Tip of the iceberg? Three novel TOPLESS interacting effectors of the gall-inducing fungusUstilago maydis

Summary Ustilago maydis is a biotrophic pathogen causing smut disease in maize. It secretes a cocktail of effector proteins during its biotrophic stages in the host plant, which target different host proteins. One such class of proteins we identified previously is TOPLESS (TPL) and TOPLESS RELATED (TPR) transcriptional corepressors. Here we screen 297 U. maydis effector candidates for their ability to interact with maize TPL protein RAMOSA 1 ENHANCER LOCUS 2 Like 2 (RELK2) and their ability to induce auxin signaling and thereby identified three novel TPL /TPR interacting effector proteins (Tip6, Tip7 and Tip8). Two of them, Tip6 and Tip7 contain a classical ethylene-responsive element binding factor-associated amphiphilic repression (EAR) motif and interact with maize TPL protein RELK2 in nuclear compartments, whereas Tip8 lacks known TPL interaction motifs and its overexpression in non-host plant leads to cell death indicating recognition of the effector. By using structural modeling, we show an interaction of Tip6 and Tip7 with the previously crystallized EAR motif binding domain of RELK2. Furthermore, by infection assays with an octuple deletion mutant of U. maydis , we demonstrate a role of Tips in U. maydis virulence. Our findings suggest the TOPLESS class of corepressors as a major hub of U. maydis effector proteins.