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Corn Smut
Huitlacoche
Scientific classification
Kingdom: Fungi
Subkingdom: Dikarya
Phylum: Basidiomycota
Subphylum: Ustilaginomycotina
Class: Ustilaginomycetes
Order: Ustilaginales
Genus: Ustilago
Species: U. maydis
Binomial name
Ustilago maydis
(Persoon) Roussel

Corn smut is a disease of maize caused by the pathogenic plant fungus Ustilago maydis. U. maydis causes smut disease on maize (Zea mays) and teosinte (Euchlena mexicana). Known in Latin America as huitlacoche (literally, "raven's excrement"), it is eaten, usually as a filling in quesadillas and other tortilla-based foods.

Contents

Characteristics

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 name Ustilago comes from the Latin word ustilare (to burn).

Culinary uses

Corn tortilla with huitlacoche

Considered a pest in most of the United States, smut feeds off the corn plant and decreases the yield. Usually smut-infected crops are destroyed. Some farmers may also choose to prepare corn silage out of the smutted corn. However, in Mexico corn smut is called huitlacoche ([witɬakotʃe], sometimes spelled cuitlacoche), a Nahuatl word reportedly meaning raven's excrement[1]. It is considered a delicacy, even being preserved and sold for a higher price than corn.[2] 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. 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. Regardless, the cursory show of interest is significant because the USDA has spent a considerable amount of time and money trying to eradicate huitlacoche in the United States. Moreover, in 1989 the James Beard Foundation held a high-profile huitlacoche dinner. This dinner famously tried to get Americans to eat more of it by renaming it the Mexican truffle.

Huitlacoche grows best during times of drought in a 78°F to 93°F (25°C–34°C) temperature range. Aztecs purposely inoculated corn with the spores by scratching their corn plants at the soil level with a knife—thereby allowing the water-borne spores easy entrance into the plant.

Life cycle

Ustilago maydis diploid teleospores
Ustilago maydis haploid sporidia

When grown in the lab on very simple media, it behaves like baker's yeast, forming single cells going by the name 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 in order to grow and differentiate and cannot be maintained in the laboratory.

Proliferation of the fungus inside the plant leads to disease symptoms as chlorosis, anthocyanin formation, reduced growth and the appearance of tumors harboring the developing teliospores.[3][4]

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; these are the sporidia, completing the life cycle.

Uses

Native Americans of the American Southwest, including the Zuni tribe, have used corn smut to induce labor. It has similar medicinal effects to ergot, but weaker, due to the presence of the chemical ustilagine.[5]

Model organism

The yeast-like growth of Ustilago 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.[6]

U. maydis is not only used to study plant disease. In 1996, a study on U. maydis genetics lead to the discovery of synthesis-dependent strand annealing, a method of homologous recombination used in DNA repair.[7]. 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.

Availability

Huitlacoche can be purchased as a canned good in some markets and via the internet. Some farmers markets and organic growers are endeavoring to bring fresh huitlacoche to their customers and local food service trade. In Mexico it is mostly consumed fresh and can be purchased at restaurants, street or farmer's markets throughout the country.

Footnotes

  1. ^ Gourmet Sleuth: Mexican Corn Truffle
  2. ^ Uribe, Monica Ortiz (2009-08-20). "In Mexico, Tar-Like Fungus Considered Delicacy". http://www.npr.org/templates/story/story.php?storyId=111789560&sc=fb&cc=fp. Retrieved 2009-08-20.  
  3. ^ Banuett, F. (1995). "Genetics of Ustilago Maydis, A Fungal Pathogen that Induces Tumors in Maize". Annual Reviews in Genetics 29 (1): 179–208. doi:10.1146/annurev.ge.29.120195.001143.  
  4. ^ Christensen, J.J. (1963). [/scholar?hl=en&lr=&ie=UTF-8&sa=G&oi=qs&q=corn+smut+caused+by+ustilago+maydis+author:r-kahmann "Corn smut caused by Ustilago maydis. Monograph no. 2"]. Amer. Phytopath. Society. /scholar?hl=en&lr=&ie=UTF-8&sa=G&oi=qs&q=corn+smut+caused+by+ustilago+maydis+author:r-kahmann. Retrieved 2008-07-08.  
  5. ^ O'Dowd, Michael J. (2001). The History of Medications for Women. Taylor & Francis. ISBN 1-85070-002-8.   p. 410, via Google Books
  6. ^ Kämper J, Kahmann R, Bölker M, et al. (November 2006). "Insights from the genome of the biotrophic fungal plant pathogen Ustilago maydis". Nature 444 (7115): 97–101. doi:10.1038/nature05248. PMID 17080091.  
  7. ^ Ferguson, DO; Holloman, HK (1996). "Recombinational repair of gaps in DNA is asymmetric in Ustilago maydis and can be explained by a migrating D-loop model". Pnas USA 99 (11): 5419–5424. doi:10.1073/pnas.93.11.5419. PMID 8643590.  

References

  • McGee, Harold (2004). On Food and Cooking (Revised Edition). Scribner. ISBN 0-684-80001-2.   p 349, "Huitlacoche, or Corn Smut".

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