Hint: for the most part, thanks to coincidence
The attractiveness of many floral scents for people is a good side effect. We have not even been in the world when they appeared. But the perfume available on the market, although they try, seldom smells like flowers. Expensive fashionable bottles with names like “jasmine” or “gardenia” can smell beautifully - but this is just a pitifully similar to real flowers.
One reason for this is that flowers usually produce a mixture of a very large number of volatile molecules - up to a thousand species. Some of them belong to related chemical groups, and even if they differ only slightly in structure, they can produce very different odors. In colors related to each other, volatile molecules can vary both in proportions (which reflect different regulation of genes) and in chemical structure (which reflects the activity of genes associated with the production of enzymes necessary for synthesis). It is quite difficult to establish exactly which components of this mixture are important for attracting insects, birds, or for creating smells that are pleasant to people. This is especially difficult because our sense of smell depends on a complex set of nerve cells and is different in different people. The production of aromas depends on the genes of the plants, and the ability of animals, including us, to pick up these aromas, depends on the genes of animals.
As in the case of color, the chemistry of volatile compounds that affect odor depends on the presence of genes that encode protein enzymes. These enzymes work consistently to create complex odorous molecules from their preceding molecules, whose presence depends on another set of genes and enzymes. The relative amount of different molecules depends, in turn, on the third set of genes encoding RNA and proteins, which are important for regulating and modulating the genes necessary for the production of odors.
When we smell a rose, we perceive a mixture of several hundred different molecules. And each of them is the result of a set of genes and enzymes encoded by them, causing certain chemical reactions in rose petals. Many volatile molecules are derived from the amino acid
phenylalanine .
Plants create phenylalanine from simpler molecules through a set of genes encoding the protein enzymes necessary for this. Phenylalanine is a close relative of
tyrosine , an amino acid used by plants to create
betalain pigments, and is also an aromatic compound, having a ring of carbon atoms. The difference in their chemical structure consists only in the fact that tyrosine has additional oxygen (in the form of a –OH group connected to a carbon ring). Mammals generally produce tyrosine from phenylalanine (plants use a different path for this). The list of pleasant-smelling molecules derived from phenylalanine and tyrosine is rather long.
Plants create phenylalanine and tyrosine to be able to create proteins. But evolution, another opportunist, uses amino acids for other purposes. Each of the methods depends on the appearance in the course of evolution of one or several additional genes encoding enzymes that create both flavors and proteins with RNA, which are necessary for the inclusion of genes contained in the lobes at the right moment. Many aromatic volatile molecules arose as a result of the fact that copies of the genes underwent mutations — we have already seen such a scheme several times. This is one of the most effective ways to create variants of genes that natural selection can work with.
To produce a volatile aromatic substance from the amino acid phenylalanine or tyrosine, it is necessary to perform a chemical surgery on an amino acid using one or more reactions, the catalysts of which are certain enzymes. One of these reactions removes the amino group (–NH2) from the amino acid. If phenylalanine served as the initial molecule, then
cinnamic acid is obtained; if the starting molecule was tyrosine, then coumarin acid would result. The only difference between these acids is that coumarin acid has the same additional oxygen atom in the form of the –OH group, like tyrosine. And most of the fragrances of flowers, although not all, originate in the form of one of these two molecules.
The name of cinnamic acid should not be a secret - it is she who gives cinnamon a familiar smell. Cinnamon is the dried bark of evergreen trees from the genus of cinnamon of the laurel family, which reminds us that many plants have other parts besides petals that produce aromas. The enzyme that removes an amino group from phenylalanine to produce an acid is called
PAL , and it is encoded by a gene of the same name. Most plants have several PAL genes. The lab model
rezuhovidki has four PAL genes, and they show different degrees of activity, working in different parts of the plant. Possession of several PAL genes makes sense, since the same phenylalanine minus the amino group, for example, cinnamic acid, produces many plant molecules, and not only volatile ones. Among them are
lignin , a large molecule found in wood, and pigments that give color to flowers. Some plants use PAL to start a long chain of reactions leading to the production of
chalcone , a molecule that eventually turns into an anthocyanin dye.
