Silica Nanoparticles and Synthetic Communities Enhance Soybean Salt Tolerance through 5-Aminovaleric Acid-Mediated <i>Sphingomonas</i> Recruitment

We first constructed functionally complementary synthetic communities (SynC) based on functional traits and phylogenetic similarity to core taxa enriched under silica nanoparticles (SiO 2 NPs) treatment. Then, we evaluated their combined effects with SiO 2 NPs in alleviating soybean salt stress. In a 37-day pot experiment under 200 mM NaCl, combined application reduced the shoot Na + /K + ratio by 24.7%–33.2% compared to either treatment alone. Furthermore, the rhizosphere microbiome was markedly restructured, with an enrichment of Sphingomonas, which was identified as a major contributor to salt tolerance through ion homeostasis and niacin and nicotinamide metabolism. Notably, 5-aminovaleric acid (5-AVA) was significantly positively correlated to the abundance of Sphingomonas . Exogenous application of 5-AVA enriched Sphingomonas populations by 33.9% and enhanced soybean growth under salt stress. Our findings demonstrate that the combined application enhances soybean salt tolerance by altering the root exudate composition and enriching beneficial bacteria, providing a promising strategy for advancing sustainable agriculture under salt stress.