Regulation of Morphology of Corn Smut, Ustilago maydis

Basidiomycota, in contrast to other fungi such as Ascomycota, produce basidia that yield four sexual spores called basidiospores. U. maydis is part of this phylum. Its mating-type is determined by a tetrapolar system with two unrelated loci, a and b. There are two idiomorphs for the a locus, a1 and a2. Haploid U. maydis cells have either the 4.5kb a1 locus with genes mfa1, pra1, and rfa2, or the 8kb a2 locus with genes mfa2, pra2, lga2, and rga2. mfa1 and mfa2 encode pheromone precursors, pra1 and pra2 genes encode pheromone receptors for the a2 and a1 pheromone, respectively. rfa2, lga2, and rga2 are thought to function within mitochondria. The pheromone encoded by one idiomorph will bind to the receptor of the opposite cell type, activating a signaling cascade that induces G2 arrest and the formation of conjugation hyphae. The b locus contains two genes, bE and bW, and regulates the switch to the pathogenic filamentous stage, as well as tumor induction and the formation of teliospores. The complex mating-type regulation is not specific to U. maydis, and other Basidiomycetes such as Schizophyllum commune and Coprinus cinereus.

Promycelium undergoes meiosis to produce saprophytic haploid cells. When in contact with corn, these sporidia exchange pheromones and become conjugative hyphae. These fuse to form a dikaryote, which is able to being intracellular invasion. Tumors are induced in which the fungi proliferates. Spores are formed and spread in the air and form a promycelium.

Higher fungi, like U. maydis, make ideal genetic models because they are easy to mate, transform, and select for. Observing metabolism, virulence, genotype is easier because they tend to be linked to readily apparent morphology. U. maydis’ relatedness to animal cells makes their study even more relevant to humans. Not only do does it have microtubule organization, nuclear migration, and nuclear envelop breakdown like in humans, U. maydis has homologues of Homo sapiens proteins that other, “higher” genetic models lack, such as Brh2, a BRCA2 (Breast Cancer Type 2 susceptibility protein) homologue. In vivo studies of Brh2 made it possible for geneticists to understand the function of BRCA2 in DNA repair and tumor suppression in humans. There is no doubt of U. maydis’ importance as a genetic model to study other complex mammalian cell processes. 

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