Melatonin mitigates nickel oxide nanoparticles induced phytotoxicity in soybean by reducing metal accumulation, enhancing antioxidant defense and promoting nitrogen assimilation

Heavy metals like nickel (Ni) from anthropogenic activities damage plant growth, posing challenges to agriculture. Melatonin (ME), a potent bio-regulator, has shown promise in alleviating stress induced by heavy metals. However, the mechanisms through which ME alleviates NiO-NPs phytotoxicity remain unclear. Our results showed that NiO-NPs reduced root and shoot length as well as biomass by 14 %, 12 %, 21 %, and 14 %, respectively, compared to control. However, the combined effect of ME (75 µM) and NiO-NPs (100 mg kg^-1) significantly increased these parameters by 12-28 % compared to NiO-NPs. Moreover, co-exposure of ME (75 µM) and NiO-NPs notably decreased the Ni contents in root and shoot compared to NiO-NPs treatment. This reduction was associated with enhanced levels of phytohormones (ABA, JA, SA, and GA4) and secondary metabolite production, showing a 12-32 % improvement compared to NiO-NPs alone. ME further enhanced SOD, POD, CAT, and APX activities by 14-21 % while reducing oxidative enzymes (MDA, H₂O₂) by 17-21 %. Similarly, ME (75 µM) upregulated POD, CAT, and APX gene expression by 1.33-1.6-fold, while SOD was downregulated. Additionally, ME improved nodule formation (14 %), N₂ content (19-21 %), N₂-assimilation enzymes (UE, NR, GS, GOGAT, GDH) by 19-29 %, and nutrient balance in roots (16-24 %) and shoots (19-25 %). These findings provide insights into ME's role in mitigating NiO-NPs toxicity and enhancing N₂-acquisition in soybeans, offering strategies for sustainable agriculture.