Mechanisms of Fe biofortification and mitigation of Cd accumulation in rice (Oryza sativa L.) grown hydroponically with Fe chelate fertilization

Synergistic effects of silicon and goethite co-application in alleviating cadmium stress in rice (Oryza sativa L.): Insights into plant growth and iron plaque formation mechanisms

Rice is one of the most important staple food crops; however, it is prone to cadmium (Cd) accumulation, which has negative health effects. Therefore, methods to reduce Cd uptake by rice are necessary. At present, there is limited research on the effects of co-application of silicon (Si) and goethite in mitigating Cd stress in rice. Furthermore, the specific mechanisms underlying the effects of their combined application on iron plaque formation in rice roots remain unclear. Therefore, this study analyzed the effects of the combined application of Si and goethite on the biomass, physiological stress indicators, Cd concentration, and iron plaques of rice using hydroponic experiments. The results revealed that co-treatment with both Si and goethite increased the plant height and dry weight, superoxide dismutase and catalase activities, photosynthetic pigment concentration, and root activity. Moreover, this treatment decreased the malondialdehyde concentration, repaired epidermal cells, reduced the Cd concentration in the roots by 57.2 %, and increased the number of iron plaques and Cd concentration by 150.9 % and 266.2 % in the amorphous and crystalline fractions, respectively. The Cd/Fe ratio in amorphous iron plaques also increased. Our findings suggest that goethite serves as a raw material for iron plaque formation, while Si enhances the oxidation capacity of rice roots. The application of a combination of Si and goethite increases the quantity and quality of iron plaques, enhancing its Cd fixation capacity. This study provides theoretical evidence for the effective inhibition of Cd uptake by iron plaques in rice, providing insights into methods for the remediation of Cd contamination.

Safe utilization evaluation of two typical traditional Chinese medicinal materials in Cd-contaminated soil based on the analysis of Cd transfer and AHP model

Due to the increasing scarcity of wild resources, most traditional Chinese medicinal materials (TCMMs) in the market are produced via artificial cultivation. The widespread pollution of cadmium (Cd) in soil limits the safe cultivation and use of TCMMs. This study investigated Cd accumulation, distribution, and the medicinal component content under simulated field conditions to clarify the differences in the Cd absorption, transfer and detoxification mechanisms of Polygonatum cyrtonema Hua and Bletilla striata, and provide the preliminary safe utilization conditions of TCMMs based on the analytic hierarchy process (AHP). The results showed that the Cd content of P. cyrtonema Hua was lower than the safety threshold under a high soil Cd concentration of 0.91 mg/kg (Cd-L), while B. striata was safe only at a low Cd concentration of 0.25 mg/kg (CK). Cd at 0.91 mg/kg induced hormesis affecting the net increase in biomass and medicinal component content for both TCMMs, while P. cyrtonema Hua showed better potential for safe utilization. Additionally, P. cyrtonema Hua had stronger resistance to Cd stress, exhibiting superior characteristics for synergistic absorption of Cd with mineral elements, transfer to nonmedical part and safer fixation forms in subcellular components. In contrast, B. striata showed insufficient Cd tolerance, and Cd was easily accumulated in organelles to inhibit plant growth. Our findings may attract more attention to the safe cultivation of TCMMs and provide insight into guidance for the safe utilization of slightly Cd-contaminated soil.

Does elevated CO2 enhance the arsenic uptake by rice? Yes or maybe: Evidences from FACE experiments

Elevated CO₂ (eCO₂) strongly affects rice yield and quality in arsenic (As) paddy soils. However, understanding of the As accumulation in rice under coupled stress of eCO₂ and soil As is still limited while data are scarce. It greatly limits the prediction for future rice safety. This study investigated the As uptake by rice grown in different As paddy soils under two CO₂ conditions (ambient and ambient +200 μmol mol^-1) in the free-air CO₂ enrichment (FACE) system. Results showed that eCO₂ lowered soil Eh at the tillering stage and caused higher concentrations of dissolved As and Fe²⁺ in soil pore water. Compared with the control, the increased As transfer abilities in rice straws under eCO₂ contributed to the higher As accumulation in rice grains, and their total As concentrations were increased by 10.3-31.2%. Besides, the increased amounts of iron plaque (IP) under eCO₂ failed to effectively inhibit the As uptake by rice due to the difference in critical stage between As immobilized by IP (mainly in maturing stage) and uptake by rice roots (about 50% contribution before filling stage). Risk assessments suggest that eCO₂ enhanced the human health risks of As intake from rice grains produced in low-As paddy soils (<30 mg kg^-1). In order to alleviate the As threats to rice under eCO₂, we consider that proper soil drainage before filling stage to improve soil Eh can serve as an effective way to reduce As uptake by rice. Pursuing appropriate rice varieties to reduce the As transfer ability may be the other positive strategy.

Biochar and nano-ferric oxide synergistically alleviate cadmium toxicity of muskmelon

Cadmium is toxic to plants. The accumulation of cadmium in edible plants such as muskmelon may affect the safe production of crops and result in human health problem. Thus effective measures are urgently needed for soil remediation. This work aims to investigate the effects of nano-ferric oxide and biochar alone or mixture on muskmelon under cadmium stress. The results of growth and physiological indexes showed that compared with the application of cadmium alone, the composite treatment (biochar and nano-ferric oxide) decreased malondialdehyde content by 59.12% and ascorbate peroxidase activity increased by 276.6%. Their addition can increase the stress resistance of plants. The results of soil analysis and cadmium content determination in plants showed that the composite treatment was beneficial to reduce the cadmium content in various parts of muskmelon. In the presence of high concentration of cadmium, the Target Hazard Quotient value of peel and flesh of muskmelon in the composite treatment was less than 1, which means the edible risk was greatly reduced. Furthermore, the addition of composite treatment increased the content of effective components; the contents of polyphenols, flavonoids, and saponins in the flesh of the compound treatment were increased by 99.73%, 143.07%, and 18.78% compared with the cadmium treatment. The results provide a technical reference for the further application of biochar combined with nano-ferric oxide in the field of soil heavy metal remediation, and provide a theoretical basis for further research on reducing the toxicity of cadmium to plants and improving the edible quality of crops.

