Iron plaque decreases cadmium accumulation in Oryza sativa L. and serves as a source of iron

Green manuring reduces cadmium accumulation in rice: Roles of iron plaque and dissolved organic matter

In southern China, winter green manure is widely used in rice cropping systems for improving grain yields and soil fertility. Cd pollution has recently been reported in some of these paddy fields. Research on the in-depth understanding of how green manuring affects Cd absorption in rice is limited. This study aimed to investigate the impacts of different green manures, including single plantation and mixed plantation on the absorption of Cd by rice and explore the underlying mechanisms. Pot experiments demonstrated that compared with winter fallow-rice, green manuring treatments considerably decreased rice Cd content, promoted the conversion of bioavailable Cd fraction into a more stable form, induced the formation of iron plaque, and increased the content of humic-like fraction (HF) in soil dissolved organic matter (DOM). Treatment with mixed plantation resulted in a greater decrease in rice Cd content and an increase in HF and iron plaque contents than single plantation. Hydroponic experiments confirmed that both iron plaque and green manure-derived DOM significantly reduced the Cd content in rice seedlings. In conclusion, green manure incorporation is an efficient measure for the safe utilization of Cd-contaminated soil, and mixed plantation of different green manures exerts stronger effects.

Role of iron manganese plaque in the safe production of rice (Oryza sativa L.) grains: Field evidence at plot and regional scales in cadmium-contaminated paddy soils

The relationship between iron manganese plaque (IP) and cadmium (Cd) accumulation by rice in the microenvironment of rice rhizosphere at varying field scales needs to be further explored. In this study, we selected different rice varieties and implemented tailored amendments to ensure the safe production of rice grains in heavily Cd-contaminated farmland situated around an E-waste dismantling site. Through regional surveys, we elucidated the role of IP in facilitating safe rice production. The selection of low-Cd accumulating rice varieties and application of appropriate amendments with sufficient dosages allowed for the effective reduction of Cd transport from soil to rice, resulting in a safe concentration of Cd in rice grains. Analysis using a random forest algorithm indicated that iron (Fe) played a more pivotal role than manganese in soil-rice systems in mitigating Cd accumulation in brown rice. The presence of Fe in IP (IP-Fe) at a low loading mass was unfavorable to the Cd-safe production of rice, while at an IP-Fe loading mass of 52 g/kg, the Cd content in brown rice decreased to a safe level. Furthermore, precipitation, coprecipitation, and complexation of surface functional groups contributed to Cd fixation on IP, as indicated by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, electron probe microanalysis, and Fourier-transform infrared spectroscopy with attenuated total reflection. Our results highlighted the key role of IP in the production of Cd-safe rice at different field scales.

Exploring the mechanism of Cd uptake and translocation in rice: Future perspectives of rice safety

Cadmium (Cd) contamination in rice fields has been recognized as a severe global agro-environmental issue. To reach the goal of controlling Cd risk, we must pay more attention and obtain an in-depth understanding of the environmental behavior, uptake and translocation of Cd in soil-rice systems. However, to date, these aspects still lack sufficient exploration and summary. Here, we critically reviewed (i) the processes and transfer proteins of Cd uptake/transport in the soil-rice system, (ii) a series of soil and other environmental factors affecting the bioavailability of Cd in paddies, and (iii) the latest advances in regard to remediation strategies while producing rice. We propose that the correlation between the bioavailability of Cd and environmental factors must be further explored to develop low Cd accumulation and efficient remediation strategies in the future. Second, the mechanism of Cd uptake in rice mediated by elevated CO2 also needs to be given more attention. Meanwhile, more scientific planting methods (direct seeding and intercropping) and suitable rice with low Cd accumulation are important measures to ensure the safety of rice consumption. In addition, the relevant Cd efflux transporters in rice have yet to be revealed, which will promote molecular breeding techniques to address the current Cd-contaminated soil-rice system. The potential for efficient, durable, and low-cost soil remediation technologies and foliar amendments to limit Cd uptake by rice needs to be examined in the future. Conventional breeding procedures combined with molecular marker techniques for screening rice varieties with low Cd accumulation could be a more practical approach to select for desirable agronomic traits with low risk.

Nano zero-valent iron enhances the absorption and transport of chromium in rice (Oryza sativa L.): Implication for Cr risks management in paddy fields

Chromium (Cr) accumulating in soil caused serious pollution to cultivated land. At present, nano zero-valent iron (nZVI) is considered to be a promising remediation material for Cr-contaminated soil. However, the nZVI impact on the behavior of Cr in the soil-rice system under high natural geological background value remains unknown. We studied the effects of nZVI on the migration and transformation of Cr in paddy soil-rice by pot experiment. Three different doses of nZVI (0, 0.001 % and 0.1 % (w/w)) treatments and one dose of 0.1 % (w/w) nZVI treatment without plant rice were set up. Under continuous flooding conditions, nZVI significantly increased rice biomass compared with the control. At the same time, nZVI significantly promoted the reduction of Fe in the soil, increased the concentration of oxalate Fe and bioavailable Cr, then facilitated the absorption of Cr in rice roots and the transportation to the aboveground part. In addition, the enrichment of Fe(III)-reducing bacteria and sulfate-reducing bacteria in soil provided electron donors for Cr oxidation, which helps to form bioavailable Cr that is easily absorbed by plants. The results of this study can provide scientific basis and technical support for the remediation of Cr -polluted paddy soil with high geological background.

