“Golden Corn” — Ancient, new, or both?
No picnic is complete without corn on the cob. Its glistening golden kernels are irresistible — and tasty. This humble marvel is one of America’s greatest and most ancient treasures. For thousands of years it’s provided sustenance and joy for humanity. We owe an unparalleled debt to those who helped shape this gift to us — the plant, the ears, the kernels. Our earliest farmers were the ones. Long before the pyramids of Egypt were built, ancient farmers in the Americas had already domesticated maize from its wild-grass ancestor. They did so by selecting desirable traits year after year. In this simple-but-powerful way, they used genetics to develop corn on the cob from a wild weed.
But did early farmers select for “golden corn” without knowing its nutritional advantage? Or was white maize preferred because of its aroma and flavor? Right now, scientists are learning how those colors came to be.
Most US Corn Was White
The corn-belt was dominated by white-kernel lines until about 1930, when health benefits of vitamin A and yellow corn were revealed. Almost overnight, US growers switched to golden corn. Now, white kernels are re-emerging for their distinctive features, and new research is helping us understand the nature of yellow corn versus white.
Vitamin A and Healthy Corn
Discovery of vitamin A from “golden corn” had a profound effect on demand for yellow kernels. The health advantages for humans and livestock were quickly recognized. The healthy yellow and orange pigments were named “carotenoids” because of their abundance in carrots. Mothers began urging children to eat carrots for better vision. The carotenoids in carrots and golden corn are the sources of the vitamin A made from these pigments and they play roles in animal vision and health.
Seeding Healthful Awareness
As awareness of healthful golden kernels spread, seed companies saw an opportunity to profit from the distinctive yellow product. Prior to this time, most corn came from saved seeds of open-pollinated plants. This meant that essentially any plant could cross its genetics with its neighbors. Inbreeding often resulted. When the source of vitamin A was found to be carotenoids, seed companies focused on the genetics of yellow corn. They soon advertised the new yellow-corn feature in their hybrids. The beautiful, uniform-golden ears spawned a new industry. In addition to their health advantages, the hybrid plants were more productive than the open-pollinated, often inbred varieties. White-kernel corn was soon a thing of the past, largely grown as a novelty.
Although much was gained by transition to “golden corn,” advantages of white kernels are being rediscovered. Among these are distinctive taste and aroma of fresh corn products.
Some Kernels are Whiter than White
They’re “mighty white.” We now know why. Recent work from the University of Florida shows that these super-white kernels have enzymes that break down the yellow carotenoid pigment in kernels. When these pigments are metabolized, they’re converted to colorless products that include possible flavors and aromas. This conversion of carotenoids is regulated by a gene called Ccd1 for “Carotenoid cleavage dioxygenase 1.” The name comes from the unusual way carotenoids are cleaved (a dioxygenase). Because no one knew the basis for this white-kernel trait, it has simply been called “white cap” for over 100 years.
The new study suggests that “white-cap” was extremely important to genetic improvement of maize from its ancient relatives. Data indicate that humans likely identified “white-cap” corn, planted some of it each year, and thus added its genetics to future lines of corn. Eventually this continuous selection maintained and enhanced a white-kernel trait. The genetic alterations trace back through many locations, which suggests a long-term human interest in cultivating both white and yellow maize. While hundreds of generations of farmer-breeders deliberately picked and planted the white kernels to skew the genetics toward white ears, we have only now begun to understand the genetic basis of this white-kernel trait.
Still more fascinating is the molecular basis for this genetic variant. It was recently shown that the most snow-white “white-cap” kernels can carry anywhere from one to twenty-three copies of the Ccd1 gene. And — — The more copies there are, the whiter the kernel.
The “White Cap” Trait Formed in a Remarkable Way
Analysis of sequences from modern and primitive lines indicated that unusual pieces of DNA called “jumping genes” or “transposable elements” were involved. These pieces of DNA can pick up from their positions in the double-helix and move to spots elsewhere in the genome. In the case of the Ccd1 gene for carotenoid cleavage, jumping genes inserted nearby and began making copies of it. This, plus other events, led to multiple copies of the gene regulating conversion of yellow kernels to white.
Lots of Duplication
An especially interesting part of the story is that whiter and whiter kernels resulted wherever more gene duplications had accumulated. Early growers selected for these. By this straight-forward process, early farmers were genetically engineering whiter and whiter corn. As they chose whiter kernels, they were also choosing genetics for more copies of the genes that metabolized the yellow pigment.
Corn has a wonderful genetic history shaped by humans that selected the traits that they found desirable. One of these was “white cap” kernels, and now molecular tools show the basis of changes in DNA that ancient farmers perpetuated by selecting the whitest corn. “White-cap” maize thus gives us a new view into the association between our own cultural history, our health, and how jumping, duplicating genes can affect important crop traits.
The genetic basis of corn kernel color was recently published in the journal Genetics as:
Bao-Cai Tan, Jiahn-Chou Guan, Shou Ding, Shan Wu, Jonathan W Saunders, Karen E Koch, Donald R McCarty (2017) Structure and origin of the White Cap locus and its role in the evolution of grain color in maize.
