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Significance

Economic significance

Economic losses from common smut have been relatively minor in field corn grown in the United States since the early 1940s when hybrids were widely adopted and levels of host resistance were improved dramatically. Nevertheless, yield can be reduced occasionally when smut is prevalent and severe. Losses are greatest when apical meristems of very young seedlings are infected which often results in plant death or barren plants. Ear galls also can be economically important in field corn depending on the severity of infection (i.e., the number of kernels per ear replaced by smut galls). Economically significant outbreaks of common smut in field corn usually are associated with severe thunderstorms that occur soon after seedlings emerge or plant stresses (e.g., drought) that result in asynchronous timing of silk emergence and pollen production. Prior to the development of hybrid corn, losses due to common smut were estimated to range up to 10% annually because the disease sometimes was quite prevalent. For example, incidence of infected plants was 6 to 18% when corn fields near Ames, Iowa were surveyed from 1930 to 1934. Other occurrences of over 75% incidence of smut infection of open-pollinated cultivars were reported occasionally prior to the development of hybrids.

Common smut is of greater economic consequence in sweet corn than in field corn. When sweet corn is grown for fresh market (e.g., roadside stands, farmers’ markets, retail stores), ears with a single smut gall usually are not marketable because of “cosmetic injury” (i.e., lack of consumer acceptance). Hence, even a very low incidence of common smut can result in substantial losses. When sweet corn is grown for processing (i.e., canning, freezing), fields with moderate amounts of ear galls often are not harvested. In fresh market and processing sweet corn crops, galls on plant tissues other than ears can cause secondary losses if the crop is mechanically harvested because husk leaves of ears become covered with teliospores (Figure 26). In processing crops, additional costs are incurred to remove teliospores that contaminate husk leaves. In fresh market corn, some consumers will not accept sooty, black teliospores on husk leaves even though ears are not damaged.

Figure 26

In areas of the world where open-pollinated corn cultivars are grown or where hybrid corn is developed from smut-susceptible germplasm, common smut continues to be problematic. For example, incidence of smut infection was greater than 50% in 1976 in several areas of Germany where hybrids derived from European flint corn were prevalent.

Model organism for basic biological studies

Ustilago maydis has been used by investigators as a model organism to study a variety of interesting biological phenomena, including fungal mating type, fungal dimorphism, plant-pathogen interactions, and genetic recombination and repair. It has an intriguing feature of a life cycle that has both biotrophic and saprophytic stages. Other features that make the fungus a particularly attractive experimental organism include: a) the saprophytic stage is haploid, and thus easily manipulated genetically, b) meiotic progeny are easily analyzed, c) the entire life cycle can be completed in young plants in about 3 weeks, and d) diploid strains can be artificially constructed in the laboratory. Only a few examples of these studies can be mentioned here.

In the 1960s and 70s, R. Holliday built on the work of previous workers when he carefully developed standard media and techniques for working with U. maydis. He also produced an impressive array of mutants of various kinds. Some of these mutants were especially susceptible to additional mutations, because they were defective in their ability to repair their DNA. Holliday used these mutants to develop a model for how homologous recombination can occur between paired DNA molecules. The “Holliday structure” and Holliday’s model of genetic recombination are described in virtually every college genetics textbook. The work on recombination has continued to bear fruit, most notably in the lab of W. Holloman. Women with inherited mutations in a human tumor suppressor gene known as BRCA2 are more likely to develop breast and ovarian cancer, compared to women without the mutations. Workers in Holloman’s lab found the homolog of BRCA2 in U. maydis, and were able to study how the products of this tumor suppressor gene are involved in making repairs to damaged DNA.

Mating compatibility systems in filamentous basidiomycetes are characterized by multiple alleles at two genetic loci; strains with unlike alleles at both loci are said to be compatible, and can mate and begin sexual development. The multiplicity of possible combinations of alleles has fascinated scientists studying mushrooms and other basidiomycetes for many years, but the molecular interactions were not understood until recently. Mating compatibility in U. maydis was well-described by Christensen and previous workers, and it is easy to assay haploid isolates for mating compatibility using a charcoal agar assay first described by Day and Anagnostakis (Figure 27). In the late 1980s and early 1990s, S. Leong, R. Kahmann, and co-workers took advantage of earlier work, and the fact that U. maydis was easily grown as a haploid yeast in liquid culture when they developed and used a genetic transformation system that allowed them to clone and describe the mating compatibility genes in U. maydis. The presence of a pheromone-receptor combination at one locus coupled with a transcriptional regulator at the other locus, first elucidated in U. maydis, is the common theme in basidiomycete mating systems.

Figure 27

Most recently, the entire genome of U. maydis has been sequenced as part of an international collaboration. The genome and its annotation are publicly available at the Ustilago maydis database maintained by the Munich Information Center for Protein Sequences (MIPS) and online at http://mips.gsf.de/projects/fungi/ustilago.html. The availability of the genomic sequence and ancillary tools are beginning to bear fruit for scientists interested in host-pathogen interactions.

Cuitlacoche: edible galls of Ustilago maydis

Cuitlacoche (or huitlacoche) is the native Mexican name given to young, edible galls that form when ears of corn are infected by Ustilago maydis. In central Mexico, cuitlacoche is a highly prized delicacy that has been eaten since Pre-Columbian times. Traditional maize growers gather and market cuitlacoche following natural infection. About 400 to 500 tons of cuitlacoche are sold annually during July and August at markets in Mexico City (Figure 28a-d). More than 100 tons are processed by companies that sell the specialty mushroom canned or lyophilized (Figure 29a,b).

Figure 28a	Figure 28b
Figure 28c	Figure 28d
Figure 29a	Figure 29b

Concurrent with an expanding market in the U. S. for other types of specialty mushrooms such as Pleurotus (oyster), Lentinula (shiitake), Flammulina (enoki) and Morchella (morel), epicureans in North America increasingly view cuitlacoche as a gourmet fungus that is part of a growing market for haute Mexican cuisine. Cuitlacoche is served in soups, appetizers and entrees at many fashionable Mexican restaurants in major metropolitan areas in the United States, such as Topolobampo in Chicago and Rosa Mexicano in New York City and Washington DC (Figure 30a-c). Recipes for cuitlacoche are available on the internet and in gourmet Mexican cookbooks, such as Rick Bayless’s Mexican Kitchen and Diana Kennedy’s The Art of Mexican Cooking. Canned cuitlacoche is sold on the internet and sometimes is referred to as “maize mushrooms” or “Mexican truffles”. Fresh or frozen cuitlacoche occasionally is available at farmers’ markets or from local suppliers in the U.S. (Figure 31a-c).

Figure 30a	Figure 30b	Figure 30c
Figure 31a	Figure 31b	Figure 31c

Methods to cultivate cuitlacoche as a cash crop and various other aspects of cuitlacoche production systems have been studied in the U.S. and Mexico during the past 15 years. Essential aspects of efficient cultivation of cuitlacoche include: efficient methods of inoculation, rates of gall enlargement and teliospore formation, optimal times for inoculation and harvest, production practices that optimize infection, gall size, and yield, and, possibly most important, proper post-harvest handling and marketing.

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