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The Chemistry of Color

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Source: Science

Genetic researchers have studied plant pigments for a variety of reasons in recent years, perhaps chief among them proven health benefits to humans. The carotenoids, for instance, increase levels of vitamin A; together with anthocyanins and other so-called flavonoids, they are also thought to provide some degree of protection against cancer and heart disease. But the ornamental value of chemical pigments, too, has stirred considerable interest. Scientists have worked hard to find ways to make violets red and roses blue. And in last Friday's issue of Science, biomolecular engineers from Tohoku University in Japan described for the first time the enzyme responsible for making some flowers--such as snapdragons and cosmos--bright yellow.

Toru Nakayama and his colleagues set out to identify the enzyme that turns chemicals called chalcones into aurones, flavonoids found in the yellow flowers of many popular plants. They began with 32 kilograms of snapdragon buds, honing in on and purifying a 39-kilodalton, copper-containing glycoprotein, which they named aureusidin synthesase. Additional work revealed that the complementary DNA encoding this enzyme is expressed in the petals of aurone-containing plants, and in greater amounts in yellow varieties than in pink varieties. DNA sequence analysis also showed that aureusidin synthesase belongs to the family of plant polyphenol oxidases (PPOs), which occur in all higher plants and cause them to turn brown when exposed to air. "The physiological function of PPO in plants remains to be established," the authors write. But "this report demonstrates the participation of a PPO homolog in flower coloration."

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