — these are hallmarks of the products of duplicated genes, which arise when a gene doubles in number and one of the copies then undergoes a minor mutation. Indeed, Scalliet et al. show that, although all of the 13 European rose species they examined carry an OOMT-like gene, only the Chinese species, which are evolutionarily younger, carry two types of this gene. The authors' further phylogenetic analysis strongly hints that, in Chinese species, the second OOMT gene arose through a duplication event.
Roses are not unique in acquiring new functions through gene duplication; this is thought to be a fundamental mechanism for generating diversity between and within organisms. But it isn't always possible to trace such changes, which makes Scalliet and colleagues' work of interest not only to plant biologists but also to evolutionary scientists.
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Shadan, S. Scent of a rose. Nature 452, 947 (2008). https://doi.org/10.1038/452947a