| Corn smut | |
|---|---|
 Huitlacoche | |
| Common names | huitlacoche (Mexico), blister smut of maize, boil smut of maize, common smut of maize |
| Causal agents | Mycosarcoma maydis |
| Hosts | maize andteosinte |
| EPPO Code | USTIMA |
| Distribution | Worldwide, where corn is grown[1][2] |
| Corn smut | |
|---|---|
 | |
| Mycosarcoma maydis diploid teleospores | |
Scientific classification | |
| Kingdom: | Fungi |
| Division: | Basidiomycota |
| Class: | Ustilaginomycetes |
| Order: | Ustilaginales |
| Family: | Ustilaginaceae |
| Genus: | Mycosarcoma |
| Species: | M. maydis |
| Binomial name | |
| Mycosarcoma maydis | |
| Synonyms | |
Corn smut is aplant disease caused by thepathogenicfungusMycosarcoma maydis,synonymUstilago maydis. One of several cereal crop pathogens calledsmut, the fungus formsgalls on all above-ground parts of corn species such asmaize andteosinte. The infected corn is edible; inMexico, it is considered adelicacy, calledhuitlacoche,[5] often eaten as a filling inquesadillas and other tortilla-based dishes, as well as in soups.[6]
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InMexico, corn smut is known ashuitlacoche (Spanish pronunciation:[(ɡ)witlaˈkotʃe], sometimes spelledcuitlacoche). This word entered Spanish in Mexico fromClassical Nahuatl, though the Nahuatl words from whichhuitlacoche is derived are debated. In modern Nahuatl, the word forhuitlacoche iscuitlacochin (Nahuatl pronunciation:[kʷit͡ɬɑˈkot͡ʃin]), and some sources deemcuitlacochi to be the classical form.[7]
Some sources wrongly give the etymology as coming from the Nahuatl wordscuitlatl[ˈkʷit͡ɬɑt͡ɬ] ("excrement" or "rear-end", actually meaning "excrescence") andcochtli[ˈkot͡ʃt͡ɬi] ("sleeping", fromcochi "to sleep"), thus giving a combined mis-meaning of "sleeping/hibernating excrement",[7][8] but actually meaning "sleeping excrescence", referring to the fact that the fungus grows between the kernels of corn and impedes them from developing, thus they remain "sleeping".
A second group of sources deem the word to mean "raven's excrement."[9][10] These sources appear to be combining the wordcuitlacoche for "thrasher"[11] withcuitla, meaning "excrement," actually meaning "excrescence". However, the avian meaning ofcuitlacoche derives from the Nahuatl word "song"cuīcatl[ˈkʷiːkɑt͡ɬ], from the verb "to sing"cuīca[ˈkʷiːkɑ].[7] This root then clashes with this reconstruction's second claim that the segmentcuitla- comes fromcuitla ("excrement").
One source derives the meaning as "corn excrescence," usingcuītla again and "corn"tlaōlli[t͡ɬɑˈoːlːi].[12] This requires the linguistically unlikely evolution oftlaōlli "corn" intotlacoche.
InPeru, it is known aschumo orpacho.
Mycosarcoma maydis is the best known and studied fungus of the Ustilaginomycetes, a sub class of basidiomycota, and is often used as the exemplar species when talking about its entire class.[13]
Formerly it was placed in genusUstilago, but in 2016, it was placed in genusMycosarcoma.[14]
The fungus infects all parts of the host plant by invading the ovaries of its host. The infection causes the corn kernels to swell up into tumor-likegalls, wherein the tissues, texture, and developmental pattern aremushroom-like. The galls grow to 4 to 5 inches in diameter. These galls are made up ofhypertrophied cells of the infected plant, along with resulting fungal threads and blue-blackspores.[15] These dark-colored spores give the cob a burned, scorched appearance.[citation needed]
When grown in the lab on very simplemedia,M. maydis behaves like baker'syeast, forming singlecells calledsporidia. These cells multiply by budding off daughter cells. When two compatible sporidia meet on the surface of the plant, however, they switch to a different mode of growth. First, they produce one or anotherpheromone, and begin producing one or the other type ofpheromone receptor - this depends onmating typea orb, as determined by alleles at two unlinked matingloci. If this signaling is successful, they then send out conjugation tubes to find each other,[13] after which they fuse and make ahypha to enter the corn plant. Hyphae growing in the plant aredikaryotic; they possess two haploid nuclei per hyphal compartment. In contrast to sporidia, the dikaryotic phase ofM. maydis only occurs during successful infection of a corn plant, and cannot be maintained in the laboratory.
Proliferation of the fungus inside the plant leads to disease symptoms such aschlorosis,anthocyanin formation, reduced growth, and the appearance of tumors harboring the developingteliospores. These teliospores help to overwinter the pathogen into the next season. They survive in the soil.[16][17]
Mature tumors release spores that are dispersed by rain and wind. Under appropriate conditions, ametabasidium is formed in whichmeiosis occurs. Resultinghaploid nuclei migrate into elongated singlecells. These cells detach from themetabasidium to become the sporidia, thus completing the life cycle.
Plants have evolved efficient defense systems against pathogenic microbes. A rapid plant defense reaction after pathogen attack is theoxidative burst, which involves the production of reactive oxygen species at the site of the attempted invasion. As a pathogen,M. maydis can respond by anoxidative stress response, regulated by geneYAP1. This response protectsM. maydis from the host attack and is necessary for the pathogen's virulence.[18] Furthermore,M. maydis has a well-established recombinational DNA repair system.[19] 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 proteins, and is a streamlined version of the mammalian Breast Cancer 2 (BRCA2) protein. When any of these proteins is inactivated, sensitivity ofM. 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 inM. 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.
