Scientists dig up carrot genome

An international team of researchers has successfully deciphered the entire carrot genome, uncovering its evolutionary origins along the way.

Led by University of Wisconsin-Madison’s Massimo Iorizzo, the genome, published in Nature Genetics, also reveals the genes behind the vegetable’s nutritional value and bright orange colour.

The authors believe carrots have existed for millions of years, but the first cultivated carrot crop only appeared around 1,100 years ago in Asia. Back then, carrots were yellow and purple. It wasn’t until the 1500s that orange carrots appeared on the scene, but they were more nutritious than their yellow and purple counterparts.

This is because yellow, orange and red fruits and vegetables contain carotenoids, such as alpha- and beta-carotene, which humans convert to vitamin A.

So why did carrots take on an orange hue?

Plants usually acquire energy through photosynthesis, which uses sunlight, water and carbon dioxide and pigments, such as carotenoids. But root-like plants, being underground, have no need for light, so the photosynthetic pigments such carotenoids are used elsewhere.

“The accumulation of orange pigments is an accumulation that wouldn’t normally happen,” says study senior author and University of Wisconsin-Madison geneticist Phil Simon. “It’s a repurposing of genes plants usually use when growing in light.”

To see what the genetic basis for the carrot’s colour was, along with clues to its evolutionary past, Simon and his team used a standard orange carrot (Daucus carota) as their subject. They identified more than 32,000 genes, around 10,000 of which are unique to the carrot, on nine chromosomes.

They then sequenced the genomes of 35 different carrot specimens, wild and cultivated, in an attempt to understand how carrots evolved into those we know today.

Eventually, they found the gene responsible for the high amounts of beta-carotene called DCAR_032551. It is responsible for the difference in colour and nutrition between white carrots and yellow or orange ones. So, the paler orange a carrot, the lower levels of carotenoids it contains.

Carotenoid levels have been on the rise for the past 40 years, thanks to agricultural techniques, with modern-day carrots containing as much as 50% more than their ancestors. Future crops will be even more nutritious and hardy, says Allen Van Deynze from the University of California, Davis, and part of the research team.

“The genomic information has already been made available to assist in improving carrot traits as enhanced levels of beta-carotene, drought tolerance and disease resistance.

“Going forward, the genome will serve as the basis for molecular breeding of the carrot.”