Mating in the Smut Fungi: From a to b to the Downstream Cascades

Regine Kahmann; Jan Schirawski

doi:10.1128/9781555815837.ch22

Chapter 22 : Mating in the Smut Fungi: From a to b to the Downstream Cascades

Authors: Regine Kahmann¹, Jan Schirawski²

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Affiliations: 1: Max Planck Institute for Terrestrial Microbiology, 35043 Marburg, Germany; 2: Max Planck Institute for Terrestrial Microbiology, 35043 Marburg, Germany

Content Type: Monograph

Publication Year: 2007

Category: Fungi and Fungal Pathogenesis

Book DOI: 10.1128/9781555815837

Chapter DOI: 10.1128/9781555815837.ch22

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Abstract:

Smut fungi induce disease only in their dikaryotic stage, which is generated by mating. Mating is regulated by two loci, which harbor conserved genes. In the a locus these genes specify pheromones and receptors, while in the b locus two transcription factors are encoded. This chapter focuses on the signaling cascades, which coordinate cyclic AMP (cAMP) and mitogen-activated protein kinase (MAPK) signaling in response to pheromone, as well as the transcriptional cascade triggered by the products of the b locus. The first part of this chapter highlights variations in the organization of the mating-type loci in Ustilago maydis, Sporisorium reilianum, and U. hordei, their consequences for mating, and finally evolutionary implication. The chapter reviews the signaling pathway underlying pheromone perception as well as the regulatory cascade triggered by the homeodomain heterodimer. Pheromone perception in smut fungi elicits the formation of conjugation hyphae. The connection between mating and cell cycle in U. maydis may also provide an explanation for the observation that the pheromone-induced formation of conjugation tubes is prf1 independent. One of the driving forces for many of the studies described in this chapter has been the assumption that finding the complete set of targets for the bE/bW heterodimer will explain sexual development and its connection to pathogenesis.

Citation: Kahmann R, Schirawski J. 2007. Mating in the Smut Fungi: From a to b to the Downstream Cascades, p 377-387. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch22

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Mating in the Smut Fungi: From a to b to the Downstream Cascades

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Figure 22.1

Genetic organization of the mating-type loci in smut fungi. Genes are indicated by arrows with the arrow denoting direction of transcription. Related genes are denoted by the same pattern, and respective gene functions are explained in the lower part of the figure. Asterisks (*) indicate that the relative order and orientation of these genes have not been determined. In U. maydis and S. reilianum the a- and b-specific sequences reside on different chromosomes, while they are linked in U. hordei by spacer regions (which are not drawn to scale and whose length is indicated). The black bars on top of the figure indicate the region of the a locus (that expands to different lengths in the different loci, indicated by a broken line) from the lba gene to the rba gene and the region of the b locus, which covers the two homeodomain protein genes, bE and bW. Sequence information was obtained from the following accession numbers: AF043940, AM118080, AACP01000083, AACP01000013, AJ884588, AJ884583, AJ884590, AJ884585, AJ884589, AJ884584, U37796, M84182, AF184070, AF184069, and Z18531.

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Figure 22.1

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Figure 22.2

Pheromone-triggered signaling cascades in U. maydis. Lipopeptide pheromones Mfa1/2 bind to their cognate seven-transmembrane receptors present on either a1 (Pra1) or a2 (Pra2) cells. The left section displays the MAPK module consisting of Kpp4/Ubc4, Fuz7/Ubc5, and Kpp2/Ubc3 and includes Ras2 and Ubc2 as likely upstream components funneling into the MAPK module. The right-hand side depicts the known components of the cAMP signaling cascade. PKA signaling and MAPK signaling converge on Prf1. Phosphorylation of Prf1 by Adr1 and Kpp2/Ubc3 is indicated by black and gray circles, respectively, and the genes activated by the different forms are indicated. The transcriptional regulation of prf1 is shown in the lower left section, with triangles denoting binding sites for Rop1 (RRS), black bars denoting binding sites for Prf1 (PRE), and an open bar denoting the upstream activating sequence (UAS). Broken lines depict hypothetical interactions. Details are described in the text.

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Figure 22.2

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Tables

Mating in the Smut Fungi: From a to b to the Downstream Cascades

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Table 22.1

Targets of regulation by the bE/bW heterodimer

Table 22.1

Targets of regulation by the bE/bW heterodimer