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Abstract

Plant pathogenic fungi colonizing living plant tissue secrete a cocktail
of effector proteins to suppress plant immunity and reprogramme host
cells. Although many of these effectors function inside host cells,
delivery systems used by pathogenic bacteria to translocate effectors
into host cells have not been detected in fungi. Here, we show that five
unrelated effectors and two membrane proteins from Ustilago maydis, a
biotrophic fungus causing smut disease in corn, form a stable protein
complex. All seven genes appear co-regulated and are only expressed
during colonization. Single mutants arrest in the epidermal layer, fail
to suppress host defence responses and fail to induce non-host
resistance, two reactions that likely depend on translocated effectors.
The complex is anchored in the fungal membrane, protrudes into host
cells and likely contacts channel-forming plant plasma membrane
proteins. Constitutive expression of all seven complex members resulted
in a surface-exposed form in cultured U. maydis cells. As orthologues of
the complex-forming proteins are conserved in smut fungi, the complex
may become an interesting fungicide target.
This study reports that five effectors and two transmembrane proteins
from the plant pathogenic fungus Ustilago maydis form a stable cell
surface-exposed protein complex required for virulence.