Powdery mildew fungi are important pathogens on many plant species, and they cause serious threats to crop production, which often reduce yield dramatically. The powdery mildew fungus, Blumeria graminis f.sp. hordei (Bgh), and its interaction with barley is a well-studied model system, in which many molecular details and tools are available. There are more than 500 identified Candidate Secreted Effector Proteins (CSEPs) encoded from Bgh genome, which manipulate host processes. Thereby they are essential for pathogen virulence, for instance down-regulate the defence mechanisms in the plant and influence its membrane trafficking processes. However, the CSEPs targets and the process of manipulation are still little known. In this PhD study, yeast 2-hybrid (Y2H) was performed to screen an existing barley cDNA library. The barley multivesicular body-associated MON1 was identified as an interactor of CSEP0162, which was confirmed in planta by BiFC and subcellular colocalization in barley protoplast. MON1 are highly conserved protein in a stable expression level. The complex of MON1 and CCZ1 functions as GEF of the multivesicular Bodies (MVBs) Rab7 protein and mediates the Rab5-Rab7 transfer of MVBs. In this study, I validated that MON1 was important for plant resistance to powdery mildew. Knockdown of HvMON1 inhibited the encasement formation, and HvMON1 was required for resistance and hypersensitive response mediated by the CNL, Mla3. These conclusions were also verified in Arabidopsis. Interestingly, I was able to show that the petiteness and lethality of Arabidopsis mutant mon1-1 (Col-0) was partially immunity-dependent. In summary, this work shows the importance of MON1involved membrane trafficking process for plant resistance and suggests a model where CSEP0162 manipulates the process of membrane trafficking in cytosol to hinder plant immunity.