P-166 - The small secreted protein and avirulence factor AvrStb6 interferes with bacterial growth during plant infection

Zymoseptoria tritici, the causal agent of septoria tritici blotch (STB), is a major wheat pathogen in Europe. While the wheat resistance gene Stb6 detects the fungal effector AvrStb6, strong selection pressure has not led to its loss, suggesting a critical role in virulence. Structural predictions (AlphaFold3) and sequence analysis (AMAPEC v1.0, FoldSeek) indicate AvrStb6 is a putative antimicrobial peptide (AMP) with cysteine-rich domains resembling bacterial predatory proteins. Electrostatic potential analysis revealed two positively charged regions, a characteristic feature of AMPs that interact with bacterial membranes. To experimentally validate this, an electrochemiluminescence-based liposome assay confirmed AvrStb6’s pore-forming activity and induction of a significant ECL emission. Growth inhibition assays further demonstrated AvrStb6 inhibition of bacterial growth, particularly against wheat phyllosphere bacteria antagonistic to Z. tritici. Amplicon sequencing from wheat apoplastic fluid infected with Z. tritici wild-type or ∆AvrStb6 mutant revealed significant microbiome shifts in the presence of AvrStb6. These findings suggest AvrStb6 modulates the wheat microbiome to enhance fungal virulence. Ongoing work aims to determine whether its antimicrobial activity also contributes to plant immune activation beyond Stb6 recognition.