carrots

Carrots (Photo by Nick Fewings on Unsplash)

RALEIGH, N.C. — Have you ever wondered why carrots are always orange? Researchers from North Carolina State University and the University of Wisconsin-Madison have cracked that question by unraveling the genetic mystery behind the orange hue of carrots. They discovered that for a carrot to be orange, three specific genes must be in a “recessive” or “turned off” state.

“Normally, to make some function, you need genes to be turned on,” says study co-corresponding author Massimo Iorizzo, an associate professor of horticultural science with North Carolina State University’s Plants for Human Health Institute, in a university release. “In the case of the orange carrot, the genes that regulate orange carotenoids – the precursor of vitamin A that have been shown to provide health benefits – need to be turned off.”

Massimo Iorizzo examines orange carrots to learn more about their pigmentation and domestication
Massimo Iorizzo examines orange carrots to learn more about their pigmentation and domestication. (Photo courtesy of Massimo Iorizzo)

Carotenoids are natural pigments responsible for the colors of many fruits and vegetables. In carrots, especially the orange variety, they are present in large amounts. These carotenoids are beneficial for our health as they can reduce the risk of diseases such as eye disease. In fact, orange carrots stand out as the top source of pro-vitamin A in the typical American diet.

The research journey didn’t start with this study. Back in 2016, these scientists unveiled the first carrot genome sequence, which provided insights into the yellow carrot’s coloration.

In their latest endeavor, the team analyzed 630 carrot genomes. They used a method called “selective sweeps,” which involves comparing the genetic data of different carrot groups to spot commonalities. A significant discovery was that many genes connected with flowering were “selected,” primarily to delay the process. When a carrot plant flowers, the taproot (the part we eat) becomes hard and inedible.

“We found many genes involved in flowering regulation that were selected in multiple populations in orange carrot, likely to adapt to different geographic regions,” notes Iorizzo.

The study also offered a historical perspective on the domestication of the carrot. Carrots were tamed around the 9th or 10th century in western and central Asia.

“Purple carrots were common in central Asia along with yellow carrots,” says Iorizzo. “Both were brought to Europe, but yellow carrots were more popular, likely due to their taste.”

White and yellow carrots are at the base of the orange carrot’s phylogenic tree
White and yellow carrots are at the base of the orange carrot’s phylogenic tree. (Photo courtesy of Massimo Iorizzo and Becky Kirkland, NC State University)

It’s believed that the orange carrot emerged in western Europe in the 15th or 16th century. They might have originated from a cross between white and yellow carrots.

“This study basically reconstructed the chronology of when carrot was domesticated and then orange carrot was selected,” explains Iorizzo. “Orange carrot could have resulted from white and yellow carrot crosses, as white and yellow carrots are at the base of the phylogenetic tree for the orange carrot.”

The appeal of the orange carrot grew because of its color and sweeter taste. Its rise in northern Europe during the 16th and 17th centuries aligns with its depiction in artworks from that time. The health benefits of carotenoids, precursors of vitamin A, further propelled the orange carrot’s popularity by the late 19th and early 20th centuries.

The study is published in the journal Nature Plants.

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