Sho1 and Msb2-Related Proteins Regulate Appressorium Development in the Smut Fungus Ustilago maydis    

Abstract

The dimorphic fungus Ustilago maydis switches from budding to hyphal growth on the plant surface. In response to hydrophobicity and hydroxy fatty acids, U. maydis develops infection structures called appressoria. Here, we report that, unlike in Saccharomyces cerevisiae and other fungi where Sho1 (synthetic high osmolarity sensitive) and Msb2 (multicopy suppressor of a budding defect) regulate stress responses and pseudohyphal growth, Sho1 and Msb2-like proteins play a key role during appressorium differentiation in U. maydis. Sho1 was identified through a two-hybrid screen as an interaction partner of the mitogen-activated protein (MAP) kinase Kpp6. Epistasis analysis revealed that sho1 and msb2 act upstream of the MAP kinases kpp2 and kpp6. Furthermore, Sho1 was shown to destabilize Kpp6 through direct interaction with the unique N-terminal domain in Kpp6, indicating a role of Sho1 in fine-tuning Kpp6 activity. Morphological differentiation in response to a hydrophobic surface was strongly attenuated in sho1 msb2 mutants, while hydroxy fatty acid-induced differentiation was unaffected. These data suggest that Sho1 and the transmembrane mucin Msb2 are involved in plant surface sensing in U. maydis.

Programmed cell cycle arrest is required for infection of corn plants by the fungus Ustilago maydis

“…adopting the appressorium differentiation program). We found that both WT and single mutants were able to produce appressoria at a ratio comparable to those found in other studies for these in vitro conditions (Berndt et al, 2010;Freitag et al, 2011;Lanver et al, 2010;Mendoza-Mendoza et al, 2009). In the hsl1 tef1 chk1Δ strain, however, we rarely found GFP-positive filaments (Fig.…”

“…The appressorium formation resulted in a localized area of secretion, where plant cell wall-degrading enzymes that help the penetration of the cuticle (Schirawski et al, 2005) and specific effector proteins required for the precise signaling occurring during infection (Djamei and Kahmann, 2012) are concentrated. As a consequence, appressorium formation is essential during the infective process in U. maydis, and mutants affected in this step are severely impaired in their virulence (Berndt et al, 2010;Fernandez-Alvarez et al, 2012;Freitag et al, 2011;Lanver et al, 2010). Therefore, it appears that the lack of virulence of non-cell cycle-arrested filaments is a consequence of the inability to produce appressoria.…”

“…Mutants lacking these elements, or those unable to process the surface receptors correctly (i.e. showing defects in glycosylation), are severely impaired in their capacity to produce appressoria and subsequently show dramatic defects in virulence (Fernandez-Alvarez et al, 2012;Lanver et al, 2010). Cell cycle regulation could affect the transmission pathway or it could interfere with elements downstream of the signaling cascade.…”

“…Eventually, the hypha stops polar growth in response to unidentified plant signals and forms the appressorium, a structure that adheres to the plant surface to achieve penetration (Snetselaar and Mims, 1993). Appressorium formation is mandatory for the infection to proceed, as U. maydis mutant strains unable to produce functional appressoria are avirulent (Berndt et al, 2010;FernandezAlvarez et al, 2009;Freitag et al, 2011;Lanver et al, 2010). Once the filament enters the plant, the cell cycle is reactivated.…”

Mitogen-Activated Protein Kinase Signaling in Plant-Interacting Fungi: Distinct Messages from Conserved Messengers

“…Many questions regarding fungal MAPK signaling remain unanswered, and upcoming studies should, for instance, focus on the understanding of upstream mechanisms leading to MAPK activation. Recently, homologs of the yeast osmosensors Sho1 and Msb2 (Figure 1) have been shown to act upstream of pathogenesis-related MAPK cascades in U. maydis ( Figure 5; Lanver et al, 2010), M. oryzae (Figure 6; Liu et al, 2011), and F. oxysporum (Pérez-Nadales and Di Pietro, 2011). Along with the previously identified G protein-coupled receptor Pth11 from M. oryzae (Figure 6; DeZwaan et al, 1999;Kulkarni et al, 2003), these candidate receptors likely ensure MAPK cascade activation following recognition of specific ligands.…”