Sulfur nanoparticles enhance Cd-phytoremediation in <i>Salix chaenomeloides</i> via alleviating phytotoxicity and modulating rhizosphere microbiota

Sulfur application can enhance plant tolerance to toxic metal stress, but the the underlying physiological and microecological mechanisms in woody plants remain poorly understood. In this study, Salix chaenomeloides was treated with sulfur nanoparticles (S NPs) at dosages of 60 and 120 mg∙kg-1 under cadmium (Cd) stress. We evaluated the in planta physiological responses and rhizosphere microbial processes mediating Cd tolerance and phytoremediation efficiency. Sulfur application (sulfate and S NPs) significantly stimulated Salix growth, increasing above-ground and root biomass by 15.2-28.6% and 26.9-49.5%, respectively, compared to Cd alone treatment. Notably, S NPs outperformed sulfate in alleviating Cd-induced oxidative damage, reducing malondialdehyde and superoxide anion contents by 22.4-23.8% and 82.9-84.5%, respectively, compared to Cd alone. Under Cd stress, 120 mg∙kg-1 S NPs application significantly increased glutathione and oxidized-glutathione contents by 8.96% and 33.0%, respectively, compared to Cd alone. S NPs significantly upregulated the expression of genes involved in metal transport and sulfur metabolism. The abundance of the Cd tolerance gene czcC and sulfur transformation genes (cysH, dsrA and soxB) was significantly increased by S NPs application under Cd stress. The expression levels of functional genes linked to the S-cycle showed a noteworthy positive relationship with the translocation factor of Cd in shoots and the total accumulation of Cd. These findings demonstrate that S NPs represent a safe and sustainable strategy to enhance Cd phytoextraction and mitigate phytotoxicity in Salix.