Identification of a small secreted protein, PlSSP, that contributes to the symbiotic association of Phomopsis liquidambaris with rice under nitrogen starvation.

Endophytic fungi are crucial for enhancing plant growth and stress tolerance. Phomopsis liquidambaris B3, a broad-spectrum endophytic fungus, significantly improves plant nitrogen uptake and growth under nitrogen-limited conditions. In this study, we identified a small secreted protein, PlSSP, which localizes to the cytoplasmic matrix of host cells and modulates plant immune responses. Using proteomic and transcriptomic approaches, we found that PlSSP upregulates key defense-related genes, including members of the PR and WRKY families, as well as genes involved in reactive oxygen species scavenging and nitrogen assimilation. Structural analysis revealed PlSSP's secondary and thermal stability features, which likely contribute to its functional interaction with host cellular components. Functional analyses demonstrated that PlSSP expression correlates with increased fungal colonization and rice biomass accumulation under nitrogen-starved conditions. These results advance our understanding of how P. liquidambaris promotes plant resilience and nutrient uptake, providing insights with potential applications in sustainable agriculture.