Corn smut (Ustilago maydis) is a pathogenic plant fungus that causes smut disease on maize and teosinte (Euchlena mexicana). The fungus forms galls on all above-ground parts of corn species, and is known in Mexico as huitlacoche; it is eaten, usually as a filling, in quesadillas and other tortilla-based foods, and soups.
|Common names:||huitlacoche (Mexico), blister smut of maize, boil smut of maize, common smut of maize|
|Causal agents:||Ustilago maydis|
|hosts:||maize and teosinte|
Corn smut is a plant disease caused by the pathogenic fungus Ustilago maydis that causes smut on maize and teosinte (Euchlena mexicana). The fungus forms galls on all above-ground parts of corn species, and is known in Mexico as the food delicacy huitlacoche; it is eaten, usually as a filling, in quesadillas and other tortilla-based foods, and soups.
In Mexico, corn smut is known as huitlacoche [wit͡ɬakot͡ʃe], sometimes spelled cuitlacoche). This word entered Spanish in Mexico from classical Nahuatl, though the Nahuatl words from which huitlacoche is derived is debated. In modern Nahuatl, the word for huitlacoche is cuitlacochin [kʷit͡ɬɑˈkot͡ʃin], and some sources deem cuitlacochi to be the classical form.
Some sources give the etymology as coming from the Nahuatl words cuitla [ˈkʷit͡ɬɑ] ("excrement" or "rear-end") and cochtli [ˈkot͡ʃt͡ɬi] ("sleeping", from cochi="to sleep"), thus giving a combined meaning of "sleeping/hibernating excrement".
A second group of sources deem the word to mean "raven's excrement". These sources appear to be combining the word cuitlacoche for "thrasher" with cuitla, meaning "excrement". However, the avian meaning of cuitlacoche derives from the Nahuatl word "song" cuīcatl [ˈkʷiːkɑt͡ɬ], itself from the verb "to sing" cuīca [ˈkʷiːkɑ]. This root then clashes with this reconstruction's second claim that the segment cuitla- comes from cuitla ("excrement").
Although it can infect any part of the plant, it usually enters the ovaries and replaces the normal kernels of the cobs with large, distorted tumors analogous to mushrooms. These tumors, or "galls", are made up of much-enlarged cells of the infected plant, fungal threads, and blue-black spores The spores give the cob a burned, scorched appearance. The generic name Ustilago comes from the Latin word ustilare (to burn).
When grown in the lab on very simple media, it behaves like baker's yeast, forming single cells called sporidia. These cells multiply by budding off daughter cells. When two compatible sporidia meet on the surface of the plant, they switch to a different mode of growth. First, they send out conjugation tubes to find each other, after which they fuse and make a hypha to enter the maize plant. Hyphae growing in the plant are dikaryotic; they possess two haploid nuclei per hyphal compartment. In contrast to sporidia, the dikaryotic phase of U. maydis requires infection of the plant to grow and differentiate, and cannot be maintained in the laboratory.
Proliferation of the fungus inside the plant leads to disease symptoms such as chlorosis, anthocyanin formation, reduced growth and the appearance of tumors harboring the developing teliospores.
Mature spores are released from the tumors and spread by rain and wind. Under appropriate conditions, a probasidium is formed in which meiosis occurs. Resulting haploid nuclei migrate into elongated single cells. These cells detach from the probasidium to become the sporidia, thus completing the life cycle.