Inhibition Roles of Calcium in Cadmium Uptake and Translocation in Rice: A Review

Cadmium (Cd) contamination in rice grains is posing a significant threat to global food security. To restrict the transport of Cd in the soil-rice system, an efficient way is to use the ionomics strategy. Since calcium (Ca) and Cd have similar ionic radii, their uptake and translocation may be linked in multiple aspects in rice. However, the underlying antagonistic mechanisms are still not fully understood. Therefore, we first summarized the current knowledge on the physiological and molecular footprints of Cd translocation in plants and then explored the potential antagonistic points between Ca and Cd in rice, including exchange adsorption on roots, plant cell-wall composition, co-transporter gene expression, and transpiration inhibition. This review provides suggestions for Ca/Cd interaction studies on rice and introduces ionomics research as a means of better controlling the accumulation of Cd in plants.

Mineralogical characteristics of root iron plaque and its functional mechanism for regulating Cr phytoextraction of hyperaccumulator Leersia hexandra Swartz

Leersia hexandra Swartz (L. hexandra) is a promising hyperaccumulator for Cr pollution remediation, but whether its Cr phytoextraction is subject to the root surface-attached iron plaque (IP) remains unclear. In this research, the natural and artificial IPs were proven to be comprised of small amounts of exchangeable Fe as well as carbonate Fe, and dominantly Fe minerals involving amorphous two-line ferrihydrite (Fh), poorly crystalline lepidocrocite (Le) and highly crystalline goethite (Go). The Fe content in the artificial IPs augmented with increasing induced Fe(II) concentration, and the 50 mg/L Fe(II) led to the identical Fe content and different component proportions of artificial IP (Fe50) and natural IP. Fh was consisted of highly aggregated nanoparticles, and the aging of Fh caused its phase conversion to rod-like Le and Go. The Cr(VI) adsorption results of Fe minerals corroborated the coordination of Cr(VI) onto the Fh surface and the significantly greater equilibrium Cr(VI) adsorption amount of Fh over Le and Go. The greatest Cr(VI) reduction capacity of Fh among three Fe minerals was found to be related to its most abundant surface-adsorbed Fe(II) content. The results of hydroponic experiment of L. hexandra showed that the presence of IP facilitated the Cr(VI) removal by L. hexandra during the cultivation period of 10-45 days, and consequently, compared to the Fe0 group (without IP), around 60% of increase in the Cr accumulation of shoots was achieved by Fe50 group. The findings of this work are conductive to furthering our understanding of IP-regulated Cr phytoextraction of L. hexandra.

Silicon facilitated the physical barrier and adsorption of cadmium of iron plaque by changing the biochemical composition to reduce cadmium absorption of rice roots

Silicon effectively inhibits cadmium (Cd) uptake in rice, iron plaque on root surface was the primary link and first interface of Cd entering into rice root. To elucidate the mechanism of iron plaque under silicon treatment on root Cd uptake, the morphological characteristics of iron plaque, mechanisms of Cd adsorption of iron plaque and effect of iron plaque on Cd uptake by rice roots of Yuzhenxiang (YZX) and Xiangwanxian (XWX) rice varieties were studied by employing energy spectrum analysis technique, non-invasive micro-test technique, and isothermal-kinetic adsorption method. Scanning electron microscopy-X-ray energy dispersive (SEM-EDX) analysis showed that denser crystal structure of iron plaque was observed at Si treatment, silicon promoted the thickening of iron plaque and strengthened the isolation of iron plaque to Cd, which reduced the Cd content of white roots of YZX and XWX varieties by 30.2% and 20.9% respectively. However, the blocking effect of iron plaque on Cd was weakened under silicon treatment with iron plaque removed, Cd content in iron plaque of YZX and XWX cultivars was significantly decreased by 36.3% and 18.4%, Cd concentrations in white root and shoot was significantly increased, and the influxes of Cd²⁺ at elongation and maturation zone of root were increased in multiples. The results of adsorption test showed that the adsorption process of iron plaque was mainly a monolayer adsorption completed by boundary diffusion. The X-ray photoelectron spectroscopy (XPS) results demonstrated that silicon changed the biochemical composition of iron plaque and increased the density of the carbon-oxygen bound groups on iron plaque, which is the most likely reasons for the higher affinity of Cd adsorption ability of iron plaque observed in the silicon treated iron plaque. This study suggested the silicon-facilitated iron plaque have played critical effects in controlling the Cd accumulation in rice roots by changing the morphology and chemical composition of iron plaque.

