Making scents of flowers: it's time for science to close its eyes and sniff - Brief Article

Science News, July 27, 2002 by Susan Milius

One fragrance outperformed all the rest, luring lots of midges and stingless bees. This winning concoction came from the variety called Rim-100, selected from Mexican plants that still look much like cacao's wild ancestors. The researchers speculated that over the years, cacao breeders had maximized some desirable properties of the crop but lost much of the scent. Such feeble fragrance, the researchers suggest, might explain why many plantation varieties attract few insect visitors.

Scent scientists could do farming a great service if they could put some aromas back into crops, says Natalia Dudareva of Purdue University in West Lafayette, Ind. Three-quarters of crops depend on insect pollinators, and the number of their visits influences the size of the fruit. Dudareva muses that better-smelling plants could score more pollination and produce better yields. A watermelon flower needs about a dozen visits from pollen-carrying insects to develop a real whopper of a fruit, and a strawberry needs some 25 visits to reach prime berry size.

While being bred for other characteristics, many ornamental flowers have lost their scent. After all, some modern roses smell about as exciting as iceberg lettuce. Dudareva says that people would prefer to buy fragrant flowers.

Geneticists are beginning to tease out the basic mechanisms that give flowers their scents. Eran Pichersky of the University of Michigan in Ann Arbor and his colleagues reported the first floral scent gene in 1996. It came from a fragrant, Western wildflower, Clarkia breweri. The gene encodes an enzyme critical to making linalool, an alcohol that powers many strong flower odors, including lavender.

Since then, researchers have been finding other scent genes in Clarkia as well as in snapdragons, roses, and some other species. Pichersky estimates that the worldwide score for finding scent genes hangs somewhere around 10.

Finding the genes has been the easy part. Exploiting them to enhance floral aroma is much harder, says Dudareva.

Pichersky managed to get one of the Clarkia aroma genes to work in a tomato plant to produce small amounts of the scented compound linalool in the fruit--resulting in a faint, sweet, floral smell. Enhanced fragrance might increase the appeal of store-bought tomatoes, the researchers hoped.

A team in the Netherlands put the same Clarkia gene into a petunia. Unfortunately, other compounds in the flower bound to the linalool and made it too heavy to waft away.

Even more challenges await those who seek to transplant fragrances, says Dudareva. For example, snapdragons churn out most of their odor between 9 a.m. and 4 p.m., the standard workday for bees. Even plants growing in darkness maintain this rhythm. In 2000, Dudareva and her colleagues reported that the enzyme for a compound that gives the snapdragon fragrance its snap functions around the clock. However, the benzoic acid that the enzyme works on waxes and wanes, creating the daily rhythm. Putting the enzyme gene into a plant may not mean a thing if it doesn't have enough raw material.