Field of Science


How Stress Makes Oranges Better for You

Though Sicily may seem like a relaxing oasis, it's really a stressful climate where rogue elements can turn you bloody--whether you have a run-in with the mafia, or you're an orange. New research shows why the Italian blood orange prefers this hostile environment to your backyard. With a little coercion, though, we might someday convince this extra-healthy fruit to move abroad.

A variety of the sweet orange Citrus sinensis, the blood orange has eerie red flesh and is grown most successfully around Sicily. To develop their trademark color, the oranges need to ripen in a climate where the nights are much colder than the days. They're cultivated in a few other places outside of Italy, and their color can be enhanced by storing them in the cold after picking them. But in general, the blood orange is an inflexible character.

The blood orange's pickiness makes it hard to mass produce and get onto grocery store shelves. A team of researchers from the United Kingdom, Italy, and China (where another variety of blood orange grows) set out to find what makes the orange so finicky.

Pigments called anthocyanins give the fruit its gory look. These same pigments are responsible for the deep purplish hues of blueberries, eggplant peels, and Japanese maples. Combing through the blood orange's DNA, the researchers found a gene they named Ruby that turns on the fruit's anthocyanin machinery.

When Ruby is activated, the plant makes anthocyanin and the orange turns red. To demonstrate this, the researchers snuck the Ruby gene into a tobacco plant. Tobacco leaves normally make anthocyanin only in small amounts. But with Ruby added to their genes, tobacco plants cranked up the pigment's production and sprouted reddish leaves.

If Ruby is the foreman who hits the ON button at the anthocyanin factory, he apparently needs a cold snap to get him out of bed--because without cold nighttime temperatures, blood oranges don't turn bloody. The researchers discovered that the element waking Ruby up is a kind of rogue gene called a transposon.

Also called "jumping genes," transposons are chunks of DNA that can hop around a genome and insert themselves wherever they like. At some point in the blood orange's evolution, a tranposon stuck itself right in front of Ruby and became the on-switch for the on-switch.

Ordinarily, the plant suppresses the transposon. It's not a good idea, after all, to let wandering genes start bossing around the rest of your DNA. But transposons often get turned on when plants are stressed. Scientists think this may be a desperate trick plants evolved to use when times are tough: The normal order of business isn't working, so plants set their rogue genes free to see if they have any useful innovations. When blood orange trees are stressed by cold temperatures, they release their hold on the transposon in front of Ruby. The transposon wakes up the factory foreman, and you know the rest.

Now that we've found the secret to making blood oranges bloody, senior author Cathie Martin says genetic engineers could create a new variety that doesn't need the cold at all. Scientists could tweak the orange's genome so that Ruby is active all the time, keeping the pigment factory going in any temperature.

Imagining a job for a task force of tree psychologists, I asked Martin if we could grow unmodified blood orange trees in warm climates and just stress them out some other way. But she said that probably wouldn't work. You also can't create a blood orange by chilling regular "blonde" oranges or orange trees--this particular team of rogue gene and factory foreman is specific to this variety of Citrus sinensis.

Of course, you could always just stick to the fruits that grow easily in your climate. But Martin says blood oranges are even better for us than regular oranges. "There are many examples of...dietary anthocyanins having a beneficial effect on health," she says, "especially for cardiovascular disease and obesity." In mice, blood orange juice (but not regular orange juice) limits weight gain and prevents obesity.

If these pigments are as healthful as they seem--and especially if climate change is going to make the tree's home turf less comfortable--maybe it's worth pursuing a way to get the blood orange out of Sicily.

Butelli, E., Licciardello, C., Zhang, Y., Liu, J., Mackay, S., Bailey, P., Reforgiato-Recupero, G., & Martin, C. (2012). Retrotransposons Control Fruit-Specific, Cold-Dependent Accumulation of Anthocyanins in Blood Oranges THE PLANT CELL ONLINE DOI: 10.1105/tpc.111.095232

Image: ccharmon/Flickr


  1. You seem to be using Citrus sinensis as meaning the blood orange. Citrus sinensis is the species name of the sweet orange with many varieties apart from the bloody ones - including navels, common and acidless.

  2. Thanks for the taxonomical help! I've fixed it.

  3. bloody oranges are common in many parts other than Sicily. in Spainthey are called Bull´s Blood oranges


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