Speciation and distribution of chromium (III) in rice root tip and mature zone: The significant impact of root exudation and iron plaque on chromium bioavailability

Evidence on the contribution of root regions with varied maturity levels in iron plaque (IP) formation and root exudation of metabolites and their consequences for uptake and bioavailability of chromium (Cr) remains unknown. Therefore, we applied combined nanoscale secondary ion mass spectrometry (NanoSIMS) and synchrotron-based techniques, micro-X-ray fluorescence (µ-XRF) and micro-X-ray absorption near-edge structure (µ-XANES) to examine the speciation and localisation of Cr and the distribution of (micro-) nutrients in rice root tip and mature region. µ-XRF mapping revealed that the distribution of Cr and (micro-) nutrients varied between root regions. Cr K-edge XANES analysis at Cr hotspots attributed the dominant speciation of Cr in outer (epidermal and sub-epidermal) cell layers of the root tips and mature root to Cr(III)-FA (fulvic acid-like anions) (58-64%) and Cr(III)-Fh (amorphous ferrihydrite) (83-87%) complexes, respectively. The co-occurrence of a high proportion of Cr(III)-FA species and strong co-location signals of ⁵²Cr¹⁶O and ¹³C¹⁴N in the mature root epidermis relative to the sub-epidermis indicated an association of Cr with active root surfaces, where the dissolution of IP and release of their associated Cr are likely subject to the mediation of organic anions. The results of NanoSIMS (poor ⁵²Cr¹⁶O and ¹³C¹⁴N signals), dissolution (no IP dissolution) and µ-XANES (64% in sub-epidermis >58% in the epidermis for Cr(III)-FA species) analyses of root tips may be indicative of the possible re-uptake of Cr by this region. The results of this research work highlight the significance of IP and organic anions in rice root systems on the bioavailability and dynamics of heavy metals (e.g. Cr).

Iron-based nanomaterials reduce cadmium toxicity in rice (Oryza sativa L.) by modulating phytohormones, phytochelatin, cadmium transport genes and iron plaque formation

Rice is known to accumulate cadmium (Cd) in its grains, causing a severe threat to billions of people worldwide. The possible phytotoxicity and mechanism of 50-200 mg/L hydroxyapatite NPs (nHA), iron oxide NPs (nFe2O3) or nano zero valent iron (nZVI) co-exposed with Cd (100 μM) in rice seedlings were investigated. Three types of nanoparticles significantly reduced the bioaccumulation of Cd in rice shoots by 16-63%, with nZVI showing the greatest effect, followed by nHA and nFe2O3. A decrease in Cd content in the roots was observed only in the nZVI treatment, with values ranging from 8 to 19%. Correspondingly, nZVI showed the best results in promoting plant growth, increasing rice plant height, shoot and root biomass by 13%, 29% and 42%. In vitro studies showed that nZVI reduced the content of Cd in the solution by 20-52% through adsorption, which might have contributed to the immobilization of Cd in root. Importantly, the nZVI treatment resulted in 267% more iron plaques on the root surface, which acted as a barrier to hinder the entry of Cd. Moreover, all three nanoparticles significantly reduced the oxidative stress induced by Cd by regulating phytohormones, phytochelatin, inorganic homeostasis and the expression of genes associated with Cd uptake and transport. Overall, this study elucidates for the first time the multiple complementing mechanisms for some nanoparticles to reduce Cd uptake and transport in rice and provides theoretical basis for applying nanoparticles for reducing Cd accumulation in edible plants.

Simultaneous alleviation of Cd availability in contaminated soil and accumulation in rice (Oryza sativa L.) by Fe-Mn oxide-modified biochar

Fe-Mn oxide-modified biochar (BC-FM) was used to remediate Cd-contaminated soil and mitigate Cd accumulation in rice. The roles of Fe and Mn in soil Cd immobilization and in controlling Cd uptake by rice were investigated via X-ray photoelectron spectroscopy (XPS) characterization and chemical analysis. Fe and Mn loaded on BC-FM increased the removal efficiencies of CaCl₂ extractable Cd in soil and Cd in pore water compared to those in only biochar (BC)-treated soil, with maximum removal rates at 67.9 % and 77.8 %, respectively. The XPS results indicated that the redox reactions of the Fe-Mn oxides on BC-FM surface affected Cd immobilization in the soil. The Fe (II/III) components on BC-FM were primarily converted to Fe₃O₄ in the soil system, which may form stable complexes with Cd²⁺ (Fe-O-Cd) during the entire rice growth period, and Cd may be bound to MnO or Mn₂O₃ in the form of CdMn₂O₄. The excellent adsorption performance of BC-FM enhanced by Fe-Mn oxides reduced the available Cd in the soil and stimulated Fe and Mn transport in rice, thereby inhibiting Cd accumulation in the aerial parts of rice. Cd concentrations in brown rice under BC-FM treatments reached the national safety standard (0.2 mg/kg, GB2762-2017). And BC-FM significantly increased the biomass of brown rice with a maximum rate of 26.8 %. These findings suggest that BC-FM could be used as an efficient material for Cd-contaminated soil remediation, and Fe-Mn plays important role in immobilizing Cd in soil and reducing Cd transport in rice.