The role of nickel in cadmium accumulation in rice

Rice is one of the world's staple foods. Cadmium (Cd) levels in paddy soil are still increasing, and "Cd-contaminated rice" is a frequent occurrence, posing a serious threat to human health. Therefore, Cd contamination in rice is a key issue in agricultural production that needs to be addressed urgently. The Cd accumulation in rice is closely related to other elements. In this study, the impact of nickel (Ni) on the uptake and accumulation of Cd in rice was revealed, and the mechanism was discussed. Statistical analysis of field data showed that Cd concentration in rice grains decreased exponentially with increasing Ni concentration in paddy soils, which was verified by the hydroponic experiments. Under 5 μmol/L Cd exposure conditions, the addition of Ni (100 μmol/L) reduced the Cd contents in roots, stems, and leaves by 81.6 %, 60.6 %, and 65.9 %, respectively. With the presence of Ni, the amount of iron plaque decreased, and the Cd content in the iron plaque was reduced due to the competition between Ni and Cd for adsorption sites. In addition, the migration of Cd from stems to leaves was reduced. At the same time, the distribution of Cd in the cell was altered, and the concentration of Cd in the root cell walls increased with increasing Ni addition under 5 μmol/L Cd exposure. These findings highlight the critical role of Ni in inhibiting Cd accumulation in rice, and provide important information for understanding the effects of coexisting elements in Cd-contaminated soils on Cd accumulation in crops.

Effects of elevated CO2 on the Cd uptake by rice in Cd-contaminated paddy soils

The rising atmospheric CO₂ is a major driver for climate change, directly affects rice production. Cadmium (Cd) in paddy soils also serves as a persistent concern. Currently, few studies consider the rice response to coupled stresses of elevated CO₂ (eCO₂) and soil Cd. Experimental evidence understanding the effects and mechanisms of eCO₂ on Cd uptake by rice is lacking yet. In a free-air CO₂ enrichment (FACE) system, a 3-year pot experiment was conducted to explore the Cd uptake by rice under two CO₂ conditions (ambient and ambient + 200 µmol·mol^-1) using combinations of in-situ Cd-contaminated soils and associated rice varieties. Results showed that more low-crystalline Fe oxides (Feh) in iron plaque (IP) were deposited on root surface with the increased dissolved Fe²⁺ due to lower soil redox status under eCO₂. The Cd accumulation in rice was hindered due to more Cd associated with Feh (Feh-Cd) rather than uptake by roots. Taken together, the relative effects of eCO₂ on Cd uptake by rice were consistent across years under different Cd-contaminated soils. Our findings will help to better understand the Cd uptake by rice under future climate conditions, and thus push the development of climate-crop-soil models and accurate prediction for food security.

FeCl3 and Fe2(SO4)3 differentially reduce Cd uptake and accumulation in Polish wheat (Triticum polonicum L.) seedlings by exporting Cd from roots and limiting Cd binding in the root cell walls

Wheat grown in cadmium (Cd)-contaminated soils easily accumulates more Cd in edible parts than the Chinese safety limit (0.1 mg/kg). FeCl₃ and Fe₂(SO₄)₃ have been used to extract Cd from Cd-contaminated soils. Thus, we hypothesized that FeCl₃ and Fe₂(SO₄)₃, used as iron (Fe) fertilizers, can reduce Cd uptake and accumulation in wheat. Here, a hydroponic experiment was performed with three FeCl₃ and Fe₂(SO₄)₃ concentrations under 80 μM CdCl₂ stress on dwarf Polish wheat (Triticum polonicum L., 2n = 4x = 28, AABB) seedlings. Compared with Fe deficiency, FeCl₃ and Fe₂(SO₄)₃ additions competitively reduced Cd concentrations. The reductions were not associated with changes in dry weight and root morphological parameters. FeCl₃ and Fe₂(SO₄)₃ additions reduced Cd concentrations in the following order from smallest to largest reduction: 25 μM Fe₂(SO₄)₃ < 200 μM FeCl₃ < 50 μM FeCl₃ < 100 μM Fe₂(SO₄)₃. Investigation of subcellular distributions showed that the four Fe fertilizers differentially reduced Cd binding in the root cell walls and enhanced root sucrose and trehalose. Cd chemical form analysis revealed that Fe fertilizer addition also differentially reduced root F_E, F_W, and F_NaCl. Transcriptomic analysis revealed that addition of FeCl₃ and Fe₂(SO₄)₃ differentially up-regulated several genes that hydrolyze cell wall polysaccharides and metal transporter genes for Cd uptake (IRT1 and CAX19) and export (ZIP1, ABCG11, ABCG14, ABCG28, ABCG37, ABCG44, and ABCG48) reducing Cd uptake and accumulation. Our results demonstrated that FeCl₃ and Fe₂(SO₄)₃ can reduce Cd accumulation in wheat, and 50 μM FeCl₃ is the most effective treatment.

