July 11, 2000 WEST LAFAYETTE, Ind. – To Juliet, a rose by any other name would smell as sweet, but today the love-struck teen would be hard pressed to find much scent at all.
When it comes to fragrance, today's roses and flowers are practically lifeless compared to the blooms Shakespeare was familiar with. Unfortunately for gardeners, modern plant breeding has caused many of the most popular flowers to lose much of the scent of their ancestors. As plants have been bred to maximize color, shape, and other characteristics, such as long-lasting blooms after cutting, the scents have mysteriously disappeared.
Now, Natalia Dudareva, assistant professor of reproductive biology in the Department of Horticulture at Purdue University, has found new insights into the biology of floral scents – insights that might result in sweeter smelling roses, plus a bouquet of other benefits.
Improving floral scent is a goal of the $20 billion per year horticulture industry, but it is also important to agriculture. Almost three-fourths of all crops depend on insect pollinators attracted by floral scents. Honey bees alone are responsible for pollinating one third of U.S. crops.
Boosting floral scents would not only make flower beds more aesthetically pleasing, it would also improve the yield and quality of many crops.
Plants use floral scents to attract pollinators or to repel harmful insects. Floral scents begin as oils that are produced by the petals in most plants. Because these oils evaporate easily in warm weather, scientists call them volatile compounds. The aroma of a flower may contain as few as seven to ten different oils, as in snapdragon or petunia, or as many as 100 different chemicals, as is the case with orchids.
Dudareva has been studying floral scents in snapdragon, which she selected because it is one of the few modern plants that have a strong floral scent. "Most scientific work has been done on a Californian plant called Clarkia breweri, but I was wanting to use another plant model to know if similar mechanisms were found in other plant species."
Dudareva and colleagues found that both the moth-pollinated Clarkia and the bee-pollinated snapdragon produce floral scents using the same biochemical pathway. They also discovered that the base material, or substrate, used by snapdragon to produce the fragrant oils is regulated by a separate gene.
"This suggests that there may be basic genetic regulatory mechanisms responsible for floral scent production in different plant species," Dudareva says. "This is important for applied research. Now we know that floral scent production is regulated in a similar way in different plants and that the substrate is involved in the regulation of scent production. But it will also make the engineering of transgenic plants more complicated because before you genetically modify a plant you have to check to see if the substrate is produced in the petals."
Dudareva says the lack of substrate may be why previous attempts to genetically engineer plants to produce more scent have failed.
These new insights may allow scientists to genetically engineer plants to produce stronger scents, or even to produce their scents during certain times of the day.
"The floral industry is interested in creating flowers that are fragrant in the evening, when people come home from work," Dudareva says. "Petunia is one flower that releases its scent in the evening, so we should be able to do this. But before we can make flowers more fragrant in the evening, we need to increase the scent overall." Dudareva says most people prefer moth-pollinated flowers, which they consider "sweet-smelling," but many moth-pollinated flowers produce scents only at night, and not earlier in the evening.
A scientific paper explaining the biochemistry and genetics of floral scent production in snapdragon is published in the June issue of the journal Plant Cell. The research was funded by the National Science Foundation and by grants from the Fred Gloeckner Foundation, Inc.
Dudareva's research also has found that snapdragons have evolved into a close relationship with the bumblebee. Snapdragons release four times more scent during the day, when the bees are active, than at night. In fact, 24-hour graphs of scent production by snapdragons and bee activity show bell-shaped curves that line up almost exactly on top of one another.
Snapdragon flowers are tubes made up of five petals. The floral scents are released only on the lobes of the petals, and not in the tube. To reach the nectar, a bee must land on the lower lobe of the flower and insert himself partially into the tube. As he does this, the back of the bee brushes against the upper lobe of the snapdragon flower. When a bee does this, scent molecules are deposited on his back and legs, and he carries the scent back to the hive, attracting more bees to the plant.
In snapdragon, as in most flowering plants, newly opened blossoms don't produce as much scent as mature blossoms. "These flowers aren't ready to be pollinated because they are immature, and the lack of scent makes them less attractive to pollinators," Dudareva says.
Conversely, once many plants have been pollinated, they decrease the amount of scent and even the quality of the scent itself. However, some plants benefit from repeated visits from pollinators. Many fruits must be pollinated multiple times to produce fruit, and the number of pollinations helps determine the size and quality of the fruit. "For watermelon it takes about 12 times to have quality fruit, and it takes 25 pollinations for strawberries to maximize berry size," Dudareva says.
Improving the floral scents of fruits trees and plants would be a benefit for fruit farmers, Dudareva says."Plants didn't evolve to produce their scents for the benefit of humans," Dudareva adds, "but floral scents sometimes influenced the decisions humans made about which plants to cultivate."
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