Petals show their yellow colour

Alternate single and double bonds help give colour

Japanese scientists have revealed how some plants achieve a brilliant yellow colour.

The researchers have identified the enzyme and the possible biochemical reactions that produce the molecule responsible for yellowness.

It took 32 kilograms of snapdragon buds and many hours of laboratory work to make the discovery.

The new knowledge could now be used to engineer new types of ornamental flowers.

Energy levels

"Yellow varieties of some popular ornamental flowers, such as Petunia, Pelargonium, Eustoma, and Impatiens, are not known," Dr Toru Nakayama, from Tohoku University, told BBC News Online.

"If the gene of the 'yellow enzyme' is available, it would be a useful 'tool' to create yellow flowers of these plants by genetic engineering approaches."

Most floral colours arise from a group of molecules called flavonoids. In snapdragons, cosmos, and other flowers, yellowness comes from the aurone flavonoid.

Like the other flavonoids, it displays many of the molecular features that give colour, such as carbon rings and alternate single and double bonds.

These features create a unique environment that determines the energy levels available to electrons moving around the system and the precise wavelength (colour) of light emitted when the molecule is excited.

Laborious work

Dr Nakayama and his colleagues report in the journal Science that they have now discovered how the aurone flavonoid is synthesised in plant petals.

They have identified the key enzyme, named aureusidin synthase, which belongs to a family of enzymes whose function had previously been a mystery.

However, achieving the breakthrough proved quite a struggle. The enzyme occurs in very small quantities and isolating samples of sufficient size required 32 kg of buds from 16,000 stems of snapdragon plants.

"It was really laborious but essential to accomplish this work," Dr Nakayama said.

"We prepared a 'juice' from snapdragon buds from which we could finally isolate 90 micrograms (approximately 1/10000 gram) of pure enzyme from the 'bud extracts' by using the advanced techniques for protein purification."