Characterization of the Sta1 effector which decorates biotrophic hyphae
The sta1 gene encodes a virulence promoting effector which is secreted by budding cells but becomes bound to the cell wall of ﬁlamentous hyphae. This is the ﬁrst effector that shows such a preference in cell wall binding. sta1 mutants show a dramatic reduction of virulence and are altered in tissue colonization in areas close to the vascular bundles. Functional orthologues of Sta1 are found in related smut pathogens infecting monocot and dicot plants. When Sta1 is constitutively expressed, fungal ﬁlaments become susceptible to Congo red, β‐glucanase, and chitinase, suggesting that Sta1 alters the structure of the fungal cell wall. Constitutive or delayed expression of sta1 during plant colonization negatively affects virulence, implicating that Sta1 protein is needed during a speciﬁc stage of colonization. The modiﬁcation of the hyphal cell wall by Sta1 may be necessary to protect hyphae from being recognized by the plant immune system and to allow accommodation of U. maydis in speciﬁc leaf tissues.