Plants, in general, have evolved efficient defense systems against pathogenic microbes. A rapid plant defense reaction after pathogen attack is the oxidative burst, which involves the production of reactive oxygen species at the site of the attempted invasion. As a pathogen, U. maydis can respond to such an oxidative burst by an oxidative stress response, regulated by gene yap1. This response protects U. maydis from the host attack, and is necessary for the pathogen’s virulence. Furthermore, U. maydis has a well-established recombinational DNA repair system. This repair system involves a homolog of Rad51 that has a very similar sequence and size to its mammalian counterparts. This system also involves a protein, Rec2 that is more distantly related to Rad51, and Brh2 protein that is a streamlined version of the mammalian Breast Cancer 2 (BRCA2) protein. When any of these proteins is inactivated, sensitivity of U. maydis to DNA damaging agents is increased. Also mitotic recombination becomes deficient, mutation frequency increases and meiosis fails to complete. These observations suggest that recombinational repair during mitosis and meiosis in U. maydis may assist the pathogen in surviving DNA damage arising from the host’s oxidative defensive response to infection, as well as from other DNA damaging agents.
The yeast-like growth of U. maydis makes it an appealing model organism for research, although its relevance in nature is unknown. The fungus is exceptionally well-suited for genetic modification. This allows researchers to study the interaction between the fungus and its host with relative ease. The availability of the entire genome is another advantage of this fungus as model organism.
U. maydis is not only used to study plant disease, but it also is used to study plant genetics. In 1996, a study on U. maydis genetics led to the discovery of synthesis-dependent strand annealing, a method of homologous recombination used in DNA repair. Other studies in the fungus have also investigated the role of the cytoskeleton in polarized growth. It is largely due to work with U. maydis that the function of the breast-cancer gene BRCA2 is now known.
Smut feeds on the corn plant and decreases the yield. Smut-infected crops are often destroyed, although some farmers use them to prepare silage. The smut is a delicacy in Mexico, where it is known as huitlacoche, even being preserved and sold for a significantly higher price than uninfected corn. The consumption of corn smut originates from ancient Aztec cuisine. For culinary use, the galls are harvested while still immature — fully mature galls are dry and almost entirely spore-filled. The immature galls, gathered two to three weeks after an ear of corn is infected, still retain moisture and, when cooked, have a flavor described as mushroom-like, sweet, savory, woody, and earthy. Flavor compounds include sotolon and vanillin, as well as the sugar glucose.
The fungus has had difficulty entering into the American and European diets as most farmers see it as blight, despite attempts by government and high profile chefs to introduce it. In the mid-1990s, due to demand created by high-end restaurants, Pennsylvania and Florida farms were allowed by the United States Department of Agriculture (USDA) to intentionally infect corn with huitlacoche. Most observers consider the program to have had little impact, although the initiative is still in progress. The cursory show of interest is significant because the USDA has spent a considerable amount of time and money trying to eradicate corn smut in the United States. Moreover, in 1989, the James Beard Foundation held a high-profile huitlacoche dinner, prepared by Josefina Howard, chef at Rosa Mexicano restaurant. This dinner tried to get Americans to eat more of it by renaming it the Mexican truffle.
Native Americans of the American Southwest, including the Zuni people, have used corn smut in an attempt to induce labor. It has similar medicinal effects to ergot, but weaker, due to the presence of the chemical ustilagine.
Recipes of Mexico
A favorite and simple Mexican-style succotash can be made from chorizo, onions, garlic, serrano peppers, huitlacoche, and shrimp with salsa taquera. The mild, earthy flavors of the huitlacoche blend nicely with the fats of the chorizo and bond to mellow out the heat from the peppers and salsa.
Another Mayan favorite on the Riviera Maya (Cancun to Tulum) is to add huitlacoche to omelettes. Once again, its earthy flavors bond with the fats that cook the eggs to mellow the flavors into a truffle-like taste.
An important thing to note about huitlacoche is that the blueish color transforms into the recognizable black color only with heat. Any dish with huitlacoche must include a slow simmer of the fungus until it becomes black which also removes most of the starch of the corn and what is left is a black oily paste.
In Mexico, huitlacoche is mostly consumed fresh and can be purchased at restaurants or street or farmer's markets throughout the country and, to a much lesser extent, can also be purchased as a canned good in some markets and via the internet. Farmers in the countryside have been known to spread the spores around intentionally to create more of the fungus. In some parts of the country, they call the fungus "hongo de maiz," i.e. "maize fungus." 
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