Save the Flowers
Would-be scent engineers aim to resurrect lost floral fragrance
Ivan Amato
Vince Agnes, as well-appointed as the flowers that he has been selling for more than 60 years in his shop in Silver Spring, Md., remembers when all his roses smelled as good as they looked. When he opened for business in the 1940s, there were only a few varieties: red, white, yellow, and pink, he recalls. "Now, there are thousands," Agnes says, " but only a few have a lot of scent."
No one knows what's responsible for this waning of fragrance by roses and other ornamental-flower varieties, including carnations and chrysanthemums, but scientists who investigate floral scent suspect that the flower breeding that's led to an estimated 18,000 rose cultivars in an ever-widening spectrum has run roughshod over fragrance.
"Pigment compounds are derived from the same biochemical precursors [as scent compounds are], so it makes sense that if you make more of one you get less of the other," notes floral-scent biochemist and geneticist Eran Pichersky of the University of Michigan in Ann Arbor.
Floral scent may be dwindling because breeders for the $30 billion ornamental-flower industry pay scant attention to this most emblematic attribute of flowers. "In order of [commercial] priority, color is number 1 through 10," says Alan Blowers, head of flower biotechnology for Ball Helix, a biotech company in West Chicago, Ill., devoted to the ornamental-plant industry. Beyond color, breeders have been targeting improvements in flower longevity, shape, size, disease resistance, and other traits likely to improve the growers' bottom lines.
Fragrance is different. It's invisible, and its sensory impression is as subjective as taste. And, as it turns out, fragrance is a genetically complex trait that's difficult to manipulate by ordinary breeding methods. Despite those obstacles, Blowers predicts, "fragrance will become important again," as the molecular biology underlying floral scent becomes better understood.
With a nose both for understanding the molecular origins of floral scents and for engineering what could be blockbuster flower varieties, researchers have been teasing out the complex biochemical orchestration underlying one of life's simplest pleasures. They've been uncovering fragrance-related genes, the enzymes encoded by those genes, the in-cell reactions that these enzymes catalyze, and the fragrant performance of all this molecular biologya vast aromatic harmony of alcohols, aldehydes, fatty acids, terpenoids, benzenoids, and other volatile, and therefore sniffable, chemicals.
In the past few years, flower scientists have assembled enough knowledge and technology to consider resurrecting scents in flowers that have lost them or engineering plants that produce scents never before experienced by a bee, beetle, or gardener.
The researchers "have pushed the envelope in terms of our eventual ability to change floral scent," says Michael Dobres, head of the Philadelphia biotech company NovaFlora, which is developing genetic methods for controlling various traits of ornamental flowers.
Deconstructing scent
The plant world perfumes, or sometimes stinks up, the environment with a vast roster of volatile organic chemicals. Scientists have so far identified about 1,000 of these compounds emanating from petals, leaves, and other tissues.
In their initial work on engineering scents back into plants, scientists have enlisted the petunia (above), snapdragon (below), and a wildflower called Clarkia breweri (bottom).
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"There could be up to 50, maybe 100, chemicals involved in a particular scent," says Pichersky.
Usually, only a few of the volatile chemicals in a fragrance are obviously noticeable to human noses. One whiff of 2-phenylethanol, for instance, and images of roses come to mind, even though scores of volatile chemicals contribute to the fully detailed scent of roses. Like harmonics that help the ear distinguish a middle C played on a piano from one played on a violin, the minor chemical components of a scent provide the olfactory subtleties that individualize the scent of a particular rose variety.
Pichersky, who grew up on a kibbutz growing flowers and other crops in his native Israel and now lives on a 30-acre farm outside Ann Arbor, has been gardening all his life. He has made it his mission to uncover as much as he can about the biosynthesis of floral scents and the biological roles that these scents play. In 1996, he and his colleagues in Michigan were the first to discover a gene that produces a floral scent.
Not only do the volatiles in botanical scents attract pollinators and delight the human nose, he notes, but they also serve to protect plants from pathogens and pests. For example, when some plants come under attack by munching caterpillars, they emit specific chemicals as clarion calls for parasitic wasps. The wasps alight on the marauding caterpillars and lay eggs, which hatch into larvae that eat the caterpillars alive. "It's a chemical arms race out there," says Pichersky.
As the first step in analyzing the complex biochemical choreography behind a floral scent, Pichersky and his coworkers in 1994 worked out the amino acid sequence of the enzyme linalool synthase from petals of Clarkia breweri, a purple wildflower native to California. They then used that information to identify the enzyme's gene.
Through painstaking biochemical analysis, the researchers discovered that this enzyme converts the substrate geranyl pyrophosphate into linalool, a volatile compound with what Pichersky describes as a "wine-sweet" smell. Geranyl pyrophosphate was already known as an intermediate in the metabolic pathway that produces cholesterol compounds.
Since then, Pichersky's group and others have uncovered about 25 more floral-scent genes. Natalia Dudareva, a former postdoc student of Pichersky who now runs her own floral-scent laboratory at Purdue University in West Lafayette, Ind., estimates that the present list of known scent genes and their associated enzymes can account for the cellular synthesis of no more than 5 percent of the plant volatiles that scientists have identified.... [last part of article omitted]
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