Agronomic biofortification of food crops: An emerging opportunity for global food and nutritional security

Fortification of food with mineral micronutrients and micronutrient supplementation occupied the center stage during the two-year-long Corona Pandemic, highlighting the urgent need to focus on micronutrition. Focus has also been intensified on the biofortification (natural assimilation) of mineral micronutrients into food crops using various techniques like agronomic, genetic, or transgenic. Agronomic biofortification is a time-tested method and has been found useful in the fortification of several nutrients in several crops, yet the nutrient use and uptake efficiency of crops has been noted to vary due to different growing conditions like soil type, crop management, fertilizer type, etc. Agronomic biofortification can be an important tool in achieving nutritional security and its importance has recently increased because of climate change related issues, and pandemics such as COVID-19. The introduction of high specialty fertilizers like nano-fertilizers, chelated fertilizers, and water-soluble fertilizers that have high nutrient uptake efficiency and better nutrient translocation to the consumable parts of a crop plant has further improved the effectiveness of agronomic biofortification. Several new agronomic biofortification techniques like nutripriming, foliar application, soilless activation, and mechanized application techniques have further increased the relevance of agronomic biofortification. These new technological advances, along with an increased realization of mineral micronutrient nutrition have reinforced the relevance of agronomic biofortification for global food and nutritional security. The review highlights the advances made in the field of agronomic biofortification via the improved new fertilizer forms, and the emerging techniques that achieve better micronutrient use efficiency of crop plants.

Flooding-induced rhizosphere Clostridium assemblage prevents root-to-shoot cadmium translocation in rice by promoting the formation of root apoplastic barriers

Water managements are the most effective agricultural practices for restraining cadmium (Cd) uptake and translocation in rice, which closely correlated with rhizosphere assembly of beneficial microbiome. However, the role of the assemblage of specific microbiota in controlling root-to-shoot Cd translocation in rice remains scarcely clear. The aim of this study was to ascertain how water managements shaped rhizosphere microbiome and mediated root-to-shoot Cd translocation. To disentangle the acting mechanisms of water managements, we performed an experiment monitoring Cd uptake and transport in rice and changes in soil microbial communities in response to continuously flooding and moistening irrigation. Continuously flooding changed rhizosphere microbial communities, leading to the increased abundance of anaerobic bacteria such as Clostridium populations. Weighted gene co-expression network analysis (WGCNA) showed that a dominant OTU163, corresponding to Clostridium sp. CSP1, exhibited a strong negative correlation with root-to-shoot Cd translocation. An integrated analysis of transcriptome and metabolome further indicated that the Clostridium-secreted butyric acid was involved in the regulation of phenylpropanoid pathway in rice roots. The formation of endodermal suberized barriers and lignified xylems was remarkably enhanced in the Clostridium-treated roots, which led to more Cd retained in root cell wall and less Cd in the xylem sap. Collectively, our results indicate that the development of root apoplastic barriers can be orchestrated by beneficial Clostridium strains that are assembled by host plants grown under flooding regime, thereby inhibiting root-to-shoot Cd translocation.

Fe fortification limits rice Cd accumulation by promoting root cell wall chelation and reducing the mobility of Cd in xylem

Fe biofortification and Cd mitigation in rice is essential for human health, thus the effects of fertilization with Fe on Cd uptake and distribution in rice need to be comprehensively studied. In this study, we investigated the roles of root iron (Fe)/manganese (Mn) plaques, root cell wall, organic acid, and expressions of Cd-transport related genes in restricting Cd uptake and translocation. The rice plants were exposed to 1 μM CdCl₂ with or without the addition of three doses of Fe at 5, 50, and 500 μM EDTA-Na₂Fe. The results showed that increasing supply of Fe remarkably reduced Cd accumulation in the shoots, mainly because of inhibited translocation of Cd from roots to shoots. As compared to 5 μM Fe treatment, 500 μM Fe significantly increased the ionic soluble pectin (ISP) content and decreased citric acid (CA) in the roots, thereby providing more Cd-binding sites in the cell wall of roots and reducing the mobility of Cd in xylem. Plant Fe status-mediated CA act as the main chelator for Cd mobilization, rather than through decreasing the pH. However, the plants supplied with low Fe or excess Fe facilitated the uptake of Cd in rice roots, as low Fe up-regulated the expression of Cd-transport related genes and excess Fe enhanced Cd enrichment on the root by iron plaque. Importantly, soil fertilization with Fe strongly reduced Cd accumulation in rice grain. Thus, optimizing the soil environmental Fe could effectively reduce Cd accumulation in the shoots by immobilizing Cd in the roots.

CF1 reduces grain-cadmium levels in rice (Oryza sativa)

Rice (Oryza sativa) is a leading source of dietary cadmium (Cd), a non-essential heavy metal that poses a serious threat to human health. There are significant variations in grain-Cd levels in natural rice populations, which make the breeding of low-Cd rice a cost-effective way to mitigate grain-Cd accumulation. However, the genetic factors that regulate grain-Cd accumulation have yet to be fully established, thereby hindering the development of low-Cd varieties. Here, we reported a low-Cd quantitative trait locus, CF1, that has the potential to reduce Cd accumulation in rice grains. CF1 is allelic to the metal transporter OsYSL2, which transports Fe from the roots to the shoots. However, it is incapable of binding Cd, and thus, reduces grain-Cd levels indirectly rather than directly in the form of upward delivery. Further analysis showed that high expression levels of CF1 improve Fe nutrition in the shoots, subsequently inhibiting Cd uptake by systemically inhibiting expression of the main Cd uptake gene OsNramp5 in the roots. Compared with the CF1 allele from '02428' (CF1⁰²⁴²⁸ ), higher expression levels of CF1 from 'TQ' (CF1^TQ ) increased the Fe contents and decreased Cd levels in rice grains. In natural rice populations, CF1^TQ was found to be a minor allele, while CF1⁰²⁴²⁸ is present in most japonica rice, suggesting that CF1^TQ could be widely integrated into the japonica rice genome to generate low-Cd varieties. Overall, these results broaden our mechanistic understanding of the natural variation in grain-Cd accumulation, supporting marker-assisted selection of low-Cd rice.

