Seed Nanopriming as a Strategy to Mitigate Salinity Stress: Physiological, Biochemical, and Molecular Mechanisms

Salinity is a significant abiotic factor limiting agricultural productivity worldwide, affecting nearly one-third of global agricultural land. The problem is becoming more severe through climate change, improper irrigation, and urbanization-induced soil alkalization. Salinity stress reduces seed germination through osmotic imbalance and ionic toxicity, which impair water uptake and metabolic activation. Seed priming technologies, including hydropriming, chemopriming, biopriming, and nanopriming, have emerged as strategies to mitigate salinity stress and improve crop establishment. Nanopriming involves soaking seeds in, or coating them with, a formulation containing nanoparticles (1-100 nm). This review discusses the mechanisms underlying nanopriming-mediated enhancement of seed performance under salinity stress. Physiologically, nanopriming improves osmotic homeostasis by regulating seed imbibition, increasing water potential, and enhancing ionic balance through reduced Na⁺ and Cl⁻ accumulation and improved K⁺ uptake. It modulates hormonal balance by decreasing abscisic acid (ABA) while increasing gibberellic acid (GA), promoting germination. Biochemically, nanopriming enhances metabolic activation through increased α-amylase activity and mitigates oxidative damage by reducing reactive oxygen species (ROS) accumulation and strengthening antioxidant defense systems, including enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR)) and non-enzymatic components. At the molecular level, nanopriming activates stress-responsive signaling pathways involving ABA, GA, ethylene, Ca²⁺ signalling, ROS-mediated regulation, mitogen-activated protein kinase (MAPK)-cascades, and endoplasmic reticulum (ER)-stress responses. These pathways upregulate genes related to aquaporins, ion transporters, osmolyte biosynthesis, antioxidant enzymes, and growth regulation. Nanoprimed seeds exhibit increasedgermination percentages, faster germination rates, improved vigour indices, enhanced growth, and increased stress tolerance. Despite these benefits, research gaps remain. Field-scale evaluations are limited, and understanding of nanoparticle fate in soil ecosystems is insufficient. Inappropriate nanoparticle parameters may induce phytotoxicity, oxidative damage, and ecological risks. Some studies report excessive ROS production and nanoparticle accumulation in plants, raising food safety concerns. While nanopriming represents an innovative strategy for enhancing seed germination under salinity stress, further research is needed to optimize formulations, establish safety thresholds, and develop standardized protocols for sustainable agricultural application.