快猫短视频

A ROSE by any other name would smell as sweet-unless it came from the flower shop on the corner, that is. Plunge your nose into the petals of those dozen red long-stems from your sweetheart this Valentine鈥檚 Day, and experience all the intoxicating aroma of a bowl of salad. Sadly, Shakespeare鈥檚 perfumed symbols of love are not what they used to be.

It鈥檚 not just Valentine鈥檚 bouquets that have lost their bouquet. Visit your florist and check out the whole array of cut flowers. If you are lucky, you will find a lavender rose such as sterling silver or lavande. Their perfume is wonderful. But most of the rest-red, yellow and white roses, petunias, tulips and carnations-might as well be made of silk. Where has all the scent gone?

No one set out to create an odourless flower. Instead, scent slipped away by accident while flower breeders crafted brightly coloured, long-lasting flowers to satisfy the cut-flower market. But now two molecular biologists are trying to recoup the loss. Natalia Dudareva of Purdue University in West Lafayette, Indiana, and Elan Pichersky of the University of Michigan in Ann Arbor are tracking down the genes flowers use to make scent.

By restoring those genes to scentless commercial varieties, they hope to revive their dormant fragrance or even create new ones. Flower companies are already taking an interest, and at least one has doctored petunias with genes for citrus and pine odours. If we鈥檙e lucky, a dazzling array of fragrant flowers may greet us at the florist of the future. 鈥淧eople like scent. It makes them feel good,鈥� says Dudareva. 鈥淎nd if we can understand how scent works in plants, we should be able to put it back into flowers that have lost it.鈥�

Perfumers have been isolating scent compounds from flowers for centuries, and more recently biochemists have worked out many of the molecular pathways by which they form. Many scent compounds probably started out as plant defences. For example, compounds called terpenes that give juniper, oregano and basil foliage their characteristic odours drive herbivores away from the stems of some plants but attract pollinators to the flowers of others. Other terpenes that are antibacterial agents for trees also turn up in flowers-for example, piney pinene in columbine and citrusy limonene in lavender.

Many scents lie just a few biochemical steps away from other important molecules in the plant. Clove and cinnamon oils-chemical names eugenol and methyl cinnamate-form part of the scent of carnations and are also just one chemical reaction away from the precursor of lignin, the main component of wood. In nearly all plants salicylic acid turns on cellular defences against viruses. Add a methyl group to it and you get oil of wintergreen, part of the fragrance of jasmine.

But until recently, no one had looked at the genes underlying these biochemical links. So we didn鈥檛 know for certain what had gone wrong in flowers that had lost their scent. Pichersky suspected that breeders might have accidentally damaged the genes encoding the enzymes that produce scent compounds from their more common precursors. If during breeding for, say, stem length, a damaged version of the gene for a scent enzyme turned up in the flower with the longest stem, the breeder would select that flower and cross it to itself to make a pure variety. As a result the damaged scent gene would inadvertently be preserved.

This is plausible because scent doesn鈥檛 matter much to flower breeders. John Dolan, a long-time rose breeder and consultant in California鈥檚 Central Valley near San Francisco, says breeders would like their roses to smell nice, but economics get in the way. 鈥淲e have 26 different characteristics to consider in making a rose,鈥� he told 快猫短视频 while overseeing a harvest for shipment. 鈥淩oses per bush, vase life, colour, form, thorns and so on. In the marketplace, all these things trump scent.鈥�

In fact, fragrance and long vase life seem to be incompatible goals, says Dolan. The scentless vendela rose variety, for example, keeps more than a week once you get it home, but the fragrant lavande lasts only half as long. 鈥淧eople like a good fragrance,鈥� he says, 鈥渂ut when they have to choose between vase life and fragrance, they take vase life every time.鈥�

That suggests the demise of scent was more than mere accident. Breeders may actively, though indirectly, select against scented flowers. Since pigments such as red or blue anthocyanins branch off the same pathway as scents like phenyl ethanol, the main fragrance in roses, genetic changes might favour the pigment pathway at the expense of scents. For example, breeding for redder roses might select for regulatory genes that trigger the pigment-making genes instead of the scent-making ones. Alternatively, flowers bred to last longer might steal the metabolic energy they need to stay fresh from non-essential pathways like fragrance production. 鈥淢aking scent is energetically very costly,鈥� says Dudareva. 鈥淪o maybe it鈥檚 what flowers lose first when you breed them for other traits.鈥�

With these possibilities in mind, Pichersky set out six years ago to find the genes responsible for scents. His first target was the gene for an enzyme that makes a scent compound called linalool, a key component of oil of bergamot found in Earl Grey tea and one of the major scent compounds produced by the California wildflower Clarkia breweri.

