Polysaccharide PSLP-1 from Polygonatum sibiricum Stems and Leaves Alleviates Depressive-like Behaviors and Modulates Gut Microbiota and Tryptophan Metabolism along the Gut–Brain Axis

The stems and leaves of Polygonatum sibiricum Red. are underused food byproducts whose polysaccharides and bioactivities remain poorly characterized. In this study, a novel homogeneous polysaccharide, PSLP-1 (4.287 kDa), was isolated and identified as a fructan-rich fraction with a →1)-β-d-Fruf-(2→ backbone and C6 side-chain substitution. In a chronic restraint stress mouse model, PSLP-1 showed clear antidepressant-like activity by reducing immobility time in both the forced swim and tail suspension tests. PSLP-1 was also associated with restoration of tryptophan metabolism, characterized by suppression of the indoleamine 2,3-dioxygenase 1-mediated kynurenine pathway and promotion of tryptophan hydroxylase-related 5-hydroxytryptamine synthesis. These effects were accompanied by gut microbiota remodeling, including increased Akkermansia and decreased Lactobacillus and Alistipes. These findings support P. sibiricum stems and leaves as a promising source of bioactive polysaccharides.