An insight into the act of iron to impede arsenic toxicity in paddy agro-system

Surplus research on the widespread arsenic (As) revealed its disturbing role in obstructing the metabolic function of plants. Also, the predilection of As towards rice has been an interesting topic. Contrary to As, iron (Fe) is an essential micronutrient for all life forms. Past findings propound about the enhanced As-resistance in rice plants during Fe supplementation. Thus, considering the severity of As contamination and resulting exposure through rice crops, as well as the studied cross-talks between As and Fe, we found this topic of relevance. Keeping these in view, we bring this review discussing the presence of As-Fe in the paddy environment, the criticality of Fe plaque in As sequestration, and the effectiveness of various Fe forms to overcome As toxicity in rice. This type of interactive analysis for As and Fe is also crucial in the context of the involvement of Fe in cellular redox activities such as oxidative stress. Also, this piece of work highlights Fe biofortification approaches for better rice varieties with optimum intrinsic Fe and limited As. Though elaborated by others, we lastly present the acquisition and transport mechanisms of both As and Fe in rice tissues. Altogether we suggest that Fe supply and Fe plaque might be a prospective agronomical tool against As poisoning and for phytostabilization, respectively.

Chloride application weakens cadmium immobilization by lime in paddy rice soil

Contamination of agricultural products by cadmium (Cd) is a global health problem, causing chronic abnormalities. The consumption of rice, the most-consumed foods, is an important exposure route of Cd to human body. Chloride (Cl^-) is reported to increase Cd uptake by rice; however, the effect on Cd uptake and accumulation by rice in the presence of lime is not clear. Therefore, a pot culture experiment was performed to explore the influence of Cl^- on the absorption and accumulation of Cd in rice plants under lime remediation and its possible mechanisms. The results showed that Cl^- promoted Cd accumulation in rice grains, mainly because of increased Cd bioavailability in the soil and by impeding the formation of iron plaques on rice roots, which reduced chelating and precipitation of Cd. Moreover, increased overexpression of the main transporters of Cd in rice roots, including OsNramp5, OsNramp1, OsIRTs and OsHMA2, favored the upward translocation of Cd from the root to shoot and increased the transfer factors (TFs) from soil to root, root-stem, leaf to grain, and soil to grain. Therefore, the application of Cl-rich materials to Cd-contaminated rice fields should be avoided during liming of the soil for Cd immobilization.

Sustainable Management of Arsenic Translocation in the Paddy Plants (Oryza sativa L) Cultivated in the Alluvial Soil of Gangetic West Bengal, India

Rice plants are known to be more susceptible to arsenic (As) contamination during the cultivation process. Arsenic is genotoxic and can be a big threat to the rice eating people at large. Studies on an effective mitigation mechanism are the need of the hour. This work was an approach using iron (Fe³⁺) to form Fe-plaque in the plant root that could trap As. The present research was designed with several experimental set ups for rice cultivation in pot culture using different Fe doses with fertilizer in the soil, and finally, the optimum dose was selected considering the translocation ability, plant health, and molecular and stress biomarkers. The study revealed that on an increase in Fe dose, translocation factor (TF) and stress marker (malondialdehyde content) of the plant decreased gradually and encountered minimum (0.12 and 0.03 mg/kg, respectively) at the dose of 4.5gm/kg. In contrast, higher values of chlorophyll (2.5 mg/kg) and carbohydrate (2.2 mg/kg) and intact DNA content were recorded highlighting the rich health condition of the plant. Thus, the experiment supported well the fact that the dose of Fe as fortified fertilizer can be considered the most effective in reducing soil arsenic accumulation in the rice plants. This approach might save the rice eating people from harmful effects of As contamination in this region of India.

A Glucuronic Acid-Producing Endophyte Pseudomonas sp. MCS15 Reduces Cadmium Uptake in Rice by Inhibition of Ethylene Biosynthesis

Dynamic regulation of phytohormone levels is pivotal for plant adaptation to harmful conditions. It is increasingly evidenced that endophytic bacteria can regulate plant hormone levels to help their hosts counteract adverse effects imposed by abiotic and biotic stresses, but the mechanisms underlying the endophyte-induced stress resistance of plants remain largely elusive. In this study, a glucuronic acid-producing endophyte Pseudomonas sp. MCS15 alleviated cadmium (Cd) toxicity in rice plants. Inoculation with MCS15 significantly inhibited the expression of ethylene biosynthetic genes including OsACO3, OsACO4, OsACO5, OsACS2, and OsACS5 and thus reduced the content of ethylene in rice roots. In addition, the expression of iron uptake-related genes including OsIRT1, OsIRT2, OsNAS1, OsNAS2 and OsYSL15 was significantly downregulated in the MCS15-inoculated roots under Cd stress. Similarly, glucuronic acid treatment also remarkably inhibited root uptake of Cd and reduced the production of ethylene. However, treatment with 1-aminocyclopropyl carboxylic acid (ACC), a precursor of ethylene, almost abolished the MCS15 or glucuronic acid-induced inhibition of Cd accumulation in rice plants. Conversely, treatment with aminoethoxyvinyl glycine (AVG), an inhibitor of ethylene biosynthesis, markedly reduced the Cd accumulation in plants. Taken together, our results revealed that the endophytic bacteria MCS15-secreted glucuronic acid inhibited the biosynthesis of ethylene and thus weakened iron uptake-related systems in rice roots, which contributed to preventing the Cd accumulation.

Application of Exogenous Iron Alters the Microbial Community Structure and Reduces the Accumulation of Cadmium and Arsenic in Rice (Oryza sativa L.)

