Soil heavy metal pollution is a global environmental challenge, posing significant threats to eco-environment, agricultural development, and human health. In recent years, advanced and effective remediation strategies for heavy metal-contaminated soils have developed rapidly, and a systematical summarization of this progress is important. In this review paper, first, the anthropogenic sources of heavy metals in agricultural soils, including atmospheric deposition, animal manure, mineral fertilizers, and pesticides, are summarized. Second, the accumulation of heavy metals in crops as influenced by the plant characteristics and soil factors is analyzed. Then, the reducing strategies, including low-metal cultivar selection/breeding, physiological blocking, water management, and soil amendment are evaluated. Finally, the phytoremediation in terms of remediation efficiency and applicability is discussed. Therefore, this review provides helpful guidance for better selection and development of the control/remediation technologies for heavy metal-contaminated agricultural soils.

Arsenic (As) and selenium (Se) are two metalloids found in the environment. As it poses a significant threat to human health and plant growth due to its prevalence and toxicity, however Se is a required micronutrient for human health. In this study hydroponic experiments were performed to investigate whether endogenous Se can mitigate As toxicity in rice (Oryza sativa L.). We found that As uptake by rice roots increased by pretreatment with selenateSe(VI) or selenite Se(IV). However, co-application of arsenate As(V) or arsenite As(III) with selenate markedly reduced the uptake of As by roots. Co- or pretreatment with Se with five µM of As(V) or one µM of As(III) significantly decreased shoot As content. Conversely, Se pretreatment before the addition of five µM of As(III) or one µM of As(V) resulted in As accumulation in the shoot compared to As and Se co-application. As translocation to the shoot was lower whereas the transfer factor was higher upon the simultaneous application of Se and As compared to Se pretreatment. Se supplementation with As(III) or pretreatment increased antioxidant enzymes: Superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) increased in the root and shoot, but decreased glutathione (GSH) and malondialdehyde (MDA) contents in the shoot. Plants under As(V) treatment showed the same trend except that CAT content decreased in the root and shoot, while MDA content increased in the shoot. These results suggest that cultivating rice in the presence of Se can reduce the accumulation of toxic As in seedlings, thus ensuring the safety of this important crop for human consumption.

Selenite and phosphate share similar uptake mechanisms, as a phosphate transporter is involved in the selenite uptake process. However, the mechanism by which selenium (Se) transformation in plants is mediated by phosphorus (P) remains unclear. In this hydroponic study, the absorption, translocation, and biotransformation of Se in selenite-treated rice (Oryza sativa L.) seedlings were investigated under varying P nutrient status. The results showed that P-deficient cultivation increased the Se concentration in roots with Se-only treatment by 2.1 times relative to that of the P-normal condition. However, co-treating roots with additional P caused the Se concentration to decline by 20 and 73% compared to Se treatment alone under P-normal and P-deficient cultivation, respectively. A similar pattern was also observed in Se uptake by rice roots. With an Se-transfer factor elevated by 4.4 times, the shoot Se concentration was increased by 44% with additional P supply compared to the concentration under Se-only treatment of P deficiency; however, no significant differences were observed regarding P-normal cultivation. P deficiency increased the Se percentage by 28% within the cell wall, but reduced it by 60% in the soluble fraction of Se-only treated roots relative to that of the P-normal condition. Contrarily, compared with the Se-only treatment under P deficiency, additional P supply enhanced Se storage in the root soluble fraction by 1.3 times. The opposite tendency was observed for rice shoots. Moreover, P deficiency reduced the proportion of SeMet by 22%, but increased MeSeCys by 1.3 times in Se-only treated roots compared to those under the P-normal condition. Interestingly, MeSeCys was not detected when additional P was added to the two cultivation conditions. Unlike in the roots, only SeMet was generally detected in the rice shoots. The results demonstrate that the P nutrient status strongly affects the Se biofortification efficiency in rice seedlings by altering the Se subcellular distribution and speciation.

