Why are carrots orange? It seems like a simple question, but the answer lies deep within the genetic code of this popular vegetable. A groundbreaking new study delving into the genomes of over 600 carrot types has unearthed the precise genetic mechanisms that give orange carrots their signature hue. This research not only solves a long-standing mystery of plant pigmentation but also provides valuable insights into carrot domestication and the improvement of this health-boosting vegetable.
Published in Nature Plants, the study, led by researchers at North Carolina State University and the University of Wisconsin-Madison, reveals a surprising twist: the vibrant orange color is not created by genes turning on, but rather by three specific genes being turned off. These genes are responsible for regulating carotenoids, the pigments that not only give carrots their color but are also precursors to vitamin A, a vital nutrient with significant health benefits.
“Normally, to make some function, you need genes to be turned on,” explains Massimo Iorizzo, associate professor of horticultural science at NC State’s Plants for Human Health Institute and co-corresponding author of the study. “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.”
The Genetic Secret of Orange Carrots
Carrots, especially the bright orange varieties, are nutritional powerhouses, packed with carotenoids. These compounds are crucial for human health, contributing to reduced risks of conditions like eye disease. In fact, orange carrots are the most abundant source of pro-vitamin A in the American diet, making understanding their genetic makeup even more important.
Building upon previous work, including a 2016 study that mapped the first carrot genome and identified the gene for yellow carrot pigmentation, the researchers sequenced the genomes of 630 carrot accessions. This deep dive into carrot genetics allowed them to perform “selective sweeps,” identifying regions of the genome that have been heavily influenced by selection during domestication.
One key finding was the role of recessive genes in orange pigmentation. The study pinpointed three specific recessive genes that must be turned off to produce the high levels of carotenoids characteristic of orange carrots. This discovery is significant because it reveals an unusual genetic mechanism where gene inactivation, rather than activation, leads to a desired trait. These genes influence the complex interplay between carotenoid production, photosynthesis, and chloroplast development within the carrot.
Domestication Journey: From Central Asia to Orange Hue
Beyond pigmentation, the genomic analysis shed light on the fascinating history of carrot domestication. The study provides further evidence supporting the theory that carrots were first domesticated in the 9th or 10th century in western and central Asia. During this period, purple and yellow carrots were common in this region.
“Purple carrots were common in central Asia along with yellow carrots,” Iorizzo notes. “Both were brought to Europe, but yellow carrots were more popular, likely due to their taste.”
The emergence of the orange carrot is a more recent event, appearing in Western Europe around the 15th or 16th century. The researchers suggest that orange carrots may have arisen from crossbreeding between white and yellow carrots.
“This study basically reconstructed the chronology of when carrot was domesticated and then orange carrot was selected,” Iorizzo explains. “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 Rise of Orange Carrots and Health Impact
The appealing color and sweeter taste of orange carrots propelled their popularity. Farmers actively selected for these desirable traits, leading to the development of diverse orange carrot varieties in northern Europe during the 16th and 17th centuries. This timeline aligns with the appearance of different shades of orange carrots in paintings from that era, showcasing their growing prominence.
The popularity of orange carrots further surged as scientific understanding of alpha- and beta-carotenes, the vitamin A precursors, advanced in the late 19th and early 20th centuries. Interestingly, the very name “carotenoids” originates from carrots, as these pigments were first isolated from this vibrant vegetable.
This comprehensive genomic study not only unravels the genetic basis of orange carrots but also provides a detailed account of their domestication history. The findings offer valuable insights for future carrot breeding programs, potentially leading to even more nutritious and beneficial carrot varieties. By understanding the genes that make carrots orange, we can further enhance this important source of vitamin A and promote better health through diet.