Cadmium (Cd) and arsenic (As) contamination of soil has been a public concern due to their potential accumulation risk through the food chain. This study was conducted to investigate the performance of ferrous sulfate (FeSO4) and ferric oxide (Fe2O3) nanoparticle (Nano-Fe) to stabilize the concentrations of Cd and As in paddy soil. Both Fe treatments led to low extractable Cd and the contents of specifically sorbed As contents, increased (p < 0.05) the Shannon index and decreased (p < 0.05) the Simpson diversity indices compared with the control. Nano-Fe increased the relative abundances of Firmicutes and Proteobacteria and decreased the abundances of Acidobacteria and Chloroflexi. Moreover, the addition of both forms of Fe promoted the formation of Fe plaque and decreased the translocation factor index (TFs) root/soil, TFs shoot/root, and TFs grain/shoot of Cd and As. These results suggest that exogenous Fe may modify the microbial community and decrease the soil available Cd and As contents, inhibit the absorption of Cd and As by the roots and decrease the transport of Cd and As in rice grains and the risk intake in humans. These findings demonstrate that soil amendment with exogenous Fe, particularly Nano-Fe, is a potential approach to simultaneously remediate the accumulation of Cd and As from the soil to rice grain systems.

Liming and tillering application of manganese alleviates iron manganese plaque reduction and cadmium accumulation in rice (Oryza sativa L.)

The application time and soil pH are key to manganese (Mn) bioavailability, which may influence Mn effects on cadmium (Cd) accumulation in rice. Accordingly, this study investigated the effects of Mn application at different stages, alone or with basal liming, on Cd accumulation in rice through pot and field experiments. The results showed that basal Mn application maximally elevated soil dissolved Mn, and increasing Mn accumulation in rice by 140%-367% compared to the control. Additionally, basal or tillering applications had better effects on enhancing iron manganese plaque (IMP) and inhibiting CaCl₂-extractable Cd than later applications. Therefore, basal and tillering Mn reduced brown rice Cd by 24.6% and 18.9% compared to the control, respectively. Liming reduced CaCl₂-extractable Cd by 83.3% compared to the control but inhibited soil dissolved Mn (25.8%-76.6%) and IMP (28.9%-29.7%), resulting in only a 41.7% reduction in brown rice Cd. Liming combined with tillering Mn maximally reduced brown rice Cd by 67.4%, structural equation modeling revealed CaCl₂-extractable Cd and manganese plaque played the greatest positive and negative roles, respectively. Therefore, basal liming and tillering application of Mn is most effective at reducing rice Cd through inhibition of Cd bioavailability and alleviation of IMP reduction.

Influences of soil pH, iron application and rice variety on cadmium distribution in rice plant tissues

Cadmium (Cd) is a widespread environmental contaminant, and its increasing concentrations in rice poses significant risks to human health. Globally, rice is a staple food for millions of people, and consequently, effective strategies to reduce Cd accumulation in rice are needed. This study investigates the effect of soil pH (Soil 1: 4.6; Soil 2: 6.6) and iron (Fe) application (at 0, 1.0 and 2.0 g/kg) on Fe plaque formation, Cd sequestration in Fe plaques and Cd bioaccumulation in different parts of the rice plant for three different Cd-graded paddy soils (0, 1.0 and 3.0 mg/kg, respectively) using two Australian rice cultivars under glasshouse conditions. Results show that grain and straw yield declined as Cd toxicity increased, and the toxic effects of Cd were lower in the Quest cultivar than in the Langi cultivar. With applications of Cd at 1.0 mg/kg and 3.0 mg/kg, Cd concentrations in rice grown in Soil 1 were 1.09 mg/kg and 1.37 mg/kg, respectively, while those in rice grown in Soil 2 were 0.38 mg/kg and 0.52 mg/kg, respectively. Soil pH significantly affected the bioaccumulation of Cd in different parts of the rice plant. At both levels of Cd application, Cd concentration was highest in the root, followed by the stem, leaf, husk and grain. Cd was more concentrated in Fe plaques formed by the application of Fe than in rice plant tissues. The Quest cultivar had a higher ability to produce Fe plaques and a 1.3- and 1.4-times higher Cd concentration compared with the Langi cultivar in Soils 1 and 2, respectively.

Low Cd-accumulating rice intercropping with Sesbania cannabina L. reduces grain Cd while promoting phytoremediation of Cd-contaminated soil

