Molecular pathway that allows roses to smell so sweet

New research from May 2023

An international team of botanists, plant biologists and biochemists, has found an important pathway used by roses to produce their familiar sweet smell. In their study, reported in Proceedings of the National Academy of Sciences, the group tracked down the pathway that allows binding molecules to produce the chemicals required to create the aroma associated with roses.

More information: Corentin Conart et al, A cytosolic bifunctional geranyl/farnesyl diphosphate synthase provides MVA-derived GPP for geraniol biosynthesis in rose flowers, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2221440120

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I’m always ready for new information, but this one seems a bit anticlimactic.
When I first became interested in the chemistry of rose perfumes, I learned that it all came down to citronellal, citronellol and geranial. That was before I learned about the carotene derivatives, known collectively as the Rose Ketones. These make up a smaller part of the rose perfume that can be extracted, but they are far and away more potent. And on top of these, there is “Rose oxide”. This is said to have a “green smell” when sniffed alone, but it is the secret ingredient that turns sweet into “old rose”. * “Rosarian”: Rose Perfume (1885)

Thank you for posting. If you have an opportunity to respond I would be grateful for any interpretation.

The Carotenes collectively known as Rose Ketones are the yellow-orange red colorants that degrade under UV light and release potent fragrances yes? Carotene is a B vitamin? Something like Sea Buckthorn berry oil would have these fragrant components within it? The smell of Sea Buckthorn berry would be an approximation? Ketones indicate it would actually be something of a volatile solvent for some reason, like an acetone that naturally occured. Yes? That would make these elements of fragrance waft in the air for some distance? esp. when heated by sunlight? And the connection to the petal color is the leaching of this colorant/perfume dissipating into the surrounding air?

The citronellal and citronellol are the components that remind us of lemon I am presuming. One of these is an alcohol substance and one of these is a fatty oily substance?

The geranial is an essential oil that smells of geraniums?..which does smell rose-ish but it’s the presence of rose oxide that convinces our receptors that sweet essential petal oils smell of old rose?.

Is there anyway you would be able to make anecdotal examples of these scientific conclusions?

What is a popular fragrant rose that you would venture to say has an abundance of the Rose Ketones?
What is a popular fragrant rose that has a higher concentration of the citronellal and citronellol?etc. etc.

Thanks u

I will try to answer your questions.
"The Carotenes collectively known as Rose Ketones are the yellow-orange red colorants that degrade under UV light and release potent fragrances yes? "
There are many carotenes. Those in rose petals range from colorless through various tints and shades of yellow. They help attract some pollinators, and people. They also serve as anti-oxidants, capturing singlet oxygen atoms, and faded over time without making any perfume.
Sweet smelling roses generally sacrifice some of their carotenes to make perfume. These carotenes are built up from a chain of 40 carbon molecules. An enzyme attaches to this chain and snips off a 13 carbon segment, leaving a 27 carbon remainder. The enzyme then snips a 13 carbon chain, leaving a 14 carbon chain.
These fragments are not yet fragrant. However, they are somewhat soluble in water.
In very fragrant roses there is another enzyme (or more than one) that oxidize the fragments. These oxidized fragments are the “Rose ketones”.
Now the perfume is ready to send into the air.
Some roses (Velvet roses) are not very fragrant when fresh, but become so when they age or are mixed into a pot-pourri. The oxidation occurs without the help of a specialized enzyme (unless something else is going on in the pot-pourri).
Rose oil is a very old product that was mentioned by Homer in the Iliad ca. 750 BCE. Culpeper (1675) gave two recipes that surprised me because they contain red rather than the more fragrant pinks (damasks). But it does make sense because the unoxidized fragments will last longer in the olive oil for future use.

“The citronellal and citronellol are the components that remind us of lemon I am presuming. One of these is an alcohol substance and one of these is a fatty oily substance?”
The lemon scent that caught my attention in ‘Colonial White’ (also called ‘Climbing Sombreuil’) smells oily like a lemon-scented furniture polish. The flower that I found was abnormal, so I got to sniff fragrance in the tiny petaloids in the center (oily lemon) and in the petals (sweet apple). Ordinarily the flowers smell of lemon drops, a sweet lemony scent with the oily smell lost in the background.

The first two links I’m attaching give lots of information and drawings of the many, many forms of carotenes to be found in roses. The third has two recipes for ‘Marechal Niel’ perfume. I include then just to suggest how complex the perfume of a rose can be.

