Salinity is a major issue threatening global food security. Among the different strategies, nanotechnology has shown tremendous potential for improving crop production under abiotic stresses such as salinity. In this review, we discuss the environmental challenges associated with the different methods of nanomaterial application, including seed nanopriming, as well as foliar and soil/root application. Based on previous research, nanopriming uses less nanomaterials and has minimal concerns regarding environmental safety and the food chain. We discuss in detail the preventive measures for the safe and sustainable use of nanomaterials in agriculture based on the application methods. Furthermore, we summarize the role of antioxidant enzyme-triggering nanomaterials and direct reactive oxygen species (ROS) scavenging nanomaterials (nanozymes) in plant salt tolerance. Nanomaterials can improve sodium (Na⁺) and potassium (K⁺) homeostasis through various anatomical, physiological, and molecular mechanisms while improving plant salt tolerance. The role of nanomaterials in modulating plant photosynthesis and hormonal balance has been largely overlooked. We also identify research gaps and provide guidelines for future research work. This review provides guidelines for helping researchers to understand the proper design of nanoparticles (NPs) and different plant-related factors while using NPs for plant stress tolerance. These considerations will help to improve the efficient delivery of NPs into plants. Furthermore, after gaining sufficient scientific knowledge and better understanding, NPs can be integral to sustainable agriculture, while saving costs and reducing biosafety concerns and environmental pollution.

Soil salinity is a big environmental issue affecting crop production. Although seed nanopriming has been widely used to improve seed germination and seedling growth under salinity, our knowledge about the underlying mechanisms is still insufficient. Herein, we newly synthesized selenium-doped carbon dots nanoparticles coated with poly acrylic acid (poly acrylic acid coated selenium doped carbon dots, PAA@Se-CDs) and used it to prime seeds of rapeseeds. The TEM (transmission electron microscope) size and zeta potential of PAA@Se-CDs are 3.8 ± 0.2 nm and −30 mV, respectively. After 8 h priming, the PAA@Se-CDs nanoparticles were detected in the seed compartments (seed coat, cotyledon, and radicle), while no such signals were detected in the NNP (no nanoparticle control) group (SeO₂ was used as the NNP). Nanopriming with PAA@Se-CDs nanoparticles increased rapeseeds germination (20%) and seedling fresh weight (161%) under saline conditions compared to NNP control. PAA@Se-CDs nanopriming significantly enhanced endo-β-mannanase activities (255% increase, 21.55 µmol h⁻¹ g⁻¹ vs. 6.06 µmol h⁻¹ g⁻¹, at DAS 1 (DAS, days after sowing)), total soluble sugar (33.63 mg g⁻¹ FW (fresh weight) vs. 20.23 mg g⁻¹ FW) and protein contents (1.96 µg g⁻¹ FW vs. 1.0 µg g⁻¹ FW) to support the growth of germinating seedlings of rapeseeds under salt stress, in comparison with NNP control. The respiration rate and ATP content were increased by 76% and 607%, respectively. The oxidative damage of salinity due to the over-accumulation of reactive oxygen species (ROS) was alleviated by PAA@Se-CDs nanopriming by increasing the antioxidant enzyme activities (SOD (superoxide dismutase), POD (peroxidase), and CAT (catalase)). Another mechanism behind PAA@Se-CDs nanopriming improving rapeseeds salt tolerance at seedling stage was reducing sodium (Na⁺) accumulation and improving potassium (K⁺) retention, hence increasing the K⁺/Na⁺ ratio under saline conditions. Overall, our results not only showed that seed nanopriming with PAA@Se-CDs could be a good approach to improve salt tolerance, but also add more knowledge to the mechanism behind nanopriming-improved plant salt tolerance at germination and early seedling growth stage.