Comparing the effects of calcium and magnesium ions on accumulation and translocation of cadmium in rice

Rice (Oryza sativa L.) is one of China's most important food crops, and it is considered the primary source of human exposure to cadmium (Cd) pollution. Adding calcium (Ca) and magnesium (Mg) to the plant nutrient solutions reduces the accumulation of Cd in the rice, but under the same condition, which one has the better effect remains unclear. Thus, hydroponic experiments were performed to compare the effects of Ca and Mg ions with concentration gradients (0.10, 0.25, and 0.50 g/L, respectively) on the absorption, distribution, and translocation of Cd in rice. The Cd contents of roots, stems, leaves, panicles, husks, and grains in different growth stages were determined. The results revealed that the supplementation of both Ca and Mg influenced the Cd accumulation and translocation in rice tissues. The Cd concentrations of different patterns were in the following order: roots > stems > leaves ≈ panicles ≈ husks > grains. Both of Ca and Mg had an apparent antagonism with Cd in different parts of the rice plant, and the antagonism was more obvious in the high Cd stress treatments. With the addition of 0.1 g/L Ca²⁺ and Mg²⁺ ions, the grain Cd contents increased, while the application of 0.25 and 0.5 g/L Ca²⁺ and Mg²⁺ ions reduced grains Cd by 19.08-38.99%, with the average value of 26.75%. Under the same concentrations, the grain Cd content of Ca treatments was lower than that of Mg treatments by 8.74%. In the Ca (Mg)-deficient and Ca (Mg)-sufficient conditions, the husks and panicles accumulated Cd to hinder Cd translocation, respectively. Altogether, the results of this study indicated that Ca had a greater effect for decreasing rice Cd accumulation and translocation than Mg, and the panicle and husk were the important parts for reducing Cd translocation to grain, and these might be a focal point for the future research. It was possible to plant and grow rice in Cd-polluted soil and that the accumulation and translocation of Cd in rice plants could be reduced by optimizing soil nutrient elements.

Phytoremediation of Toxic Metals: A Sustainable Green Solution for Clean Environment

Contamination of aquatic ecosystems by various sources has become a major worry all over the world. Pollutants can enter the human body through the food chain from aquatic and soil habitats. These pollutants can cause various chronic diseases in humans and mortality if they collect in the body over an extended period. Although the phytoremediation technique cannot completely remove harmful materials, it is an environmentally benign, cost-effective, and natural process that has no negative effects on the environment. The main types of phytoremediation, their mechanisms, and strategies to raise the remediation rate and the use of genetically altered plants, phytoremediation plant prospects, economics, and usable plants are reviewed in this review. Several factors influence the phytoremediation process, including types of contaminants, pollutant characteristics, and plant species selection, climate considerations, flooding and aging, the effect of salt, soil parameters, and redox potential. Phytoremediation’s environmental and economic efficiency, use, and relevance are depicted in our work. Multiple recent breakthroughs in phytoremediation technologies are also mentioned in this review.

Immobilization of cadmium in soil and improved iron concentration and grain yields of maize (Zea mays L.) by chelated iron amendments

With the development of industrial and agricultural production, the cadmium (Cd) pollution in farmland soil is increasing which threatens the quality and safety of agricultural products in China. The purpose of this research is to explore the potential of chelated iron amendments for reducing the bioavailability of Cd to maize plants. The experiment of maize cultivation was carried out in pots. Maize was cultivated in brown soil collected from a northeast region in China and then artificially contaminated with Cd with an average concentration of 10 mg/kg. The potential of ethylenediamine tetra acetic acid ferric-sodium (EDTANa₂Fe) and ethylenediamine-N, N'-bis(2-hydroxyphenylacetic acid) ferric-sodium (EDDHAFe) as amendments for Cd immobilization in soils were tested. Effects of different concentrations (0.013, 0.026, 0.053, and 0.08 g/kg) of EDTANa₂Fe and EDDHAFe (calculated by iron) on the growth of maize and the Cd and Fe uptake in maize were evaluated. Principle component analysis (PCA) was performed to further analyze the overall effects of iron amendments on Cd accumulation in plants. The results showed that EDTANa₂Fe and EDDHAFe at dose of 53 mg/kg reduced available Cd in soil to the value of 5.19 and 4.59 mg/kg, respectively. EDTANa₂Fe of 80 mg/kg and EDDHAFe of 26, 53, and 80 mg/kg increased Fe concentration in grains by 34.86%, 36.88%, 41.46%, and 44.05%, respectively, while reduced the Cd content in grains by 39.82%, 42.67%, 48.41%, and 35.08%, respectively. Fe concentration in grains showed a significant negative correlation with the concentration of Cd (P < 0.05). EDTANa₂Fe of 53 mg/kg treatment increased the maize yield by 30.45% which was higher than in all other treatments. EDDHAFe treatments of 26 and 53 mg/kg increased the total root length by 39.72% and 48.68%, respectively, and increased the total root surface area by 19.95% and 22.98%, respectively. In general, the application of EDTANa₂Fe and EDDHAFe at dose of 53 mg/kg can effectively immobilize Cd in soil and reduce Cd accumulation in maize and improve Fe uptake in grains.