M. maydis is known to produce fourGα proteins, and one each ofGβ andGγ.[13]
Losses from corn smut can vary greatly; however, annual yield losses rarely exceed 2% when resistant cultivars are planted. This disease can have a large economic impact on sweet corn, specifically when smut galls replace the kernels. There are many ways to control and manage corn smut; however, corn smut cannot be controlled by any common fungicide at this time, asM. maydis infects individual corn kernels instead of infecting the entire cob, likehead smut.[20]
Some beneficial ways to contain corn smut include resistant corn plants,crop rotation, and avoiding mechanical injury to the plant. A mechanical injury can cause the corn to become easily accessible toM. maydis, enhancing infection. Additionally, clearing the planting area of debris can help control corn smut, as the teliospores from corn smut overwinter in debris. This is not the best practice, though, because corn smut can also overwinter in the soil; crop rotation is recommended. Lastly, as excess nitrogen in the soil augments infection rate, using fertilizer with low nitrogen levels or just limiting the amount of nitrogen in the soil proves to be another way to control corn smut.[21]
Although not all the conditions that favor growth ofM. maydis are known, there are certain environments where corn smut seems to thrive, depending on both abiotic and biotic factors. Hot and dry weather during pollination followed by a heavy rainy season appear to improve the pathogenicity of corn smut.[22] Furthermore, excess manure (and therefore nitrogen) in the soil also increases pathogenicity. Not only do these abiotic factors increase infectability, they also increase disease spread. High winds and heavy rain also increase disease spread as the spores of corn smut can be more easily transmitted. Other biotic factors largely have to do with the extent by which humans interact with the corn and corn smut. If corn debris is not cleared at the end of the season, the spores can overwinter in the corn fragments and live to infect another generation.[23] Finally, humans wounding the corn (with shears or other tools) present the opportunity for corn smut to easily enter the plant.
Smut feeds on the corn plant and decreases theyield. Smut-infected crops are often destroyed, although some farmers use them to preparesilage. However, the immature infected galls are still edible, and inMexico, they are highly esteemed as a delicacy. It is known ashuitlacoche and sold for a significantly higher price than uninfected corn. The consumption of corn smut in Mexico originated directly fromAztec cuisine.[24] For culinary use, thegalls are harvested while still immature — fully mature galls are dry and almost entirely spore-filled. The immature galls, gathered 2 to 3 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 includesotolon andvanillin, as well as the sugarglucose.
Huitlacoche is a source of theessential amino acidlysine, which the body requires but cannot manufacture. It also contains levels ofbeta-glucans similar to, and protein content equal or superior to, most edible fungi.[25]
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 into food products. In the mid-1990s, due to demand created by high-end restaurants,Pennsylvania andFlorida farms were allowed by theUnited States Department of Agriculture (USDA) to intentionally infect corn withhuitlacoche. 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, theJames Beard Foundation held a high-profilehuitlacoche dinner, prepared byJosefina Howard, chef at Rosa Mexicano restaurant.[26] This dinner tried to get Americans to eat more of it by renaming it the "Mexican truffle" and it is often compared totruffles in food articles describing its taste and texture.[26][27][28]
Native American tribes in North America ate corn smut as well. The North Dakota Hidatsa tribe's practice of preparing and eating corn smut is described vividly inBuffalo Bird Woman's Garden.[29]
Native Americans of the American Southwest, including theZuni people, have used corn smut in an attempt to induce labor. It has similar medicinal effects toergot, but weaker, due to the presence of the chemicalustilagine.[30]
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A simple Mexican-stylesuccotash can be made fromchorizo, onions, garlic, serrano peppers,huitlacoche, and shrimp withsalsa taquera. The mild, earthy flavors of thehuitlacoche blend nicely with the fats of the chorizo and bond to mellow out the heat from the peppers and salsa.
Another Maya favorite on theRiviera Maya (Cancún toTulum) is to addhuitlacoche to omelets. Its earthy flavors bond with the fats that cook the eggs to mellow the flavors into a truffle-like taste.
Huitlacoche is also popular in quesadillas with Mexican cheese, sautéed onions, and tomatoes.
The blueish color transforms into the recognizable black color only with heat. Any dish withhuitlacoche 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, streets, 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 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".[31]
When corn smut grows on a corn cob, it changes the nutritional worth of the corn it affects. Corn smut contains more proteins than the uninfected grains normally do. The amino acidlysine, of which corn contains very little, abounds in corn smut.[32]
The yeast-like growth ofM. maydis makes it an appealingmodel organism for research, although its relevance in nature is unknown. The fungus is exceptionally well-suited forgenetic modification. This allows researchers to study the interaction between the fungus and its host with relative ease. The availability of the entiregenome is another advantage of this fungus as a model organism.[33]
M. maydis is not only used to study plant disease, but it also is used to study plant genetics. In 1996, a study onM. maydis genetics led to the discovery of synthesis-dependent strand annealing, a method ofhomologous recombination used inDNA repair.[34] Other studies in the fungus have also investigated the role of thecytoskeleton in polarized growth.[citation needed] It is largely due to work withM. maydis that the function of the breast-cancergeneBRCA2 is now known.[35] The fungus is mostly studied as a model organism for host pathogen interaction and delivery of effectors protein.
M. maydis is able to produce a broad range of valuable chemicals such asustilagic acid,itaconic acid,malic acid, andhydroxyparaconic acid. Thus, corn smut is gaining more relevance in industrial applications.[36]
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