Gene controls cell shape, floral brilliance.

Snapdragons come in many hues, providing colorful accents for summer gardens. It stands to reason that a pale pink variety simply fills its petals with less pigment than its darker counterparts and that certain genes guide the amount of pigment produced.

But that isn't the case in at least one snapdragon, says Cathie Martin, a botanist at the John Innes Institute in Norwich, England. She and her colleagues have discovered that a single gene can ruin a floral finish, but not in the way they had expected. When snapdragons inherit a gene called mixta, they look pale in comparison to those that don't have this gene.

The mixta gene looks like other genes in other plants that researchers have studied. Those genes regulate pigment production in corn, for example, by controlling the activity of genes that code for proteins involved in such production. But the researchers detected no difference in the amounts of pigment in snapdragons with and without mixta, they report in the June 23 NATURE.

Other scientists have demonstrated that the intensity of some floral hues can depend upon the shape of the cells containing the coloring pigment. Cone-shaped surface cells focus more light onto pigments, enhancing their vividness; flat cells lead to duller, paler flowers.

Instead of controlling genes involved in pigment production, mixta guides genes that help control the amount of cell wall produced along the outer edge of cells on the petal surface. The thicker, more corrugated this surface, the more intense the coloration.

Cells with mixta fail to develop these thick walls and pointed geometries. When the researchers remove cell walls, the color differences between mixta and nonmixta petals disappear, Martin says. Never before have scientists pinpointed a single gene that could shape cells like this, she adds.

"[Mixta]'s acting very similarly, but it's doing very different things," says Martin. "Yet it's having a similar effect."

The researchers succeeded in tracking down the mixta gene because when they bred mixta snapdragons with wild snapdragons, they first obtained petals with both flat and pointed cells that corresponded to dull and bright coloration. This indicated that a piece of DNA was jumping in and out of the gene, causing cell-to-cell variation. By comparing DNA between bright and dull regions, they pinpointed this piece, called a transposable element, then used it to find the mixta gene itself.

Once geneticists understand better how conical cells develop, they may be able to use this gene or its relatives to enhance the colors of flowers that do not make pointed cells or to make plants more shade-tolerant by creating leaves with conical cells that gather light more efficiently, Martin suggests.