Paddy field pollution with Cd has become a serious problem and poses threat to public health. Intercropping is new good agricultural practice for phytoremediation in Cd contaminated soil. Field and pot experiments were conducted to examine the effects of intercropping low Cd-accumulating rice with Sesbania cannabina on plant growth, uptake of Cd by the intercropping system, and rhizosphere microecology, and to evaluate the potential remediation of Cd contaminated soil and safety production of rice. The results of in the field experiment show that, in intercropping system, the concentration of Cd in the grain of rice (0.18 mg kg^-1) was below the threshold level permitted by the National Food Safety Standard of China (GB 2762-2017, 0.20 mg kg^-1). Furthermore, the yield per plant of rice in intercropping system significantly (P < 0.05) increased by 19.71%. At the same time, the bio-concentration amount (BCA) of Cd per plant of Sesbania cannabina in intercropping system significantly (P < 0.05) increased by 46.15%. The metal removal equivalent ratio (MRER) of Cd was 1.11, indicating that the intercropping system had advantage in Cd removal. In the pot experiment, the rice intercropped with Sesbania cannabina under no barrier (IN) treatment significantly (P < 0.05) decreased the content of rhizosphere organic acids (oxalic and malic acids), and significantly (P < 0.05) increased the rhizosphere pH value and total iron plaque concentration on the root surface compared to the intercropping with plastic barrier (IN+P) treatment, which could significantly (P < 0.05) decreased the available Cd content in rhizosphere soil and the accumulation of Cd in rice organs. With this study we demonstrated that lower rhizosphere organic acids and higher iron plaque can obstruct and decreased the Cd absorbed by rice in a rice-Sesbania cannabina intercropping system. We conclude that intercropping rice with Sesbania cannabina is a promising and cost-effective agricultural practice for safe crop production and for phytoremediation in Cd-contaminated paddy soil.

Sustainable management of cadmium-contaminated soils as affected by exogenous application of nutrients: A review

Cadmium (Cd) pollution in arable land is of great concern as it impairs plant growth and further threats human health via food-chain. Exogenous supplementation of nutrients is an environmentally-friendly, cost-effective, convenient and feasible strategy for regulating Cd uptake, transport and accumulation in plants. To sustain Cd-contaminated soils management, on the one hand, a low level of the Cd-contaminated soil is expected to cultivate crops with decreased Cd accumulation as affected by exogenous nutrients application, on another hand, a high level of the Cd-contaminated soil is suggested to cultivate phytoextraction plants with increased Cd accumulation as affected by exogenous nutrients application. Nevertheless, effects of nutrients on Cd accumulation in plants are still ambiguous. Thus, data of Cd accumulation in shoots of plants as affected by exogenous application of nutrients were collected from previously published articles between 2005 and 2021 in the present study. According to the data, exogenous supply of calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn) and silicon (Si) to a larger extent decrease Cd amounts in shoots of plants. By contrast, exogenous nitrogen (N), and deficient Ca, Mg and Fe supply have a great possibility to increase Cd amounts in shoots of plants. Although exogenous application of phosphorus (P), sulfur (S), potassium (K), zinc (Zn) and selenium (Se) have a great opportunity to increase biomass, they show different effects on Cd concentrations. As a result, the odds are even for increasing and decreasing Cd amounts in shoots of plants. Taken together, exogenous application of Ca, Mg, Fe, Mn and Si might decrease Cd accumulation in plants that are recommended for crops production. Exogenous N and deficient Ca, Mg and Fe supply might increase Cd accumulation in plants that are recommended for phytoextraction plants. Exogenous application of P, S, K, Zn and Se have half a chance to increase or decrease Cd accumulation in plants. Therefore, dosages, forms and species should be taken into account when exogenous P, S, K, Zn and Se are added.

Unraveling the mechanisms controlling Cd accumulation and Cd-tolerance in Brachiaria decumbens and Panicum maximum under summer and winter weather conditions

We evaluated the mechanisms that control Cd accumulation and distribution, and the mechanisms that protect the photosynthetic apparatus of Brachiaria decumbens Stapf. cv. Basilisk and Panicum maximum Jacq. cv. Massai from Cd-induced oxidative stress, as well as the effects of simulated summer or winter conditions on these mechanisms. Both grasses were grown in unpolluted and Cd-polluted Oxisol (0.63 and 3.6 mg Cd kg^-1 soil, respectively) at summer and winter conditions. Grasses grown in the Cd-polluted Oxisol presented higher Cd concentration in their tissues in the winter conditions, but the shoot biomass production of both grasses was not affected by the experimental conditions. Cadmium was more accumulated in the root apoplast than the root symplast, contributing to increase the diameter and cell layers of the cambial region of both grasses. Roots of B. decumbens were more susceptible to disturbed nutrients uptake and nitrogen metabolism than roots of P. maximum. Both grasses translocated high amounts of Cd to their shoots resulting in oxidative stress. Oxidative stress in the leaves of both grasses was higher in summer than winter, but only in P. maximum superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities were increased. However, CO₂ assimilation was not affected due to the protection provided by reduced glutathione (GSH) and phytochelatins (PCs) that were more synthesized in shoots than roots. In summary, the root apoplast was not sufficiently effective to prevent Cd translocation from roots to shoot, but GSH and PCs provided good protection for the photosynthetic apparatus of both grasses.

Mechanisms underlying mercury detoxification in soil-plant systems after selenium application: a review

Feasible countermeasures to mitigate mercury (Hg) accumulation and its deleterious effects on crops are urgently needed worldwide. Selenium (Se) fertilizer application is a cost-effective strategy to reduce Hg concentrations, promote agro-environmental sustainability and food safety, and decrease the public health risk posed by Hg-contaminated soils and its accumulation in food crops. This holistic review focuses on the processes and detoxification mechanisms of Hg in whole soil-plant systems after Se application. The reduction of Hg bioavailability in soil, the formation of inert HgSe or/and HgSe-containing proteinaceous complexes in the rhizosphere and/or roots, and the reduction of plant root uptake and translocation of Hg in plant after Se application are systemically discussed. In addition, the positive responses in plant physiological and biochemical processes to Se application under Hg stress are presented to show the possible mechanisms for protecting the plant. However, application of high levels Se showed synergistic toxic effect with Hg and inhibited plant growth. The effectiveness of Se application methods, rates, and species on Hg detoxification is compared. This review provides a good approach for plant production in Hg-contaminated areas to meet food security demands and reduce the public health risk.