Nano-Fe3O4-modified biochar promotes the formation of iron plaque and cadmium immobilization in rice root

Rice as a paddy field crops, iron-containing materials application could induce its iron plaque formation, thereby affecting cadmium (Cd) transportation in the rhizosphere and its uptake in root. In this study, a hydroponic experiment was conducted to investigate the effects of three exogenous iron materials, namely nano-Fe₃O₄-modified biochar (BC-Fe), chelated iron (EDTA-Fe), and ferrous sulfate (FeSO₄), on the iron plaque formation on the surface of rice root, and to investigate the effects of formed iron plaque on the absorption, migration, and transportation of Cd and Fe in rice plant. The results showed that yellow-brown and brown iron plaque was formed on surface cells of the Fe-treated rice root, and some black particles were embedded in the iron plaque formed by BC-Fe. The proportion of crystallized iron plaque (31.8%-35.9%) formed by BC-Fe was much higher than that formed by EDTA-Fe and FeSO₄. The Cd concentrations in the crystallized iron plaque formed by BC-Fe were 7.64-13.0 mg·kg^-1, and increased with the increasing of Fe concentrations in the plaque. The Cd translocation factor from root to stem (TF_r-s) and the Cd translocation factor from stem to leaf (TF_s-l) with BC-Fe treatment decreased by 84.7% and 80.0%, respectively. The results demonstrated that application BC-Fe promoted the formation of iron plaque and enhanced the sequestration of Cd and Fe in roots, thus reduced the transportation and accumulation of Cd in aerial rice tissues.

Iron and copper micronutrients influences cadmium accumulation in rice grains by altering its transport and allocation

Cadmium (Cd) contamination in rice paddy fields constitutes a serious health issue in some parts of China. Here we study the potential for remediation of Cd contaminated alkaline paddy soil with low iron (Fe) and high copper (Cu) background by altering the concentrations of Fe and Cu in the growing media, which has been only seldom considered. We assessed how these two micronutrients (Cu and Fe) affect the absorption and transport of Cd in rice. Adding Cu significantly increased rice biomass and grain yield by reducing root Cd influx and Cd upward transport which, consequently, lowered Cd concentrations in roots, culms and leaves. However, excessive Cu also promoted a relatively higher Cd allocation in grains, especially under Fe deficiency, likely because Cu significantly increased the proportion of bioavailable Cd in leaves. Contrastingly, Fe did not alleviate the toxic effects of Cd on rice growth and yield, but it significantly reduced Cd transfer towards grains, which might be attributed to a sharp decrease in the proportion of bioavailable Cd in leaves. Our results demonstrated that Cd remediation may be achieved through altering Fe and Cu inputs, such that Cd accumulation in rice grains is reduced.

Application of different foliar iron fertilizers for enhancing the growth and antioxidant capacity of rice and minimizing cadmium accumulation

Iron (Fe) fertilizer can reduce cadmium (Cd) uptake and toxicity in rice, but the underlying mechanisms of Cd mitigation by different fertilizers are poorly understood. Here, pot experiments in rice were conducted to characterize the effects of four types of foliar-applied Fe fertilizer (chelated ferrous Fe, ferric Fe, ionic ferrous Fe, and ferric Fe) at three doses (20, 50, and 100 mg L^-1) on photosynthetic capacity, antioxidant ability, yield, and Cd accumulation in Cd-contaminated soil. The results showed that foliar Fe application increased the net photosynthesis rate by 19.3%, peroxidase (POD) by 18.2%, superoxide dismutase (SOD) by 26.9%, and catalase (CAT) by 19.6%, and led to a 7.2% increase in grain yield compared with the control. Moreover, foliar Fe application significantly reduced Cd accumulation by 15.9% in brown rice and decreased the translocation of Cd from roots to other plant tissues. Overall, application of moderate doses (50 mg L^-1) of chelated ferrous Fe was the most effective method for reducing Cd uptake (decreasing the Cd concentration in brown rice by 29.0%) and toxicity in rice (decreasing malondialdehyde by 23.2% and increasing POD, SOD, and CAT by 54.4%, 51.6%, and 45.7%, respectively), which may stem from the fact that chelated ferrous Fe was a more stable and bioavailable source of Fe for rice. The Cd concentration in rice had negative relationship with Fe concentration, and the translocation of Cd from root to the other tissues was reduced by the higher Fe nutrition status in leaf, suggesting that a high Fe supply may decrease Cd content by inhibiting the expression of the Fe transport system. These results indicate that foliar application of chelated ferrous Fe provides a promising alternative approach for enhancing growth and controlling Cd accumulation in rice plants. Furthermore, these results advance our understanding of the associations between plant Fe nutrition status and Cd accumulation.

Effects of Fe and Mn cations on Cd uptake by rice plant in hydroponic culture experiment

Iron (Fe) and manganese (Mn) are nutritional components of rice, plays an important role in its physiological processes and can minimize absorption of cadmium (Cd) in rice. Fe, Mn, and Cd transporters such as CAL1, OsNRAMP5, OsNRAMP1, OsIRT1, OsHMA3, and OsNAAT1 regulate uptake of Cd in rice. However, the effect of exogenous application of Fe, and Mn on the accumulation of Cd and relative expression (RE) of these transporters in rice has not been investigated. Therefore, a hydroponic culture experiment was conducted to investigate the impact of Fe and Mn on Cd uptake and RE of these transporters in rice. The results showed that the Fe and Mn application significantly decreased Cd in the roots and shoots of rice. Whereas, Cd concentration in the rice significantly increased with increasing Cd concentration in the solution. The addition of manganese in the culture medium can reduce the cadmium content of rice roots by 11.9–82.3% and shoots by 11.6–85.0%, while the addition of iron in the culture medium can reduce the cadmium content of rice roots and shoots by 26–65% and 9–683% respectively. Meanwhile, application of sufficient doses of Fe and Cd in solution culture increased RE of CAL1, OsNRAMP5, OsNRAMP1, OsIRT1, and OsNAAT1 in roots, whereas expression level of OsHMA3 was decreased. Similarly, expression level of CAL1, OsNRAMP5, and OsNRAMP1 significantly increased in roots in high Cd and Mn deficient treatments. This may be concluded that the Cd increases expression of CAL1, OsNRAMP5, OsNRAMP1, OsIRT1, and OsNAAT1 but decreases OsHMA3 expression in rice roots, which resulted in increased Cd uptake in hydroponically grown rice.

