P-426 - Structural and functional insights into MycFOLD effectors in arbuscular mycorrhizal fungi

Plant-associated organisms use various strategies to overcome plant immunity, including the secretion of effector proteins. While extensively studied in pathogenic fungi, the role of effectors in symbiotic interactions, particularly in arbuscular mycorrhizal fungi (AMF), remains largely unexplored. We had employed protein structure modeling to identify secreted proteins in Rhizophagus irregularis that resemble Fusarium oxysporum FOLD (Fusarium oxysporum f. sp. lycopersici dual domain) effectors. Although rare in pathogens, MycFOLDs in AMF belong to expanded gene families. Our RNA-seq analysis had revealed high MycFOLD expression during R. irregularis-host interactions, and in situ hybridization had confirmed MycFOLD transcript accumulation in arbuscules within plant root cells.

We have now carried out a functional characterization using transgenic Medicago hairy roots overexpressing MycFOLDs and demonstrated enhanced AMF colonization, suggesting a positive role in symbiosis. Structural comparisons and in vitro assays indicated that FOLD/MycFOLDs are similar to carbohydrate-binding effectors and can bind fungal and plant cell wall carbohydrates. Additionally, transient expression in Nicotiana leaves showed that MycFOLDs promote cell expansion when localized in the apoplast. These findings suggest MycFOLD effectors facilitate AMF colonization by interacting with carbohydrates, reducing plant immune recognition, and promoting hyphal progression in the apoplast.