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.

Acetic Acid-Producing Endophyte Lysinibacillus fusiformis Orchestrates Jasmonic Acid Signaling and Contributes to Repression of Cadmium Uptake in Tomato Plants

Diverse signaling pathways regulated by phytohormones are essential for the adaptation of plants to adverse environments. Root endophytic bacteria can manipulate hormone-related pathways to benefit their host plants under stress conditions, but the mechanisms underlying endophyte-mediated plant stress adaptation remain poorly discerned. Herein, the acetic acid-producing endophytic bacteria Lysinibacillus fusiformis Cr33 greatly reduced cadmium (Cd) accumulation in tomato plants. L. fusiformis led to a marked increase in jasmonic acid (JA) content and down-regulation of iron (Fe) uptake-related genes in Cd-exposed roots. Accordantly, acetic acid treatment considerably increased the JA content and inhibited root uptake of Cd uptake. In addition, the Cr33-inoculated roots displayed the increased availability of cell wall and rhizospheric Fe. Inoculation with Cr33 notably reduced the production of nitric oxide (NO) and suppressed Fe uptake systems in the Cd-treated roots, thereby contributing to hampering Cd absorption. Similar results were also observed for Cd-treated tomato plants in the presence of exogenous JA or acetic acid. However, chemical inhibition of JA biosynthesis greatly weakened the endophyte-alleviated Cd toxicity in the plants. Collectively, our findings indicated that the endophytic bacteria L. fusiformis effectively prevented Cd uptake in plants via the activation of acetic acid-mediated JA signaling pathways.

Rhizosphere iron and manganese-oxidizing bacteria stimulate root iron plaque formation and regulate Cd uptake of rice plants (Oryza sativa L.)

Iron plaque is the amorphous and/or crystalline layer of Fe and Mn (hydr)oxides formed on the root surface of wetland plants. It could adsorb and co-precipitate metal(loid)s at the rhizosphere, thus modulating the uptake and accumulation of metal elements in plants. In this study, the Fe(II)/Mn(II)-oxidizing bacteria Burkholderia sp. D416 (D416) and Pseudomonas sp. YGL (YGL) were isolated from Cd-contaminated rice field, both hydroponic experiment and pot experiment were performed to assess the impact of bacterial inoculation on iron plaque formation, elemental content of the plaque, plant dry mass, antioxidant enzyme activity and Cd content in rice plants. The results revealed that inoculation with D416, YGL, and D416+YGL stimulated iron plaque formation on the root surface of the hydroponic rice. The content of C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Fe and Ca in the root plaque were affected by the bacterial inoculation and varied among different plant growth stages. The pot experiment indicated that inoculation with D416 increased the root dry biomass by 58.89%, and the combined inoculation of D416 and YGL increased the dry biomass of root, shoot and grain by 16.89%, 21.66% and 23.26%, respectively. Importantly, YGL inoculation decreased the Cd translocation from root to shoot and from glume to brown rice grain by 50.00% and 50.27%, respectively, and the Cd content in shoot and brown rice grain were decreased by 20.00% and 34.48%, respectively. Taken together, the elemental content of the iron plaque and Cd content in rice plants varied among different plant growth stages and when plants were inoculated with different bacterial strains. YGL dramatically reduced the Cd content in brown rice grain, thus it could potentially be used to reduce Cd content in rice crop grown in Cd-contaminated soils.

Nano-scale zero valent iron modulates Fe/Cd transporters and immobilizes soil Cd for production of Cd free rice

In recent decades, nanoscale zero valent iron (nZVI) has been found to be a promising approach for heavy metal remediation. This study is the first report highlighting the role of nZVI to ameliorate Cadmium (Cd) stress in rice along with its effects in expressions of transporter genes, agronomic parameters and grain nutrient status. Initially, 3 concentration of Cd (10, 50, 250 μM) and nZVI (50, 100, 200 mg L^-1) were selected. PCA analysis based on growth parameters, photosynthetic pigment contents and lipid peroxidation rate confirmed that 100 mg L^-1 nZVI was most suitable for remediation of 10 μM Cd. It was evident that, nZVI can alleviate Cd-induced toxic effects by enhancing antioxidant defense mechanisms and other physiological processes in plants. nZVI treated rice seedlings also showed upregulation of phytochelatins which aided in Cd chelation within vacuoles. Study of root morphology with scanning electron microscopy and ROS imaging with confocal microscopy confirmed that nZVI could alleviate oxidative stress due to Cd uptake. In nZVI treated rice seedlings, gene expressions of iron (Fe) transporters (like, IRT1,IRT2,YSL2,YSL15) which are responsible for both Fe and Cd uptake were significantly down-regulated whereas, OsVIT1 and OsCAX4 genes were over expressed which lead to sequestration of Cd in vacuoles. Cd localization assay with leadmium proved that Cd translocation was reduced with nZVI treatment. To further validate our findings a pot experiment was carried out where it was found that nZVI could immobilize Cd in soil prevented accumulation of Cd in rice grains in addition to improving yield.