The team was familiar with the reaction that makes linalool from its unscented precursor geranyl pyrophosphate (GPP). The trick was finding the enzyme that did it. Pichersky first passed extracts of Clarkia petals over columns of resin to separate the proteins into groups according to characteristics like size and molecular charge. He tested each group until he found the one that catalysed the linalool reaction, and then purified it further until just one protein remained, which he called linalool synthase. Then with the help of standard molecular biology techniques he fished out the corresponding linalool synthase gene.

In 1995, Dudareva joined Pichersky鈥檚 lab as a graduate student. After two years and a great deal of hard work, Dudareva and several other graduate students pulled out three more genes for scent compounds in Clarkia.

When Dudareva set up her own lab at Purdue in the fall of 1997, she set her sights on snapdragon genes. 鈥淲e chose snapdragon partly because its scent is not very complicated,鈥� she says. 鈥淥rchids, for example, produce one hundred different components, so the activity of each enzyme would be very low. But in snapdragon scent there are only three major components, so the enzymes would be more abundant.鈥�

Just one year after she started, Dudareva鈥檚 lab got the first scent gene out of snapdragon. The enzyme it encodes, called S-adenosyl-l-methionine:benzoic acid carboxyl methyltransferase (BAMT), converts the relatively common compound benzoic acid to methyl benzoate, a sweet-smelling compound. She has also nearly isolated the second major enzyme, trans-b-ocimene synthase and hopes to have the third, myrcene synthase, soon. The compounds these enzymes make add spicy notes reminiscent of pepper and cardamom to the snapdragon fragrance. Once both these enzymes are isolated, the genes should follow quickly.

Already, Dudareva has seen how 鈥渂roken鈥� genes can lead to loss of scent. Using gas chromatography, she found several breeds of snapdragon that give off little or no methyl benzoate. In one of these, Potomac pink, the BAMT gene contains no fewer than 10 mutations. When she tried out the mutated protein in the test tube, she found it was completely dead.

Luckily Potomac pink still has a nice, if subdued, fragrance from its two other odorants. That鈥檚 because even a few scent genes can produce a lot of variety in the final aroma. Dudareva now grows 37 snapdragon varieties in her greenhouse, all ordered from a seed company like any home gardener. Plants bred for flower gardens are often more fragrant than cut flowers, and each one, she says, has a distinct fragrance; no two are quite the same. Some are slightly sweeter, some more spicy and some more flowery. Yet each uses the same three scent components.

To find out how snapdragons contrive such variety from so few raw materials, Dudareva collected the perfumed air, or headspace, around each of her snapdragons, and separated the different volatile components by gas chromatography. From the size of each peak in the readout, she could tell the relative amounts of each component. Each unique scent was mainly achieved by jiggling the proportions of the same three compounds, though other minor compounds may play some role.

As any perfumer knows, concentration makes all the difference. A good whiff of the nitrogenous compound indol, for example, is reminiscent of sewage. But at very low concentrations, indol smells like flowers. In fact it is a key component of jasmine.

鈥淓ven things you would normally think horrible-vomit odours, faeces odours-sometimes smell OK at low concentrations,鈥� says Charles Wysocki, a scent researcher at the Monell Chemical Senses Center in Philadelphia. Most scents stimulate several receptors in the nose but generally bind to one more tightly than the others. So when concentrations are low, there isn鈥檛 enough around to stick to the low-affinity receptors, only to the favoured receptor. As a result, the entire profile of neurons stimulated in the olfactory bulb of the brain changes.

