Mutualistic fungi of the order of Sebacinales colonize various host plants, including economically important crops like barley. Colonization leads to improved plant growth, development and increases grain yield. Additionally, the fungi afford protection against the soil-borne plant pathogen Bipolaris sorokiniana (Bs). Bs also infects and causes disease symptoms in roots of the model organism Arabidopsis thaliana, enabling us to develop a set-up to dissect the influence of host genetic factors on disease severity and plant growth. My research focusses on understanding how host genetic factors impact the benefical effects that are exerted by mutualisitc fungi and dissect their mechanistical function.