The root iron transporter 1 governs cadmium uptake in Vicia sativa roots

Cadmium is a non-essential element for plants and that inhibits plant growth and development. The Zhangye Mawan (ZM) variety of Vicia sativa is more sensitive to Cd toxicity than that Lanjian 3# (L3) variety, but the underlying mechanism is not fully understood. Here, we demonstrated that ZM showed higher Cd accumulation than L3 based on root Cd content and Cd fluorescence intensity in root protoplasts. VsRIT1, a member of the ZIP (ZRT/IRT-like protein) family, showed expression levels in ZM roots 8-fold higher than those in L3 roots under Cd exposure. VsRIT1 expression increased Cd transport and accumulation in Arabidopsis and yeast. These suggests that VsRIT1 participates in Cd uptake by V. sativa roots. Furthermore, ZM root tips have a higher capacity for transient Cd influx than L3 roots when exposed to Cd alone or Cd and iron (Fe) together, owing to the higher VsRIT1 expression in ZM. Our findings also imply that Cd may compete with Fe or/and zinc (Zn) for uptake via VsRIT1 in V. sativa or yeast.

Cadmium toxicity reduction in rice (Oryza sativa L.) through iron addition during primary reaction of photosynthesis

Cadmium (Cd) pollution is a worldwide concern due to its biotoxicity. Because Cd and Fe are closely associated during plant photosynthesis, this study aims at investigating the mechanism governing Cd toxicity during photosynthetic primary reaction in rice by adjusting Fe concentration. The results show that moderate Fe concentration (1.0 g kg^-1) added to soil can increase the stomatal conductance (Gs) and SPAD value by stimulating the stomatal opening and chlorophyll synthesis. Moderate Fe concentration can also improve the maximum fluorescence (Fm) and the maximal photochemical efficiency (Fv/Fm) to keep the high reaction center activity and electronic transfer efficiency in photosystems I and II. Thus, moderate Fe can eliminate Cd-induced decrease in Gs, intercellular CO₂ concentration (Ci) and net photosynthetic rate (Pn) as well as the disorder of antioxidative system under Cd concentration of 2.0 mg kg^-1 in the soil. When its application is increased to 2.0 g kg^-1, Fe can notably decrease Pn, and result in remarkable decrease in the biomass of shoots and grains. Decrease in Pn can be mainly attributed to high Fe concentration which can greatly destroy chloroplast structure and, meanwhile, inhibit the electron transfer between acceptor and donator in photosynthetic chain especially from quinone A (Q_A) to quinone B (Q_B). Unlike the situation under moderate Fe concentration, the high Fe application cannot mitigate the Cd-induced decrease in photosynthetic index. Our results indicate that the moderate Fe application is necessary to promote rice performance and production and, in the meantime, to inhibit Cd toxicity in the extensively polluted soils.

Foliar Application of Zn-EDTA at Early Filling Stage to Increase Grain Zn and Fe, and Reduce Grain Cd, Pb and Grain Yield in Rice (Oryza sativa L.)

The accumulation of Cd and Pb in rice grains poses a potential threat to human health, which is a subject of increasing concern across the globe. We examined the effect that foliar spraying of Zn-ethylenediaminetetraacetate (Zn-EDTA) (0.3% and 0.5% w/v) during the early-grain filling stage has on rice grain yield and Cd, Pb, Zn and Fe contents in rice tissues via a field experiment. The grain yield significantly decreased with the foliar application of 0.5% Zn-EDTA. In rice grain, foliar spraying of 0.5% Zn-EDTA significantly decreased the Cd and Pb contents, but increased the Zn and Fe contents. The main reasons for the decrease in the Cd and Pb content in grain were the inhibition of Cd and Pb by roots and the increased Fe content in grain via Zn-EDTA application. The foliar spraying of Zn-EDTA decreased the grain yield and Cd and Pb contents, while increased the Zn and Fe contents in grains.

Iron fractions responsible for the variation of Cd bioavailability in paddy soil under variable pe+pH conditions

Iron (Fe) in soil is closely related to cadmium (Cd) uptake by rice plants, and soil pe + pH significantly influences Fe redox behavior. This study aimed to explore the influential mechanisms of varying pe + pH conditions on the transformation of iron oxides in the rhizosphere and the subsequent effect on Cd accumulation in rice plants. A two-month pot experiment was conducted to investigate the effect of soil pe + pH on the fractions of iron oxides and formation of iron plaque (IP), as well as the effect of these changes on Cd uptake by rice plants (Oryza sativa L.). Different irrigation strategies, 70% water holding capacity (DY), continuous flooding (FL), and alternate flooding/drying weekly (AWD), were used to achieve various soil pe + pH levels. The results showed that low pe + pH conditions (under the FL and AWD treatments) were more beneficial to the transformation of crystalline iron oxides into amorphous forms in rhizosphere soil and the precipitation of IP on rice roots. The increase of amorphous iron oxides resulted in the reduction of Cd availability in rhizosphere soil by immobilizing more Cd on Fe oxides. Moreover, Cd adsorbed on rice root surfaces reacted with IP, inhibiting Cd soil-to-root transport. The two mechanisms combinatively functioned at decreasing Cd concentration in rice shoots by 14.1-33.1% at low pe + pH conditions compared to that of the high pe + pH (DY treatment). These results indicate that lowering soil pe + pH effectively reduced Cd accumulation in rice plants, probably through the immobilization of amorphous Fe oxides on Cd and sequestration of iron-plaque on Cd.

