P-205 - Powdery mildew requires endogenous RALF peptide function for successful host colonization

The receptor kinase FERONIA (FER) is a susceptibility factor for biotrophic powdery mildew fungi in Arabidopsis thaliana, but the molecular mechanisms underlying its role remain largely unknown. FER perceives endogenous RAPID ALKALINIZATION FACTOR (RALF) peptides, which regulate plant growth, development, and immunity through FER-LORELEI-LIKE GPI-ANCHORED PROTEIN (LLG) heterocomplexes. In addition, RALF peptides interact with LEUCINE-RICH REPEAT EXTENSIN (LRX) proteins, structural components of the cell wall. We demonstrate that FER’s RALF ligands are essential for successful powdery mildew colonization. Our findings reveal that both LLGs and LRXs contribute to fungal susceptibility and that RALF-mediated susceptibility arises from a combination of peptide signaling and cell wall-related effects. Moreover, we show that apoplastic pH homeostasis and cell wall integrity—key features of RALF function—facilitate powdery mildew reproduction. Unlike other pathogens that exploit FER for infection, powdery mildew fungi do not produce functional RALF mimics, underscoring the direct role of host RALFs in pathogenesis. Interestingly, RALF-dependent powdery mildew susceptibility is partially independent of FER. Additionally, resistance in ralf and fer mutants is not linked to disruptions in salicylic acid or jasmonic acid signaling, two key defense-related phytohormones. Collectively, our study identifies RALF signaling as a crucial mechanism for powdery mildew pathogenesis.