SNAP18 Truncation Triggers a Competitive Binding Switch Between NSF and ATG8f, Balancing Vesicular Trafficking and Autophagy for SCN Resistance in Soybean

Soybean cyst nematode (SCN, Heterodera glycines) poses the most devastating biotic threat to global soybean production. Traditional SCN resistance mediated by the Rhg1 locus predominantly relies on gene copy number expansion and elevated -SNAP protein dosage. Here, we report a novel resistance mechanism in a single-copy rhg1-c background, wherein a C-terminal 24-amino-acid truncation of SNAP18 (designated SNAP18lmm3) triggers a functional switch from vesicular trafficking to autophagic degradation. Biochemical assays and structural modeling demonstrate that this truncation severely impairs the canonical interaction between SNAP18 and N-ethylmaleimide-sensitive factor (NSF), disrupting SNARE complex recycling and inducing localized cytotoxicity. Concomitantly, the truncated SNAP18lmm3 exposes a binding interface for the autophagy-related protein ATG8f, routing the aberrant protein for selective autophagic clearance. This constitutively activated autophagic flux acts as a systemic detoxification system, preventing widespread cell death and ensuring normal plant growth under non-stressed conditions. Upon SCN infection, SNAP18lmm3 specifically hyper-accumulates within nematode-induced syncytia. This accumulation reaches levels fourfold higher than in adjacent cells, which overwhelms the local autophagic capacity and triggering targeted cell death that arrests nematode development. By elucidating this competitive molecular switch between NSF and ATG8f binding, our study establishes a "self-degrading toxin" model that resolves the inherent trade-off between plant growth and immunity. This work provides a new theoretical framework for engineering cellular homeostasis to enhance durable crop resistance against parasitic nematodes.