Thank you so much for your efforts, the clear illustrations and insights into the world of rose fragrances. It is an indescribably cosmos and a science in itself.

I’ve seen this phenomenon as well and haven’t found an explanation for it yet.Great!

I forgot to mention: It is vitamin A (retinol) that is derived from some of the 40 carbon carotenes. Vit. A is 20 Carbons long. One molecule of beta-carotene can be cut in half to make two molecules of retinol.

“The geranial is an essential oil that smells of geraniums?..which does smell rose-ish but it’s the presence of rose oxide that convinces our receptors that sweet essential petal oils smell of old rose?.”

The special, “green” scented substance that makes turns all the sweetness into “old Rose” is a terpene called “Rose oxide”. It doesn’t trick our nose. It’s more of a spice that shapes the taste, or a touch of color that brightens a room. Without it, a Damask rose might be confused with a Musk rose. Both are good, but it’s better to have both the distinctions.
Geraniol is a sweet, rosy substance found in roses and some geraniums. It is technically an alcohol. Geranial (note the -al at the end) is the aldehyde form of geraniol. I think it is not so sweet … but the Leffingwell web page will give more information. Also, geraniums are VERY diverse in the perfumes of their leaves. There are some that are definitely rose-scented. I have one now that is all lemon (or citronella, as my neighbor calls it. ) How about nutmeg? Chocolate? That’s a wonderfully smelly bunch of plants.

Thank you both for being so tolerant and gracious!

Limiting our conversation to the “Carotene” colorants for now

Carotenes can be colorless to the human eye but express as a large range of yellow colorants as well

Scientific community accepts that roses produce these substances “Carotenes” because they serve useful functions for the plant as “anti-oxidants” that reduce tissue destruction while the plants are “executing their life cycle functions”

Enzymes within the flowering “organ” of the plant (the blossom) do what all enzymes in plants do-act as “catalysts” “speed-up” some kind of chemical reaction within the tissues of the plant. These reactions are what we call “growth” “life cycle functions”

Enzymes are very large proteins. They take a lot of plant energy to make (?)

Enzymes are heritable. The “code” for an enzyme is inherited from each parent. In humans inheriting two defective codes for an enzyme will result in a genetic disorder.

It is a Multi-step procedure to break down the 40 carbon chain Carotene(?s) that exist within (some) roses. An enzyme must be inherited that will break this chain up into (2) 13 carbon chains and (1) 14 carbon chain. This is NOT enough to create sweet wafting fragrances.

IF the Rose has inherited a second Enzyme code that can break down fragments of the previous enzyme reaction THEN it will possess the sweet WAFTING fragrance. The fragments of the first enzyme reaction AFTER secondary enzyme reaction are known as ROSE KETONES-which are “Solvent like” and explain their ability to disperse so readily into the air.

From the Carotenes Vitamin A Retinol can be extracted- the skin healing/beautifying properties of Rose blossoms are probably related to this phenomenon. The Carotene carbon chains need only be split precisely in half to produce Retinol. The human skin tissue cannot take raw Rose Caotenes and precisely synthesize Retinol from it…or can it?

The information “code” for enzymes is transmitted by genes from each parent. An allele is a specific type of gene that “codes” for a trait. Is there any knowledge that indicates how many ALLELES exist that code for this “secondary enzyme reaction that produces Rose Ketones”

Does the deep dive scientific literature indicate this trait of secondary enzyme reaction as DOMINANT or RECESSIVE?

Foetida and Hugonis are two species with yellow coloration and likely high in Carotenes. Foetida has a strong “fragrance” and a deeper saturated blossom color. Hugonis is described as mild and has a much paler lemon color blossom . Foetida MAY possess the “wafting secondary enzyme reaction allele” as part of it’s unpleasant fragrance reputation. Hugonis probably does not possess this allele and besides it’s pale lemon color MIGHT indicate that it does not possess the rich carotene saturation that would result in “wafting” aroma. How problematic is this statement for you?

Thank you so so much for helping me come to an understanding! I will digest all of this in segments in time.

Here are a few examples to show how much yellow color is “hiding” in some fragrant white roses.
Rosa banksiae banksiae

Rosa banksiae lutea

Smith’s Yellow Noisette (Bred from Blush Noisette and Yellow Tea-scented)

Yellow Tea-scented

Blush Noisette

Glenn Dale (R. wichuraiana x Isabella Sprunt)

Rosa wichuraiana (fragrant white)

Isabella Sprunt (on right)

François Foucard (R. wichuraiana x l’Idéal)

l’Idéal

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