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.
Highlights
A crucial step for phytopathogenic fungi to initiate infection of their hosts is the penetration of the plant cuticle
By yeast two-hybrid screening, using the mitogen-activated protein (MAP) kinase Kpp6 from U. maydis as bait, we identified the C-terminal domain of
To determine the localization of Sho1 protein in U. maydis, a green fluorescent protein tag was C-terminally fused to sho1 in the native locus in the solopathogenic strain SG200
Summary
A crucial step for phytopathogenic fungi to initiate infection of their hosts is the penetration of the plant cuticle. The most common strategy for entry into the plant tissue is the development of specialized infection structures known as appressoria (Tucker and Talbot, 2001). The cues responsible for this differentiation range from chemical signals, such as ethylene, epicuticular waxes, and cutin monomers to the physical nature of the surface, such as hydrophobicity, hardness, and topography (Tucker and Talbot, 2001; Kumamoto, 2008). Processing of these signals depends on conserved signaling cascades.
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