How biotrophic smut fungi manage to colonize plants

Smut fungi are a large group of biotrophic pathogens that infect cereal crops and grasses. The best studied member of this group, Ustilago maydis, infects maize and induces characteristic tumor formation and anthocyanin induction. During host colonization, U. maydis establishes an extended interaction zone with the plant in which fungal hyphae are completely encased by the host plasma membrane. Interaction with the plant is largely determined by protein effectors that are conventionally secreted. Such effectors suppress host defense reactions and reprogram the host to benefit the pathogen.  Many of these effectors are novel, exist only in related smut fungi and locate to clusters in the genome. Effectors either function in the interaction zone by inhibiting plant enzymes, or translocate into host cells and modulate the activity of plant processes directly. In my presentation I will concentrate on the functional analysis of the largest effector gene cluster of U. maydis, encoding 24 effector genes. Among these are several effectors that contribute to virulence. I will focus on two of these: Tin3, an effector for which we propose a dual function in the interaction zone as well as inside plant cells, and Tin2, a transferred effector responsible for inducing anthocyanin biosynthesis. I will describe their sites of action and how and why they modulate host functions after uptake.