Another route to producing aromas from phenylalanine involves two amino acid dissections. The amino group (–NH2) and the acid group (–COOH) are removed, that is, everything that characterizes the molecule as an amino acid. The remaining molecule becomes the starting point for the production of many other aromatic molecules. The level of enzymes necessary for carrying out this surgery in rose petals is more often achieved in adult flowers at the end of the day, when it is most important to attract pollinating insects. Evolution ensured that genes become most active at the most appropriate moment.
The disclosure of the genes responsible for enzymes that remove the acid group from phenylalanine required a real detective investigation. Genetic data banks of colors were searched for sequences that, by analogy with genes known from other organisms, could produce an enzyme that removes the acid group from phenylalanine. Scientists stumbled upon a solution when they discovered plant DNA sequences similar to those of animals that remove the acid group from the DOPA molecule, or
dioxyphenylalanine , which is related to phenylalanine. The same molecule is used as a cure for Parkinson's disease. This piece of DNA was most active in plants then and in those places where and when the production of volatile molecules from phenylalanine was maximum. When the activity of these genes was suppressed as an experiment in mutated petunias, the production of aromatic substances was stopped. The same was true for the version of the genes present in the rose.
Varieties of this genetic code in petunia and roses produce enzymes that are 65% identical to those of animals that remove the acid group from DOPA, and are similar to other plant enzymes that remove the acid group from other molecules. Collectively, these genes belong to a family of related genes. So it makes sense to assume that they all descended from a common ancestor gene.
Flowering plants may have many more genes encoding the enzymes necessary for the production of other aromatics. Where did they come from? Probably most of them, and perhaps all, are associated with genes that are important for the work of other plant functions, and come from copying genes in the past. Apparently, this happened during the evolution of the genes responsible for the "tea" flavor, characteristic of the popular tea rose. When the ancient genus of Chinese roses penetrated into Europe at the end of the 18th century, it was immediately clear that they differed from European ones by smell. Many years later, this unique fragrance was associated with certain compounds. By that time, hybrids between Chinese and European roses had already been obtained. These hybrids, known as tea roses, are especially popular, and one of the reasons for this is their strong and attractive aroma, inherited from the Chinese ancestor of the hybrid. Among these flavors, one molecule (3,5-dimethoxytoluene, or DMT [3,5-dimethoxytoluene]) can make up to 90% of all aromatic molecules produced by flowers. Petals of European roses produce very few of these molecules, and sometimes they do not give out at all.
The DMT molecule is associated with other flavors of flowers, consisting of a main ring of six carbon atoms, some of which are decorated with an assortment of carbon, hydrogen, and oxygen. The ability to produce such decorated rings is given to flowers by various genes and enzymes. Two enzymes encoded by the genes of Chinese roses, and active in the petals of Chinese roses, can produce a certain modification leading to the production of DMT. Why can't European roses do this? They do not have the set of genes needed to make the necessary changes. Two very close, but different genes, lead to the corresponding chemical changes in roses, derived from the Chinese; they are called OOMT1 and OOMT2. Pure European roses have only one of these two genes, but to modify the aromatic ring in a manner leading to the appearance of DMT, both proteins are required. The 350 amino acids in OOMT1 and OOMT2 are 96% identical, and the change in just one amino acid from 350 is most likely responsible for the difference in what they can produce in the cells of the petals. All this suggests that earlier, most likely, there was only one OOMT gene that underwent duplication, after which one of the two copies underwent DNA mutations, and as a result, amino acid changes occurred in the protein enzyme encoded by it.
Which of the genes was the first? If we compare the OOMT genes of many different roses, most of them will have the OOMT2 genes, but only the descendants of Chinese roses will have OOMT1. From the features of the structure of the evolutionary tree of roses, it follows that, most likely, Chinese roses appeared later on the timeline than others. This would be conclusive evidence that OOMT2 has been around longer than OOMT1, and that OOMT2 has undergone duplication.
But getting roses with a smell attractive to humans could not be the reason for the success of this duplication of genes and mutations. Why did this gene survive and succeed? It is all about the bees: important pollinators of flowers, apparently, are able to feel DMT.
Maxine Singer received her doctorate in 1957 from Yale University. Worked on the editorial boards of the Proceedings of the National Academy of Sciences and the Journal of Biological Chemistry and Science. Received many scientific awards. There is an excerpt from the book “The Flowers and the Genes Generating them” (Blossoms: And the Genes That Make Them), Maxine F. Singer, 2018.