Selenium (Se) is an indispensable trace element for humans and other animals. Various studies have demonstrated the beneficial effects of Se on plants, including the promotion of growth, accumulation of secondary metabolites, and enhancement of antioxidant capacity, thereby improving plant stress resistance. Consequently, Se biofortification has emerged as an effective strategy to elevate Se content and nutritional quality in plants, attracting widespread attention. The mechanism of selenium (Se) at the plant secondary metabolic level has not yet been fully elucidated, and it remains an unanswered question as to how selenium affects plant secondary metabolic pathways and how these metabolic pathways respond to selenium biofortification. Although it has been shown that selenium can affect the antioxidant system and defense mechanisms in plants, detailed mechanisms of selenium’s action on plant secondary metabolic pathways, including its effects on specific metabolic enzymes and regulatory genes, still need to be revealed by further in-depth studies. The present study aims to elucidate the mechanisms of Se absorption, transport, and metabolism in plants under Se-rich conditions and to investigate the impact of various Se biofortification methods on the content of plant secondary metabolites. By integrating existing research progress, this paper will delve into the potential molecular regulatory mechanisms of Se on plant secondary metabolism, aiming to unravel the interplay between Se and plant secondary metabolism. This study provides a novel perspective and direction for future research on plant secondary metabolism and the biological utilization of Se.

The exceptional physicochemical properties of selenium nanoparticles (SeNPs) have led to their widespread development. The function of SeNPs is significantly influenced by their shape and particle size, which are in turn determined by the applied synthesis methods. This work presents a critical and comparative analysis of physical, chemical, and biosynthetic methods. The key point is to elaborate on how different methods precisely regulate the particle size, morphology, and stability that are crucial to their functional efficacy. This work emphasizes the importance of creating standardized protocols for characterizing SeNPs in order to make meaningful comparisons between the effectiveness of various studies. We further elucidate the underlying mechanisms of SeNPs’ anti-tumor, antioxidant, and antibacterial activities. A key novelty of this work lies in its systematic construction of a bridge between the synthesis, properties, functions, applications, and translational potential and its provision of a critical assessment. Finally, the review identifies and summarizes the principal challenges hindering clinical and commercial translation, including the imperative for standardized toxicological evaluation, scalable synthesis, and regulatory alignment.