Wheat (Triticum aestivum L.) is a staple food worldwide, and agronomic biofortification with selenium (Se) and zinc (Zn) is a simple and effective way to increase nutrient intake. This study aimed to evaluate the combined effects of Zn and Se on the biofortification of wheat grain. Zinc sulfate (ZnSO₄·H₂O, 1.74, 2.61 and 3.48 kg Zn hm⁻²) and sodium selenite (Na₂SeO₃, 15 and 30 g Se hm⁻²) were sprayed individually or simultaneously at key stages of wheat growth (the jointing, booting, and filling stage) under field conditions. On the basis of soil Zn application, the foliar application of Zn or Se alone greatly increased grain Zn by 12.07–71.88% (up to 41.66–64.30 mg kg⁻¹), and grain Se content by 131.81–527.21% (up to 0.21–0.50 mg kg⁻¹), while the soil application of Zn had little effect on grain Zn. Compared with the foliar application of Zn or Se alone, the co-application of Se increased the grain Zn content by 1.74–16.15%, while the co-application of Zn significantly reduced grain Se content by 25.43–86.34% and the effect was more pronounced with an increase in Zn dosage. Moreover, positive correlations were found between Zn and copper (Cu) in grains, and wheat grains could provide adequate dietary intakes of manganese (Mn), Cu, and molybdenum (Mo) for humans. In summary, the soil application of 11 kg Zn hm⁻² combined with the foliar application of 2.61 kg Zn hm⁻² and 30 g Se·hm⁻² is a feasible Zn-Se co-enrichment strategy, which would provide the recommended nutrient intake (RNI) of 113.63–124.72% (female) and 68.18–74.84% (male) of Zn and 81.30–95.85% of Se.

In this study, we evaluated zinc (Zn) and selenium (Se) biofortification in strawberry fruits under substrate and soil cultivation, along with their effects on mineral element accumulation and fruit quality. To achieve this, foliar Zn (0.1% and 0.2%) and Se (0.003% and 0.006%) fertilizers were applied separately or in combination at the initial flowering stage. The Zn and Se contents in strawberry fruits increased with the spraying dosage. Compared to the control, the Zn content in the first batch of Zn-treated strawberries increased by 36.9–109% and 27.1–102% under substrate and soil cultivation, respectively, while Se increased by 313–444% and 21.3–53.3%, respectively. However, foliar Zn application could not ensure long-term sustainability as Zn in strawberries gradually decreased in the two subsequent batches, while Se was more stable. Compared to the control, the Se content in the three batches of Se2 (0.006%)-treated strawberries grown in soil increased by 32.9%, 124%, and 109%, respectively. Meanwhile, compared to Se alone, the Zn–Se combined application decreased the Se content in strawberries by 61.2–77.6% and 24.9–45.7% under substrate and soil cultivation, respectively, while low doses of Se promoted Zn enrichment (by 8.62–40.9%) and high doses inhibited it (by 13.2–28.9%) under substrate cultivation. Moreover, the copper content in strawberries under substrate cultivation after the Se1 (0.003%) treatment was significantly higher (by 75.0%) than that in the control. A positive correlation was observed between Cu and Zn contents in strawberries under both substrate and soil cultivation. A consistent positive impact was also observed on fruit quality. The Se2 (0.006%) treatment caused an increase in ascorbic acid content (by 37.2%) in strawberry fruits. The soluble sugar content increased by 36.3% after the Zn1 (0.1%) treatment. The present study provides a practical basis for the biofortification of strawberries with Zn and Se.

An in-depth understanding of Se uptake and metabolism in plants is necessary for developing Se biofortification strategies. Thus, hydroponic experiments were conducted to investigate the associated processes and mechanisms of organic Se (selenomethionine (SeMet) and selenomethionine-oxide (SeOMet)) uptake, translocation, transformation and their interaction in wheat, in comparison to inorganic Se. The results showed that Se uptake by the roots and the root-to-shoot translocation factor under the SeMet treatment were higher than those under the selenite, selenate and SeOMet treatments. The uptake and translocation of SeMet were higher than those of SeOMet within 72 h, although the differences gradually narrowed with time. The uptake of SeMet and SeOMet was also sensitive to the aquaporin inhibitor: AgNO₃ addition resulted in 99.5% and 99.9% inhibitions of Se in the root in the SeMet and SeOMet treatments, respectively. Once absorbed by the root, they rapidly assimilated to other Se forms, and SeMet and Se-methyl-selenocysteine (MeSeCys) were the dominant species in SeMet- and SeOMet-treated plants, while notably, an unidentified Se form was also found in the root and xylem sap under the SeMet treatment. In addition, within 16 h, SeOMet inhibited the uptake and translocation of SeMet, while the inhibition was weakened with longer treatment time. Taken together, the present study provides new insights for the uptake and transformation processes of organic Se within plants.