Effects of nano-Fe3O4-modified biochar on iron plaque formation and Cd accumulation in rice (Oryza sativa L.)

Nano-Fe₃O₄-modified biochar (BC-Fe) was prepared by the coprecipitation of nano-Fe₃O₄ on a rice husk biochar surface. The effects of BC-Fe on cadmium (Cd) bioavailability in soil and on Cd accumulation and translocation in rice (Oryza sativa L. cv. 'H You 518') were investigated in a pot experiment with 7 application rates (0.05-1.6%, w/w). BC-Fe increased the biomass of the rice plants except for the roots and affected the concentration and accumulation of Cd and Fe in the plants. The Cd concentrations of brown rice were significantly decreased by 48.9%, 35.6%, and 46.5% by the 0.05%, 0.2%, and 0.4% BC-Fe treatments, respectively. Soil cation exchange capacity (CEC) increased by 9.4%-164.1% in response to the application of BC-Fe (0.05-1.6%), while the soil Cd availability decreased by 6.81%-25.0%. However, 0.8-1.6% BC-Fe treatments promoted Cd transport to leaves, which could increase the risk of Cd accumulation in brown rice. Furthermore, BC-Fe application promoted the formation of iron plaque and enhanced the root interception of Cd. The formation of iron plaque reduced the toxicity of Cd to rice roots, but this barrier effect was limited and had an interval threshold (DCB-Fe: 22.5-27.3 g·kg^-1) under BC-Fe treatments.

Cadmium absorption and translocation of amaranth (Amaranthus mangostanus L.) affected by iron deficiency

Amaranth (Amaranthus mangostanus L.) has superior capability for accumulating cadmium (Cd) and has the potential to be used for phytoremediation of Cd contaminated soils. Iron (Fe) is chemically similar to Cd and may mediate Cd-induced physiological or metabolic impacts in plants. The purpose was to investigate the model of time-dependent and concentration-dependent kinetics of Cd absorption under Fe deficiency, understanding the physiological mechanism of Cd absorption in amaranth roots. The kinetic characteristics of Cd uptake by amaranth grown in Cd enriched nutritional solution with or without Fe addition and with methanol-chloroform, carbonyl cyanide 3-chlorophenylhydrazone (CCCP), and lanthanum chloride (LaCl₃) were compared using ¹⁰⁹Cd²⁺ isotope labeling technique. The results showed that Cd uptake was time-dependent and about 90-93% of uptake occurred during the first 150 min. The kinetics of Cd uptake showed that two stages were involved. The saturation stage fitted the Michaelis-Menten model when concentrations of Cd were lower than 12.71 μmol/L and then the absorption of Cd by roots was increased linearly during the second stage. Only linear absorption was observed with methanol-chloroform treatment while the metabolic inhibitor CCCP inhibited only the saturation absorption process, and the Ca channel inhibitor LaCl₃ partially inhibited the two stages of absorption. These results indicated that the root absorption of ¹⁰⁹Cd²⁺ was enhanced under Fe deficiency which induced more Fe transporters in the root cell membrane, and the Ca channel, apoplastic and symplastic pathways enhanced the Cd absorption in roots.

Mechanisms and uncertainties of Zn supply on regulating rice Cd uptake

Application of Zinc (Zn) is considered an effective measure to reduce Cadmium (Cd) uptake and toxicity in Cd-contaminated soils for many plant species. However, interaction between Zn and Cd in rice plant is complex and uncertain. In this study, four indica rice cultivars were selected to evaluate the effect of Zn exposure in an EGTA-buffered nutrient solution under varying Zn activities and a field level of Cd activity to characterize the interaction between Zn and Cd in rice. Severe depression in shoots' biomass, tiller number, and SPAD (Soil and Plant Analyzer Development) value were found at both Zn deficiency and Zn phytotoxicity levels among four tested rice cultivars. There existed a strong antagonism interaction between Zn and Cd in both shoot and root from Zn deficiency to Zn phytotoxicity. The reduction of Cd accumulation in roots and shoots could be explained by the competition between Zn and Cd as well as the dilution effect of increasing biomass. The conflicting effect of Zn supply on Cd uptake may be attributed to the increasing transfer ratio of Cd from root to shoot with the increasing Zn²⁺ activities and the strong depression of Fe and Mn in shoots with the increasing Zn²⁺ activities as well as the variation of genotypes. Balance between Zn and Cd should be considered in field application.

Phytoremediation: Environmentally sustainable way for reclamation of heavy metal polluted soils

Soil contamination with toxic metals is a widespread environmental issue resulting from global industrialization within the past few years. Therefore, decontamination of heavy metal contaminated soils is very important to reduce the associated risks and for maintenance of environmental health and ecological restoration. Conventional techniques for reclamation of such soils are expensive and environmental non-friendly. Phytoremediation is an emerging technology implementing green plants to clean up the environment from contaminants and has been considered as a cost-effective and non-invasive alternative to the conventional remediation approaches. There are different types of phytoremediation including, phytostabilization, phytostimulation, phytotransformation, phytofiltration and phytoextraction, the latter being most extensively acknowledged for remediation of soils contaminated with toxic heavy metals. Recent literature is gathered to critically review the sources, hazardous effects of toxic heavy metals and environmentally sustainable phytoremediation technique for heavy metal polluted soils to offer widespread applicability of this green technology. Different strategies to enhance the bioavailability of heavy metals in the soil are also discussed shortly. It can be concluded that phytoremediation of heavy metal contaminated soils is a reliable tool and necessary for making the land resource accessible for crop production.

