Maize

Maize (Zea mays), commonly called corn, is a tall, sturdy annual grass that grows a single stem, reaching heights between 1.2 and 4 meters. Its long, narrow leaves emerge alternately from joints along the stalk. The plant is monoecious, bearing separate male and female flowers. At the top, a tassel holds male flowers that release wind-dispersed pollen. Lower down, the female inflorescence first appears as a silk—a bundle of tubular hairs, each connected to a carpel that, once pollinated, develops into a kernel (botanically a caryopsis, a fruit fused with the seed coat). The entire female structure becomes an ear or corncob, wrapped in multiple leafy husks. The ear leaf and those above it supply over three-quarters of the starch that fills the grain. Grains are arranged in 8 to 32 rows around the cob, with up to 1,200 kernels on a large cob. Modern commercial varieties typically have yellow or white grains, but other varieties can be orange, red, brown, blue, purple, or black. Yellow maize gets its color from carotenoids; red from anthocyanins and phlobaphenes; orange and green may combine these pigments.

Maize was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native Americans planted it alongside beans and squashes in the Three Sisters polyculture. The plant relies on humans for its propagation. After the Columbian exchange, it became a staple food worldwide, and total maize production now exceeds that of wheat and rice. Much of it is used for animal feed—either as grain (fodder) or as the whole plant, baled as forage or made into silage. Sugar-rich sweet corn varieties are grown for people to eat, while field corn goes to animal feed, cornmeal, masa, corn starch, corn syrup, corn oil, alcoholic drinks like bourbon whiskey, and chemical feedstocks including ethanol and other biofuels. Maize is grown across the globe, with more weight produced each year than any other grain. In 2020, world production reached 1.1 billion tonnes.

Maize has short-day photoperiodism: it needs nights of a certain length to flower, plus enough warm days above 10°C. Flowering control is genetic, involving the phytochrome system. Tropical cultivars grown at higher latitudes may grow tall without setting seed before winter, though rapid tall growth can be useful for biofuel. Immature shoots produce the antibiotic DIMBOA, which protects against many pests. Because of shallow roots, maize is drought-sensitive, intolerant of poor soils, and easily uprooted by strong winds. Its pollen is an allergen, but most falls within a few meters of the tassel, mainly affecting farm workers. Major insect pests include the European corn borer and corn rootworms, each causing annual losses of a billion dollars in the United States. Modern plant breeding has boosted yield, nutrition, drought tolerance, and pest and disease resistance. Much maize is now genetically modified.

As food, maize is used in Mexican tortillas and tamales, Italian polenta, and American hominy grits. Its protein is low in some essential amino acids, and its niacin is only available after alkali treatment. In pre-Columbian Mesoamerica, maize was deified as a maize god and depicted in sculptures.

Maize is diploid with 20 chromosomes. About 83% of its allelic variation comes from teosinte ancestors, largely because Zea species freely outcross. Barbara McClintock used maize to prove her transposon theory of “jumping genes,” winning the 1983 Nobel Prize in Physiology or Medicine. Maize remains a key model organism for genetics and developmental biology; the MADS-box motif is involved in flower development. The Maize Genetics and Genomics Database, funded by the USDA, supports research. The International Maize and Wheat Improvement Center maintains a large collection of maize accessions tested for insect resistance. In 2005, a consortium of the U.S. National Science Foundation, USDA, and Department of Energy began sequencing the maize genome, depositing data in GenBank. The genome was completed in 2008, and results were published in 2009. The genome, 85% transposons, contains 32,540 genes, much of it duplicated and reshuffled by helitrons.