This is why older flowers smell different from fresh blooms-the scent profile changes over the flower鈥檚 lifespan. The four main scent compounds of Clarkia are present throughout the flower鈥檚 six-day life, but the proportions change dramatically. In the first three days, linalool and methyl salicylate predominate, but then eugenol and benzyl acetate take over. The dying flower smells different again as each scent fades at a different rate.

Dudareva鈥檚 next challenge is to try to give snapdragons a brand new scent. Snapdragons do not make linalool, but its precursor, GPP, is abundant because it is also the precursor to myrcene and trans--ocimene, which they do make. So if she can get the linalool synthase gene into snapdragons, there should be plenty of raw material for it to work on.

But it鈥檚 by no means easy to get the new gene to work. A gene from Clarkia will not necessarily work in snapdragons, because the molecular switches that activate genes differ from one species to another. So Dudareva is making a hybrid gene that includes the regulatory switches from the snapdragon BAMT gene and the protein-coding region from the linalool synthase gene. If it works, the snapdragon will smell like no other snapdragon has ever smelled before. It may have a hint of oil of bergamot, but what exactly it will smell like is impossible to say.

Of course, the big money in this technology is not in snapdragons but in roses, which fetch one of the highest prices per flower on the market. Dudareva and Pichersky are already collaborating with several companies on pilot projects to put linalool synthase into roses, although the companies don鈥檛 want to be identified. So far the results have been disappointing. The modified roses showed a faint hint of linalool on the gas chromatograph, but it was nothing anyone could smell.

The problem, says Dudareva, is likely to be a lack of GPP, the enzyme鈥檚 precursor, or substrate, in rose petals. 鈥淚t鈥檚 not just put it in and you will get it,鈥� she says. 鈥淵ou need to have substrate in the right place, and the right time and the right concentration.鈥� In fact, Dudareva has found that snapdragons stop making methyl benzoate as they get older even though there is plenty of enzyme around. The reason, it turns out, is that the precursor benzoic acid gets used up in the petals.

If making more precursor is part of the answer, a Philadelphia company called Novaflora is poised to reap the benefits. They hold an exclusive licence to use the GPP synthase gene, cloned last summer by Rodney Croteau鈥檚 lab at Washington State University in Pullman. GPP is the precursor not just to linalool but to an entire class of scent compounds called monoterpenes.

Although Novaflora鈥檚 ultimate goal is to give roses back their scent, the company鈥檚 president, Michael Dobres, says he is starting with petunias and geraniums, for which routine laboratory techniques already exist. The company鈥檚 researchers have begun inserting GPP synthase and monoterpene synthase genes into these plants, he says. But since these genes normally work only in the stems and leaves, Novaflora must now make adjustments in the gene sequences so they鈥檒l be more active in the flowers. The region of each gene in question, called the promoter, normally lies just in front of the part that codes for the protein. The promoter鈥檚 sequence, which can be quite long and complex, determines when, where and how strongly the gene is expressed. 鈥淓ngineering fragrance is not the easiest thing in the world,鈥� Dobres says. 鈥淲e know we can get the genes in, but the question is whether we can get the enzymes at a high enough level.鈥�

Because Novaflora is tinkering with genes not just for floral scent compounds but for other fragrant monoterpenes as well, they may end up with some rather different smelling flowers. Imagine a citrus-scented rose, for example. But then, why not, asks Robert Raguso, who studies the evolution of floral scent at the University of South Carolina in Columbia. It offers the customer something new. 鈥淚magine starting with a flower that鈥檚 really beautiful but odourless, like cosmos, and making them smell fabulous,鈥� he says. 鈥淧eople would buy them up.鈥�

Nor would Raguso stop with monoterpenes. What about lactones, he suggests, the main aromas in peaches, pears and nuts? There no reason in principle why the enzymes for these compounds couldn鈥檛 be made to work in petals. After all, that鈥檚 how floral scent evolved in the first place, by borrowing fragrances from other parts of the plant.

Meanwhile, Dudareva and Pichersky are confident they will be able to restore natural scent to flowers that have lost it. 鈥淭he first flowers that are made will be somewhat primitive, but it鈥檚 possible to restore full aroma,鈥� says Pichersky. Maybe in 30 years, when lovers buy red roses for their Valentines, the florist will ask, 鈥淲ill that be with natural aroma, pineapple or coconut?鈥