Integrated Transcriptomic and Metabolomic Analyses Reveal Root-Centered Regulatory Networks Conferring Cadmium Tolerance in Salix

Cadmium (Cd) is a major environmental pollutant due to its high mobility and persistence in soils, facilitating entry into the food chain and threatening ecosystems and human health. However, the mechanisms that enable Salix species, well adapted for Cd remediation, to both tolerate and accumulate Cd remain elusive. Here, two Salix genotypes with contrasting Cd tolerance were examined under control and Cd stress using integrated physiological, transcriptomic, and metabolomic analyses of roots and leaves. The Cd-tolerant genotype (Salix suchowensis P294) maintained biomass under Cd stress, whereas the Cd-sensitive genotype (Salix sinopurpurea × Salix integra P646) showed a ~17% reduction. P294 accumulated more Cd in its stems (132.76 mg kg−1) and leaves (122.25 mg kg−1) than P646 (93.54 and 56.24 mg kg−1). Transcriptomics responses were stronger in roots, with 896 DEGs in P294 and 462 in P646, enriched in nitrogen metabolism, phenylpropanoid biosynthesis, and metal transport, whereas only 167 and 176 DEGs were detected in leaves for P294 and P646, respectively. Metabolomics revealed more altered metabolites in roots (125 in P294, 89 in P646), mainly organic acids, amino acids, and flavonoids, compared with leaves (46 and 66). RT-qPCR validated the root-specific upregulation of key detoxification and transport genes (ABCA7, PRX72, GSTU1, GSTU4, ZIP1). These results reveal a root-centered regulatory network underlying Cd accumulation and tolerance, integrating detoxification, redox homeostasis, and structural reinforcement, as well as providing valuable targets for genetic improvement of phytoremediation efficiency.