The impossible flower
Blue roses are the stuff of poems. Scientists are trying to make them reality
KYOTO, JAPAN—In 2004, Japanese researchers unveiled what they billed as the world’s first blue rose. The only problem with the flower: It wasn’t very blue.
Although its petals did produce a blue pigment, the overall appearance of the flower was more mauve. Even Yoshikazu Tanaka, the scientist behind the work, admits that his first thought on seeing the flower was: “could be bluer.”
Fifteen years later, he is still trying to make that bluer rose. Tanaka works at the global research center of Japanese beverage giant Suntory, which grew out of Japan’s first whisky distillery, opened in 1923. (The brand was made famous by the movie Lost in Translation, in which an aging actor played by Bill Murray shoots a whisky commercial in Tokyo.) The company decided to branch out into the cut-flower market in the 1980s after a tax hike made Japanese liquor more expensive. Company legend has it that the idea was to paint the English rose the Scottish national color, blue, as a kind of thank you for the invention of whisky, Tanaka says.
More likely, it just seemed a good business idea. After all, blue flowers are rare, including among cut flowers. Chrysanthemums, coronations, tulips—none of them naturally comes in blue. Blue orchids have usually been artificially dyed. Decades of breeding have yielded roses in every shade of yellow, pink, and red, but never blue ones.
Artists have long noted this rarity. In German romanticism, the blue flower became a symbol of longing and the unattainable. Rudyard Kipling dedicated a poem to someone tasked by his lover to find her a blue rose: “Half the world I wandered through/Seeking where such flowers grew.”
By the time he returns empty-handed, his love has died.
Scientists got their first glimpse of the complexity behind blue flowers in 1913, when German researcher Richard Willstätter announced he had isolated the blue pigment from cornflowers. It was an anthocyanin he named cyanidin. Two years later, when he isolated the pigment of red roses, it turned out to be the exact same molecule. Anthocyanins can change color depending on the acidity of a solution, so Willstätter proposed that roses had a different hue because the pH in their petals was lower than in cornflowers.
It was the first scientific theory about blue flowers. And it was wrong. Over the following decades, a different story emerged, one that was finally confirmed by x-ray crystallography in 2005. Cyanidin itself does not produce a stable blue color; instead, cornflowers combine six molecules of cyanidin with six molecules of a colorless copigment arranged around two metal ions—a huge molecular complex that stabilizes the cyanidin molecules and allows one electron to make the right energy transition. “Flowers are doing crazy chemistry to generate that blue,” says Beverley Glover, a botanist at the University of Cambridge in the United Kingdom.
Several other blue flowers have hit on the same trick, but most produce a different anthocyanin, called delphinidin, that can more easily be coaxed to appear blue. The only difference between cyanidin and delphinidin is that the latter has an extra oxygen atom on one of its rings, put there by an enzyme called flavonoid 3′,5′-hydroxylase. The entire family of roses, which includes apples and pears, lacks the enzyme, which means that delphinidin-producing roses can’t be produced through traditional breeding.
Tanaka is trying genetic engineering instead. By 1991, he and his colleagues had identified and patented the flavonoid 3′,5′-hydroxylase gene in petunias. Transferring that gene into carnations coaxed them into producing delphinidin, turning them a purplish blue. But when the team shuttled the gene into roses, using the bacterium Agrobacterium tumefaciens as a courier, they didn’t start to produce the blue pigment for some reason. It was the same gene from pansies that finally led to the delphinidinmaking—but not very blue—rose unveiled in 2004. Apparently, producing delphinidin alone wasn’t enough. Scientists had to do some crazy chemistry themselves.
Since then, Tanaka’s main strategy has been to transfer genes from bellflowers, pansies, and other blue flowers to “decorate” delphinidin chemically, hoping to hit a magic combination. Last year, he showed a visitor hundreds of tiny rose plants growing under fluorescent lights in his lab. “All of them are just to get a new blue color,” he said.
In the meantime, however, a collaboration between Tanaka and a group led by Naonobu Noda at the Institute of Vegetable and Floriculture Science in Tsukuba, Japan, has led to an indisputably blue flower: a blue chrysanthemum. In a 2017 Science Advances paper, the researchers reported that inserting the flavonoid 3′,5′-hydroxylase gene from bellflowers into red chrysanthemums, along with a gene that adds a glucose molecule, resulted in “the most blueshifted flowers” ever genetically engineered. Their idea was that the glucose would allow the flower’s natural enzymes to attach further chemical groups to delphinidin, creating a stronger blue. To their surprise, added groups weren’t necessary; instead, the glucose helped delphinidin assemble with copigments naturally produced in the flower, shifting the color to blue.
Using the exact same strategy has not worked in roses, probably because they don’t have the same copigments and have a lower pH. But Tanaka has not given up. He has tried to add genes from gentian that modify the delphinidin and genes from the genus Torenia that produce copigments. In a nod to Willstätter, he is even trying to change the pH in the rose petals.
Tanaka is confident he will develop bluer roses before his retirement, only 5 years away, but almost 30 years of pursuing his quest have also taught him to be cautious: “It is hard to say how blue they will be.”
Source: Science Mag