Regulatory Mechanisms of Exogenous Selenium Reducing Lead Accumulation in Plants: Focus on Phytochelatin Synthase (PCS)

Selenium (Se) is an essential trace element for humans and animals, with nutritional functions and abiotic stress regulation capacity, and has been confirmed to alleviate heavy metal toxicity and inhibit its accumulation in crops. Soil lead contamination has become a prominent environmental safety problem in agricultural production, particularly in South China, and lead over-standard in edible crops poses irreversible threats to the human nervous system and blood circulation through food chain transmission. As an efficient exogenous antagonist, Se can comprehensively regulate the absorption, translocation, and compartmentalization of lead in soil–plant systems. This review systematically summarizes the interactive effects of soil physicochemical properties, crop genotypes, and Se speciation on plant lead uptake, and focuses on phytochelatin synthase (PCS), the core rate-limiting enzyme for intracellular heavy metal chelation, to elucidate the molecular cascade of Se-mediated PCS-dependent lead detoxification. We further outline multi-pathway agronomic Se applications for lead reduction; analyze key limiting factors, including Se concentration, application method, and rhizosphere microbial community; and discuss contradictory results and unresolved questions in existing studies. Current evidence confirms that appropriate Se treatment increases glutathione (GSH) content via antioxidant system regulation, upregulates PCS gene transcription and activity, promotes phytochelatins (PCs) polymerization, and forms stable PC-Pb complexes sequestered in vacuoles to reduce cytoplasmic lead mobility. Additionally, Se reshapes rhizosphere microbial community composition to lower soil Pb2+ bioavailability and enhances lignin and pectin biosynthesis in root cell walls to physically block root Pb2+ influx. Nevertheless, critical knowledge gaps remain unaddressed: (1) upstream signal transduction cascades triggering Se-induced differential PCS expression; (2) precise Pb2+ binding sites and affinity of PC oligomers; (3) valence-dependent disparities in selenate, selenite, and nano-Se (SeNPs) modulating PCS activity; and (4) standardized field Se fertilization protocols tailored to staple and tropical tuber crops such as cassava. This review provides systematic theoretical reference and technical foundations for dissecting Se-Pb antagonistic molecular networks, developing Se-enriched low Pb2+ functional fertilizers, and mitigating Pb2+ contamination risk in agricultural commodities.