Grains — Past, Present, and Future

Stephen DeAngelis

July 28, 2010

One of America’s most patriotic songs praises the “amber waves of grain” that make the United States one the world’s breadbasket countries. Humans and their ancestors have probably always depended on grains as a food source. According to Wikipedia, “Cereal grains are grown in greater quantities and provide more food energy worldwide than any other type of crop.” In fact, the world’s three largest crops are all grains: rice, wheat, and maize. An article in the New York Times traces the ancestry of maize back at least 9000 years [“Tracking the Ancestry of Corn Back 9,000 Years,” by Sean B. Carroll, 24 May 2010]. Carroll writes:

“Corn is much more than great summer picnic food, however. Civilization owes much to this plant, and to the early people who first cultivated it. For most of human history, our ancestors relied entirely on hunting animals and gathering seeds, fruits, nuts, tubers and other plant parts from the wild for food. It was only about 10,000 years ago that humans in many parts of the world began raising livestock and growing food through deliberate planting. These advances provided more reliable sources of food and allowed for larger, more permanent settlements. Native Americans alone domesticated nine of the most important food crops in the world, including corn, more properly called maize (Zea mays), which now provides about 21 percent of human nutrition across the globe. But despite its abundance and importance, the biological origin of maize has been a long-running mystery. The bright yellow, mouth-watering treat we know so well does not grow in the wild anywhere on the planet, so its ancestry was not at all obvious. Recently, however, the combined detective work of botanists, geneticists and archeologists has been able to identify the wild ancestor of maize, to pinpoint where the plant originated, and to determine when early people were cultivating it and using it in their diets.”

It might have been as big of a surprise for you as it was for me to learn that maize “does not grow in the wild anywhere on the planet,” but Carroll asserts that “the greatest surprise, and the source of much past controversy in corn archaeology, was the identification of the ancestor of maize.” He continues his tale of botanical mystery:

“Many botanists did not see any connection between maize and other living plants. Some concluded that the crop plant arose through the domestication by early agriculturalists of a wild maize that was now extinct, or at least undiscovered. However, a few scientists working during the first part of the 20th century uncovered evidence that they believed linked maize to what, at first glance, would seem to be a very unlikely parent, a Mexican grass called teosinte. Looking at the skinny ears of teosinte, with just a dozen kernels wrapped inside a stone-hard casing, it is hard to see how they could be the forerunners of corn cobs with their many rows of juicy, naked kernels. Indeed, teosinte was at first classified as a closer relative of rice than of maize. But George W. Beadle, while a graduate student at Cornell University in the early 1930s, found that maize and teosinte had very similar chromosomes. Moreover, he made fertile hybrids between maize and teosinte that looked like intermediates between the two plants. He even Corn-and-teosinte_h1 reported that he could get teosinte kernels to pop. Dr. Beadle concluded that the two plants were members of the same species, with maize being the domesticated form of teosinte. Dr. Beadle went on to make other, more fundamental discoveries in genetics for which he shared the Nobel Prize in 1958. He later became chancellor and president of the University of Chicago.”

So if the puzzle was solved back in the early 1930s, what’s all the fuss about? Why, you might ask, is Carroll making such a big deal about maize’s origins now? He explains:

“Despite Dr. Beadle’s illustrious reputation, his theory still remained in doubt three decades after he proposed it. The differences between the two plants appeared to many scientists to be too great to have evolved in just a few thousand years of domestication. So, after he formally retired, Dr. Beadle returned to the issue and sought ways to gather more evidence. As a great geneticist, he knew that one way to examine the parentage of two individuals was to cross them and then to cross their offspring and see how often the parental forms appeared. He crossed maize and teosinte, then crossed the hybrids, and grew 50,000 plants. He obtained plants that resembled teosinte and maize at a frequency that indicated that just four or five genes controlled the major differences between the two plants. Dr. Beadle’s results showed that maize and teosinte were without any doubt remarkably and closely related. But to pinpoint the geographic origins of maize, more definitive forensic techniques were needed. This was DNA typing, exactly the same technology used by the courts to determine paternity.”

Just as old criminal cases didn’t have DNA evidence available several decades back, neither did Dr. Beadle. It took science time to catch up with his theories so they could be explored more completely. Carroll continues:

“In order to trace maize’s paternity, botanists led by my colleague John Doebley of the University of Wisconsin rounded up more than 60 samples of teosinte from across its entire geographic range in the Western Hemisphere and compared their DNA profile with all varieties of maize. They discovered that all maize was genetically most similar to a teosinte type from the tropical Central Balsas River Valley of southern Mexico, suggesting that this region was the ‘cradle’ of maize evolution. Furthermore, by calculating the genetic distance between modern maize and Balsas teosinte, they estimated that domestication occurred about 9,000 years ago. These genetic discoveries inspired recent archeological excavations of the Balsas region that sought evidence of maize use and to better understand the lifestyles of the people who were planting and harvesting it. Researchers led by Anthony Ranere of Temple University and Dolores Piperno of the Smithsonian National Museum of Natural History excavated caves and rock shelters in the region, searching for tools used by their inhabitants, maize starch grains and other microscopic evidence of maize.”

