Recent advances in the role of selenium and nanoselenium in modulating plant defense under biotic and abiotic stresses.

Climate change-driven abiotic and biotic stresses are emerging as a major threat to crop productivity and global food security. Consequently, there is a great need to develop effective and ecologically sustainable strategies to augment the growth, development, and yield of crop plants, especially under adverse environmental conditions. Nanoparticle-mediated precise modulation of soil-plant interactions has emerged as an eco-friendly, biocompatible, and stimulus-responsive strategy to increase crop production by ameliorating abiotic and biotic stresses. Selenium (Se), a trace element, has emerged as a promising mitigator of diverse stresses, including heavy metal toxicity, salinity, drought, pathogens, and pests. The use of selenium nanoparticles (SeNPs) has emerged as a potential strategy to enhance plant stress resilience, due to their increased biocompatibility, reduced toxicity, and greater stability, which provide advantages over inorganic forms of Se. SeNPs are readily absorbed by plants through root hairs and thereby improve plant growth, regulate physiological processes, stimulate antioxidants and redox balance, upregulate stress-responsive genes, and fortify stress tolerance mechanisms in plants. This review presents a comprehensive analysis of Se uptake pathways, its speciation, and incorporation into plant metabolic systems, as well as the diverse physiological and biochemical roles of Se and/or SeNPs in regulating plant defense mechanisms. A key focus is placed on SeNPs as a powerful tool for nano-enabled stress alleviation and biofortification in crop plants in modern agriculture. We further highlight how integrated multi-omics approaches are decoding the complex molecular networks underlying Se-mediated tolerance. However, the narrow optimal concentration window between benefit and phytotoxicity demands precise application. By bridging fundamental mechanisms with emerging nano-biotechnological applications, this review establishes the importance of Se and SeNPs as promising and sustainable, eco-friendly agents for developing climate-resilient crops, directly contributing to future food security.