Courtesy of Suntory Ltd.
More than $27 billion worth of cut flowers are sold in the global marketplace every year. Carnations and chrysanthemums are perennial favorites, but roses lead the way in total revenue. In a business driven by novelty, it's little wonder that molecular geneticists have been tinkering with the genes that give the best-selling rose its color.
Scientists at the Australian biotechnology company Florigene and Japanese partner Suntory recently announced the creation of a genetically engineered "blue" rose. Although blue in name only – the new flower is actually mauve – the efforts of the Florigene and Suntory researchers represent a crucial step in the centuries-long pursuit of a truly blue rose.
Traditional hybridization techniques failed to produce a blue rose because the gene for blueness – an enzyme in the biosynthetic pathway of the blue pigment delphinidin – isn't present in the rose genome. Simply introducing the delphinidin gene into roses isn't enough: Florigene scientists did that in the mid-1990s, and what they got was a dark burgundy rose. Pretty, but not blue.
The problem, according to John Mason, Florigene's manager of research and development, is that endogenous genes compete with delphinidin. What was needed was a way to silence the competition. Enter RNA interference (RNAi).
In the case of the blue rose, the enzyme that needed silencing was the rose's version of dihydroflavonol reductase (DFR), which many plants use to synthesize the major pigments involved in flower color, including delphinidin. The DFR in roses is especially good at converting the enzymatic precursors into orange and red pigments, but not as good at making delphinidin.
The Florigene and Suntory scientists realized they could knock out the rose DFR gene using RNAi and so remove the competing pigments. Thus, the current transgenic blue rose eventually involved a three-gene package: a synthetic RNAi gene that switched off the rose DFR, a delphinidin gene from blue pansy, and a DFR gene from iris that had an affinity for producing delphinidin. "We've now got a rose with essentially 98% delphinidin," says Mason.
So why isn't the new rose blue like the pansy? In a word, pH. According to Mason, "The acidity of the vacuoles in the epidermal layer of the petals, where most of the pigments are localized, is critical to the flower's color. If you were to make a crude extract of rose petals and raise the pH, they go blue." The difficulty, he says, is engineering a pH change in the petal, as the pH of the vacuole is a byproduct of the pH in the cell cytoplasm. "You can't perturb it very much without having a deleterious effect," says Mason.
Some scientists are considering a new approach, one that involves a completely different pigment that might not be so dependent on the petal's pH. Elizabeth Gillam of the University of Queensland, Australia, discovered that indole-containing bacteria turned blue when transfected with the gene for the human liver enzyme cytochrome P450.1
Gillam had hoped to explore the possibility that roses (which contain indole-related compounds) might also turn blue when P450 enzymes were introduced, but that research has met yet another stumbling block. "To create a genetically modified plant you have to go through a long and involved process in Australia," she says. "We're still working through the paperwork to get approval."
The Florigene and Suntory scientists haven't given up. They are also considering different approaches, including other bluing factors, in their continuing quest for a true blue rose. The current "blue" rose may reach the marketplace by 2007, according to Mason, who adds that it will be the first commercial ornamental plant produced by RNAi technology.