Interactions Among TOR And MAPK Signaling Pathways In Fusarium Graminearum

Target of Rapamycin signaling pathway (TOR signaling pathway) which is conserved in eukaryotes, coordinates cell growth in response to nutrient availability and environmental stresses. Our previous research has revealed that TOR signaling pathway regulates not only vegetative development and virulence but also in the responses to cell wall damaging stress in Fusarium graminearum. In current study, we identified the interaction between TOR signaling pathway and HOG pathway. Furthermore, we characterized the core components of Gpmkl signaling pathway. A better understanding of the interactions between TOR signaling pathway and MAPK signaling pathways was elucidated in this research. Results of current study include:1. Protein kinase FgSch9 was associated with the TOR signaling pathway in F.graminearum.The FgSCH9 deletion mutant (â–³FgSch9) was defective in aerial hyphal growth, hyphal branching and conidial germination. The mutant exhibited increased sensitivity to osmotic and oxidative stresses, cell wall-damaging agents, and to rapamycin, while showing increased thermal tolerance. We identified FgMaf1 as one of the FgSch9-interacting proteins, which plays an important role in regulating mycotoxin biosynthesis and virulence of F. graminearum. Co-immunoprecipitation and affinity capture-mass spectrometry assays showed that FgSch9 also interacts with FgTor and FgHogl. More importantly, both â–³FgSch9 and FgHog1 null mutant (â–³FgHogl) exhibited increased sensitivity to osmotic and oxidative stresses. This defect was more severe in the FgSch9/FgHog1 double mutant. These results indicated that TOR signaling pathway interacts with HOG signaling pathway to regulate the responses to osmotic stress and oxidative stress..2. Rag-GTPase FgGtrl and FgGtr2 forms a complex which is associated with TOR signaling pathway in F.graminearum. FgGtrl-FgGtr2 complex regulates fungal development, pathogenicity, and DON biosythesis in F.graminearum. Co-immunoprecipitation and affinity capture-mass spectrometry assays showed that FgGtrl-FgGtr2 complex also interacts with FgTap42(a downstream component of TORC1) and FgHog1. Furthermore, both â–³FgGtrl and â–³FgGtrl mutants exhibited increased tolerance to fungicide fludioxonil, which is an activator of the HOG pathway. These results further indicated the interaction between TOR signaling pathway and HOG signaling pathway.3. In the budding yeast, Shol works as a signaling sensor upstream of HOG pathway. In current study, we found that FgSho1 was not associated with the HOG signaling pathway, but was required for conidiation, full virulence, and DON biosynthesis in F. graminearum. Furthermore â–³FgShol exhibited an increased sensitivity towards cell wall damaging compounds. In addition, FgShol positively regulate the phosphorylation of FgMgvl(a core kinase of the CWI pathway). The yeast two-hybrid, co-immunoprecipitation, co-localization and affinity capture-mass spectrometry analyses strongly indicated that FgShol physically interacts with the MAPK module FgSte50-Stell-Ste7. Similar to the FgShol mutant, the mutants of FgSte50, FgSte11, and FgSte7 were defective in conidiation, pathogenicity, and DON biosynthesis. FgShol and the MAPK module positively regulate the phosphorylation of FgGpmkl. These results indicated the FgShol serves as a sensor of the Gpmkl and CWI pathways.4. We identified the transcription factor FgSte12 downstream of the Gpmkl pathway. The FgSTE12 deletion mutant (â–³FgSte12) was impaired in virulence and in secretion of cellulase and protease although it did not show recognizable phenotypic changes in hyphal growth, conidiation, and deoxynivalenol (DON) biosynthesis. Furthermore, we found that FgGpmkl controls the nuclear localization of FgSte12.These results indicated that the Gpmkl pathway regulates pathogenicity via the transcription factor FgSte12 in F.graminearum.Results of our study indicate that the TOR signaling pathway interacts with the MAPK signaling pathways to regulate DON biosynthesis, pathogenicity, and responses to various environmental stresses in F.graminearum.