Roses are often red, petunias are sometimes blue (or, more accurately, blueish-purple) -- and now some scientists know why the latter is true. The key is actually a genetic defect.

While blue petals are prized by horticulturalists for their rarity in nature, plants have little reason to prefer the color. Red and yellow are much more attractive to pollinating birds and bugs, and white is advantageous for standing out at night for pollinators like bats and moths. Usually, if there’s a hint of blue in a plant, it’s combined with red to make purple.

A team of scientists led by researchers from VU University in the Netherlands thinks they’ve found the cellular secret behind a blue petunia. In a paper published in the journal Cell Reports on Jan. 2, the researchers point to a defect in a cellular pumping system that normally keeps the pH of special plant cell components called vacuoles more acidic. An out-of-order pump means that the vacuoles in a flower’s petals end up less acidic. Like a pH litmus test, a less acidic petal is a bluer petal.

The revelation came through genetic comparisons of normal, reddish orange petunias, and a mutant strain that can yield a lavender or bluish-violet color. But the answer the team found isn't exactly a novel one.

"Already in the 1910s it was proposed that blue flower colors were caused by reduced acidity of the 'cell sap,'” senior author and VU University biologist Francesca Quattrocchio said in a statement. “Others figured that drastic changes in the cell sap might cause terrible deleterious defects, and proposed that blue flower colors had something to do with the formation of metal-anthocyanin complexes. Our current opinion is that both got it right."

Quattrochio and her team discovered genes that encode proteins that form a previously unheard-of cellular pump in petunias, which keeps moving positively charged protons across the membrane of the vacuole.

The team thinks their discovery could lead the way to manipulating the colors of other flowers, especially if they have the same pump-encoding genes found here. Mutations in soybean genes similar to the ones identified here also result in blueish-purple flowers, the authors say.

Many plant breeders have spent generations crafting bluer strains of popular flowers. In 2010, Texas researchers bred a special blue variety of hibiscus. The new shade bloomed after four years of work and more than 1,000 plant crossings. Developing a blue rose is something of a holy grail in the plant world, but so far you may have to settle for white roses dyed azure. Japanese company Suntory created a lavender rose in 2004 by introducing a gene for the blue pigment delphinidin from a pansy to a white rose.

SOURCE: Faraco et al. “Hyperacidification of Vacuoles by the Combined Action of Two Different P-ATPases in the Tonoplast Determines Flower Color.” Cell Reports published online 2 January 2014.

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