Carroll goes on to report that archaeological evidence coincided nicely with the DNA evidence. He says that researchers found stone milling tools with traces of maize on them and evidence that early Mesoamericans were quite adept at agriculture. He concludes:

“The most impressive aspect of the maize story is what it tells us about the capabilities of agriculturalists 9,000 years ago. These people were living in small groups and shifting their settlements seasonally. Yet they were able to transform a grass with many inconvenient, unwanted features into a high-yielding, easily harvested food crop. The domestication process must have occurred in many stages over a considerable length of time as many different, independent characteristics of the plant were modified. The most crucial step was freeing the teosinte kernels from their stony cases. Another step was developing plants where the kernels remained intact on the cobs, unlike the teosinte ears, which shatter into individual kernels. Early cultivators had to notice among their stands of plants variants in which the nutritious kernels were at least partially exposed, or whose ears held together better, or that had more rows of kernels, and they had to selectively breed them. It is estimated that the initial domestication process that produced the basic maize form required at least several hundred to perhaps a few thousand years. Every August, I thank these pioneer geneticists for their skill and patience.”

As Carroll notes, one of the challenges that farmers have always faced is the need to replant grain crops each season. That may be changing, however, as scientists work to create perennial plants to replace the annual plants that have historically been used [“Perennial grains could be biggest agricultural innovation in eons,” by Ben Coxworth, Gizmag, 29 June 2010]. Coxworth reports:

“It has pretty much become a given that grain crops, such as wheat and barley, need to be started from scratch every spring. This means farmers must buy seeds, use seeding equipment to get those seeds into the soil, then apply a lot of fertilizer and hope for weather conditions that won’t be too hot, cold, wet or dry for germination. There are such things as perennial grains, however – plants that, like the grass in your lawn, simply pick up in the spring where they left off in the fall. While perennial versions of common annual grains have seen little in the way of development, a new research paper says it’s about time they did. The advantages of cultivating perennial grains, the paper’s authors submit, could be one of the biggest advances in the 10,000-year history of agriculture.”

The only question that immediately comes to my mind is what would the introduction of perennial grain crops do to the concept of crop rotation? Coxworth continues:

“The paper, ‘Increased Food and Ecosystem Security via Perennial Grains,’ points out that perennials have longer growing seasons and longer, denser roots than annuals. Those longer roots, which can reach down 10 to 12 feet, allow the crops to reach and hold more water and nutrients, reduce erosion, and condition the soil. [see image] Because the plants grow for a greater length of time, they also sequester more carbon from the atmosphere. Annual crops, by contrast, are said to lose five times as much water as perennials, and 35 times as much nitrate – a plant nutrient that regularly leaches out of fields and pollutes waterways. Needless to say, annual crops also involve the rearing, transportation, purchase and sowing of seeds every year, which leaves definite carbon, chemical and financial footprints.”Grains-0

Don’t expect to see perennial grain crops anytime soon, Coxworth reports that it may take as long as 20 years for scientists to perfect the plants. In the meantime, scientists have other more pressing challenges concerning grain crops that need to be addressed. Perhaps the most immediate is the challenge of wheat rust [“Rust in the bread basket,” The Economist, 1 July 2010]. According to the article, “a crop-killing fungus is spreading out of Africa towards the world’s great wheat-growing areas.” The article concludes:

“Scientists … say they know how to attack stem rust. The question is how fast and how completely they can do it, and at what cost. … The new approach uses four or five weaker defences, for example reducing the area of the wheat plant that Ug99 [the new strain of rust] destroys, or slowing down the spread of the fungus. Each would be inadequate on its own. But put together, they achieve something close to full resistance. Multiple barriers also make it harder for Ug99 to overcome them all. A single gene will eventually be defeated—as Sr31 was. Knocking down a series of obstacles is much harder. So far, researchers have developed about 60 experimental wheat varieties with multiple low-resistance genes. … Scientists are also investigating how rust secretes the substances that enable it to break into the plant’s cells. If the wheat can learn to recognise this as it is happening, it might also be engineered to generate its own Ug99– resistant proteins. Separately, researchers have cracked the mystery of how stripe rust is able to overcome resistance in wheat so quickly. Until recently it was thought that stripe rust reproduces asexually. But new research found that, like stem rust, stripe rust increases its genetic variability by reproducing sexually on the leaves of another host plant (the barberry), making itself more adaptable—and more deadly. Advances like these suggest the march of wheat rust can be halted, though at a cost. That is not just money, but a trade-off of lower yields for more resistance. … But if the signs in the laboratory look propitious, out in the fields the distribution problems are formidable. Over 47,000 hectares have been planted with the new seeds. But that is only 0.1% of the total area planted to wheat in the countries on the FAO’s danger list. The astonishing spread of wheat rust makes quick containment impossible. In some places stem rust may become endemic before the outside world even spots it.”

We obviously owe a debt of gratitude to our ancestors for helping domesticate grasses that now provide us with cereal grains. But, with grain crops providing so much of the staple food needed to feed the globe, we also owe a debt of gratitude to scientists working to secure the present and future of such crops as well.