The Unfolded Protein Response and its interplay with the MAPK-mediated pheromone response pathway in Ustilago maydis
by Lara Schmitz
Date of Examination:2019-07-11
Date of issue:2019-11-27
Advisor:Prof. Dr. Kai Heimel
Referee:Prof. Dr. Kai Heimel
Referee:Prof. Dr. Gerhard H. Braus
Persistent Address:
http://dx.doi.org/10.53846/goediss-7748
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Abstract
English
Ustilago maydis is a biotrophic plant pathogen that causes the corn smut disease. Sexual and pathogenic development of the fungus are tightly connected, U. maydis relies on its host, Zea mays, to complete its life cycle. Prerequisite for plant infection is the fusion of two compatible, haploid sporidia to form the infectious dikaryotic filament. Cell/cell recognition and the subsequent fusion event are regulated by the biallelic a-mating type locus. Perception of compatible pheromone triggers a G2 cell cycle arrest and conjugation tube formation. After formation of the infectious filament, the bE/bW-heterodimer, encoded by the multiallelic b-mating type locus, controls all further steps of pathogenic development, including maintenance of the G2 cell cycle arrest, filamentous growth and formation of appressoria to penetrate the plant surface. After plant penetration, U. maydis relies on the unfolded protein response (UPR), a conserved eukaryotic signaling pathway, to ensure efficient secretion of effectors to suppress the plant defense response and establish a compatible biotrophic interaction. The UPR is specifically activated in planta and continuously active during plant infection. However, premature UPR activity interferes with formation of the dikaryotic filament by inhibiting bE, bW and rbf1 expression. In this study, I show that the UPR affects pathogenic development on multiple levels. UPR activity not only has extensive inhibitory effects on the b-dependent transcription factor network, but also interferes with the transcriptional and morphological response to pheromone. UPR activity leads to increased dephosphorylation of the MAPK Kpp2 by the dual specificity phosphatase Rok1, leading to reduced activity of the pheromone response factor 1 (Prf1), and the Prf1-regulated a- and b-mating type genes. Crosstalk between the UPR and mating type pathways provide a regulatory feedback mechanism to dampen the a- and b-mating type pathways. In this way, hypervirulence is prevented and the compatible biotrophic fungal/plant interaction is preserved. Since effector gene expression is regulated by the b-pathway, increased UPR activity results in reduced effector gene expression, thereby effectively lowering endoplasmic reticulum (ER) stress levels. Moreover, I established a novel expression system, allowing the depletion or overexpression of any given gene at defined stages of biotrophic development in planta. Proof-of-principle experiments demonstrated that a functional UPR is not only required directly after plant penetration, but also crucial to ensure continuous suppression of plant defense responses at later stages of pathogenic development in planta. In summary, I uncovered novel regulatory cross-connections between pathways regulating the ER stress response and pathogenic growth of the fungus in planta. My results strongly suggest that efficient host colonization and maintenance of fungal biotrophy are balanced by negative feedback loops between pathways, aligning the intracellular physiology to the organismic interaction between the fungus and its host plant.
Keywords: Ustilago maydis; Unfolded Protein Response (UPR); MAPK; phosphatase; mating type signaling; conditional gene expression; pathogenicity; pathogenic development
