The main models are the basidiomycete fungi Ustilago maydis and Ustilago hordei, which parasitize their respective host plants maize and barley to cause so called smut disease. The pathogens establish biotrophic interactions, in which infected plant cells infected stay alive throughout the entire disease cycle. While U. hordei infections are systemic and disease symptoms are only produced in the inflorescences, U. maydis induces the formation of plant, which can appear at basically all aerial parts of the maize plant. In U. maydis infected maize plants, the metabolism is reprogrammed and carbohydrate fluxes are redirected towards the infected tissue in which massive proliferation of fungal hyphae occurs. In the U. maydis – maize interaction, we are investigating i) how fungal effector proteins modulate host immunity and ii) which host factors are required to establish compatibility.