The model system Ustilago maydis
Ustilago maydis is a dimorphic fungus, which belongs to the Basidiomycota. It is long known that U. maydis is the causal agent of corn smut disease in maize, and due to its technical advantages U. maydis became a powerful model system in molecular plant pathology research1,2,3,4. In cell culture U. maydis grows as a yeast-like cell (named sporidium), but upon the beginning of the pathogenic live cycle the fungus undergoes a morphogenic transition and begins to grow as a filamentous hypha (Figure 1). At the beginning of the infection two yeast-like cells recognize each other and fuse in order to form another type of hypha that is able to invade corn tissue. This subsequently results in so-called tumors in stem, leaves and flowers (Figure 2). Interestingly, these U. maydis-induced tumors are known as a Mexican a delicacy, named cuitlachoche (Figure 3). Nowadays fungicides efficiently control U. maydis infections. However, depending on climate conditions infections can lead to significant grain-yield loss, and the fungus is therefore listed as a potential bio-weapon by the Ad Hoc Group of the Biological Weapons Convention of the United States of America5.
U. maydis is not only a pathogen; it has also a long-standing history as a model system for analyzing DNA recombination and repair6. In addition, we recently have shown that this fungus could serve as a more general model system for fundamental cell biology7. Our work demonstrated U. maydis cells contain a prominent microtubule cytoskeleton and that the basic machinery for hyphal growth is reminiscent of that of mammalian neurons. In addition, this fungus undergoes an open mitosis that is mainly supported by the motor protein dynein. Furthermore, a genome wide comparison of the predicted proteome revealed that U. maydis shares more proteins with humans than with Saccharomyces cerevisiae, a fungus also known as budding yeast.
1 Banuett, F. (1995) Annu Rev Genet. 29:179-208.
2 Feldbrügge et al. (2004) Curr Opin Microbiol. 7:666-672.
3 Perez-Martin (2006) Mol Genet Genomics. 276:211-229.
4 Bölker M. (2001) Microbiology, 147:1395-1401.
5 Madden & Wheelis (2003) Annu. Rev. Phytopathol. 41:155-176.
6 Holliday, R. (2004) DNA Repair, 3:671-682.
7 Steinberg and Perez-Martin, 2008, Trends Cell Biol., 18:61-67
Figure 1: Pathogenic life cycle of U. maydis. Yeast-like cells life in soil and are saprotrophic, but switch to filamentous growth upon initiation of the pathogenic cycle. Sporulation and subsequent meiosis requires infection of the maize plant (image taken from Steinberg and Perez-Martin, 2008, Trends Cell Biol. 18:61).
Figure 2: Symptoms of corn smut disease caused by Ustilago maydis (right image from Brefeld 1983, Untersuchungen aus dem Gesamtgebiet der Mykologie. 5, 67).
Figure 3: Ustilago maydis induced tumors are considered a delicacy from Mexico, named cuitlachoche. It is recommended to be enjoyed on special occasions along with omelettes, tacos, enchiladas, soups. The taste is said to be 'unique' (no comment, I never tried it). Here is a recipe if you'd like to try it!