Rice root exudates affect microbial methylmercury production in paddy soils

Microbial methylmercury (MeHg) production in contaminated soil-rice systems and its accumulation in rice pose health risks to consumers, especially those in Asia. However, the mechanism responsible for microbial MeHg production in paddy soils is far from clear. While previous studies examined the effect of soil and microbial factors on soil MeHg levels, in this work we explored the impact of rice cultivation itself on microbial MeHg production, focusing on the root exudate organic matter as a potential source of electron donors for microbial methylators. Effects of the cultivation of two rice cultivars, Heigu246 (H-rice) and Neiwuyou8015 (N-rice), on MeHg production in soils were therefore investigated in pot and batch incubation experiments. Soil MeHg levels measured in H-rice treatment during the heading and harvest stages were 18-49% higher than in the control and 23-108% higher than in N-rice treatment. Consequently, MeHg levels in grain, straw, and root were 38%, 81%, and 40% higher in H-rice than those in N-rice, which was mainly attributed to cultivar-specific MeHg production in soils. Results of the batch experiments suggested that root exudate organic matter could be responsible for MeHg production in soils during rice cultivation, by increasing the abundances of potential microbial methylators. For instance, root exudate organic matter increased copy numbers of Hg methylation genes (hgcA) in soils 4.1-fold. Furthermore, the 211% higher concentration of acetate (a key electron donor for microbial methylators) in the root exudate of H-rice could account for the higher MeHg production under H-rice than N-rice cultivation. Our results suggest that root exudate organic matter, especially acetate, as its key component, contributes to the elevated soil MeHg concentrations during rice cultivation. The proposed mechanism provides new insights into the elevated risk of MeHg production in contaminated soil-rice systems, as well as cultivar-specific MeHg bioaccumulation.

Mitigation of Cd accumulation in paddy rice (Oryza sativa L.) by Fe fertilization

Cadmium uptake in rice is believed to be mediated by the Fe transport system. Phyto-available Cd can be changed by Fe fertilization of substrates. This work investigated whether and how Fe fertilization affects mitigation of Cd accumulation in paddy rice. A 90-d soil column experiment was conducted to study the change of Cd and Fe availability in soil after Fe fertilization (ionic and chelated Fe). A low-Cd accumulating cultivar (TY116) and a high-Cd accumulating cultivar (JY841) were grown in two Cd-polluted paddy soils amended with chelated Fe fertilizers. Additionally, both cultivars were grown in hydroponics to compare Fe-related gene expression in EDDHAFe-deficient and EDDHAFe-sufficient roots. The column experiment showed that EDTANa₂Fe(II) and EDDHAFe(III) fertilization had a better mitigation effect on soil Cd availability compared to FeSO₄·7H₂O. Moreover, the field experiment demonstrated that these two chelated fertilizations could reduce Cd concentrations in brown rice by up to 80%. Iron concentrations in the brown rice were elevated by Fe chelates. Compared to EDDHAFe(III), EDTANa₂Fe(II) fertilization had a stronger mitigation effect by generating more EDTANa₂Cd(II) in the soil solution to decrease phyto-available Cd in the soil. While EDDHAFe(III) fertilization could increase soil pH and decrease soil Eh which contributed to decreasing phyto-available Cd in a contaminated soil. In the hydroponic experiment, Fe sufficiency significantly reduced Cd concentrations in above-ground organs. In some cases, the expression of OsIRT1, OsNRAMP1 and OsNRAMP5 was inhibited under Fe sufficiency relative to Fe deficiency conditions. These results suggest that mitigation of rice Cd by Fe chelate fertilization results from a decrease in available Cd in substrates and the inhibition of the expression of several Fe-related genes in the IRT and NRAMP families.

Inhibitory effects of rice residues amendment on Cd phytoavailability: A matter of Cd-organic matter interactions?

Incorporating crop residues into soils, a most common way of organic input into farmland soils, is being encouraged in many parts of the world, while its potential impacts on Cd phytoavailability are not well understood. Here, a Cd-contaminated soil was amended with rice residues (RR, i.e., straw + root mixture) or not (Control) and incubated for 81 days under laboratory-controlled conditions. During the incubation, key soil parameters (e.g., dissolved organic carbon and pH), Cd geochemical fractionation (by BCR sequential extraction), and CaCl₂ extracted Cd in soils (by 0.01 M CaCl₂ extraction) were quantified to explain the effects of RR amendment on Cd phytoavailability (assessed by 7 day-cultivation of rice seedling in soils). Besides, hydroponic experiments were designed to explore the effects of D-RR-OM (dissolved-RR-organic matter) on the uptake of Cd by rice seedlings (quantified by uptake constant rate, k_u, using stable isotope tracing technique). Our results demonstrated that RR amendment reduced Cd phytoavailability by 17-92% compared with Control during incubation, which might be explained by the interactions between Cd and RR-OM (RR-organic matter) in soil or porewater: (1) Cd immobilization due to its association with solid-RR-OM in soils, (2) Cd mobilization by D-RR-OM, and (3) Cd complexation with D-RR-OM in porewater, and thus reduced k_u of Cd. Our results suggested that dynamics of RR-OM (e.g., dissolution, decomposition and transformation) in soils, and thus interactions between Cd and solid/dissolved-RR-OM may control Cd phytoavailability under RR amendment. Information gained in this study would further our understanding about Cd phytoavailability in farming soils.