Putrescine mitigated heavy metal toxicity in sweet potato by coordinating osmotic regulation, redox balance, photosynthetic protection, detoxification pathways, and metal sequestration

Heavy metal (HM) contamination by chromium (Cr), nickel (Ni), and cadmium (Cd) severely threatens soil health, crop productivity, and food safety. Sweet potato, an important crop, is highly vulnerable to HM-induced oxidative damage and growth inhibition. Putrescine (Put) is a low-cost, effective biostimulant, yet its role in sweet potato under HM stress remains unknown. This study provides the first comprehensive evaluation of Put-mediated HM (Cr, Ni, and Cd) stress tolerance in vegetable sweet potato. Using multiple analyses, we determined that Put (200 µM) significantly alleviated HM-induced reductions in growth and biomass, root architecture, and leaf water content. Put promoted osmotic adjustment by elevating compatible solutes and upregulating key genes in proline ( P5CR ), glycine betaine ( BHD ), and sugar metabolism ( SPS, SUS , and T6PS ). It also re-established redox homeostasis by activating the ascorbate-glutathione cycle and key antioxidant enzymes. Put also preserved photosynthetic performance by maintaining pigment content, gas exchange, PSII photochemistry, stomatal traits, and chloroplast ultrastructure, supported by the enhanced expression of CAB7 and CAB13 . Simultaneously, Put strengthened antioxidant, phenylpropanoid, and detoxification systems by increasing phenolic and flavonoid accumulation, upregulating PAL and ANS , enhancing the activities of GST, GPX, and glyoxalase enzymes (Gly I and Gly II), and increasing DPPH scavenging capacity, while reducing ROS, lipid peroxidation, and methylglyoxal. Importantly, Put markedly reduced Cr, Ni, and Cd uptake and translocation while increasing phytochelatins, thereby lowering the internal burden of HMs. These findings provide a mechanistic basis for Put-based strategies to reduce HM accumulation and support safer sweet potato production in HM-contaminated soils.