P-189 - Microbial gatekeepers - beneficial fungi use effectors to mitigate pathogen-induced damage in plants

The beneficial fungal root endophyte Serendipita indica (Si) acts as a microbial gatekeeper, modulating interactions between plants and microbes. Si engages in direct antagonism with phytopathogens and significantly reduces the infection rate of Bipolaris sorokiniana (Bs) in host plants such as barley and Arabidopsis. Here we show that Bs follows tissue-specific colonization strategies, relying on distinct genetic pathways in leaves, while in roots, infection occurs independently of these genes and is associated with oxidative stress and ferroptotic-like cell death. We identify SiDLD1, a fungal effector from the Si DELD gene family, as a key player in disease mitigation. SiDLD1 is upregulated during both beneficial plant interactions and antagonistic confrontation with Bs. Functional assays reveal that SiDLD1 suppresses pathogen-induced stress responses in roots, including ROS accumulation, lipid peroxidation and tissue browning, thereby mitigating disease symptoms and restricting Bs proliferation. Additionally, SiDLD1 inhibits Bs spore germination and reproduction, highlighting its role in regulating pathogen fitness. Beyond plant-microbe interactions, SiDLD1 also acts as a broad-spectrum radical scavenger, suppressing lipid peroxidation and ferroptosis in both plant and mammalian cells. These findings provide new insights into how beneficial fungi protect plants from pathogen-induced damage and contribute to maintaining microbial balance in the rhizosphere.