Application of rapeseed residue increases soil organic matter, microbial biomass, and enzyme activity and mitigates cadmium pollution risk in paddy fields

Rapeseed (Brassica napus L.) is a winter oil crop and biodiesel resource that has been widely cultivated in the southern part of China. Applying rapeseed residue (RSD) to summer rice fields is a common agricultural practice under rice-rapeseed double cropping systems. However, in Cd-contaminated paddy fields, the influence mechanisms of this agricultural practice on the migration and distribution of Cd fractions in soil are not clear. Therefore, a field experiment was carried out to analyse the changes in soil pH, organic matter (OM), microbial biomass carbon (MBC) and nitrogen (MBN), enzyme activity (urease (UA), acid phosphatase (ACP), and dehydrogenase (DH)), Cd distribution fractions, and Cd concentration in rice tissues after RSD application. The results showed that RSD treatment significantly increased the soil OM and MBC concentrations and UA, ACP, and DH activities, decreased the soil acetic acid-extractable fraction of Cd (ACI-Cd), and increased the reducible fraction of Cd (Red-Cd). The formation of stable organic complexes and chelates upon application of RSD is a result of the high affinity of Cd for soil OM. The activities of soil ACP, DH and MBC can well reflect Cd ecotoxicity in soil, particularly the DH activity. In addition, RSD application was helpful in inducing iron plaque formation. The "barrier" effect of iron plaque resulted in reduced Cd accumulation in different tissues of rice. The health risk of rice consumption also decreased as a result of RSD application; it decreased by 0.89-30.0% and 24.1-51.7% in the two tested fields. Overall, the application of RSD was increased soil OM, microbial biomass, and enzyme activity, and these changes was instrumental in reduce the risk of cadmium pollution in rice fields.

Ferrous iron facilitates the formation of iron plaque and enhances the tolerance of Spartina alterniflora to artificial sewage stress

The ferrous iron (Fe²⁺) facilitates the formation of root Fe plaque of wetland plants, but its effect on the tolerance of wetland plants to artificial sewage stress has been seldom reported. In this study, the influences of Fe²⁺ on the formation of Fe plaque and its effects on the tolerance of Spartina alterniflora to artificial sewage stress were investigated. The artificial sewage stress decreased the plant height and chlorophyll content and significantly increased the MDA content in leaves. The symptoms of these stresses were alleviated with increasing Fe²⁺ concentration accompanied by significant increase in leaf alcohol dehydrogenase activity. The increase of Fe²⁺ concentration significantly increased the root Fe plaque content and reduced the accumulation of toxic metals in leaves of S. alterniflora. These results support our hypothesis that the exogenous Fe²⁺ supply may enhance the stress resistance of S. alterniflora to artificial sewage containing heavy metals.

Yield advantage and cadmium decreasing of rice in intercropping with water spinach under moisture management

Rice (Oryza sativa L.) intercropping with water spinach (Ipomoea aquatica Forsk) is an effective agricultural practice for safe crop production and for phytoremediation in cadmium-contaminated soil. A field and pot experiment were conducted to investigate the growth and cadmium absorption of rice intercropped with water spinach under different moisture management schemes (continuous flooding, interval flooding, and 75% field capacity). In the field experiment, the concentration of Cd in the grain of rice was significantly lower in the intercropping system than that permitted by the National Food Safety Standard of China (GB 2762-2017). Furthermore, the land equivalent ratio (1.42) was higher in the rice-water spinach intercropping system, indicating a significant advantage of the intercropping system in yield. At the same time, the bio-concentration amount (BCA) of Cd of rice and water spinach in intercropping system significantly increased by 17.99% and 31.98%, respectively (P＜0.05). However, the metal removal equivalent ratio (MRER) of Cd was 1.34, which showed the intercropping system of rice-water spinach had advantage in Cd removal. In the pot experiment, the total iron plaque concentration on the root surface of rice and the pH of the rhizosphere soil were higher under continuous flooding (T_CF) than under the control conditions (75% field capacity, T_CK), which could significantly decrease the available Cd in the rhizosphere soil and the accumulation of Cd in rice organs. So, this study demonstrated that iron plaque can obstruct and decrease the Cd absorbed by rice in a rice-water spinach intercropping system combined with water management. The intercropping rice with water spinach can achieve the goal of remediation while producing for farmland contaminated by Cd.

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.

Comparison of Cd subcellular distribution and Cd detoxification between low/high Cd-accumulative rice cultivars and sea rice

Salt-tolerant rice cultivar (sea rice) is a research hotspot worldwide due to its high yield in high salinity soil. However, knowledge regarding the cadmium (Cd) effects on the growth of sea rice is limited. To determine the short-term and long-term impact of Cd stress, relatively low/high Cd-accumulative rice cultivars and sea rice were grown to compare their growth responses to Cd stress over time. The results showed that sea rice presented the highest Cd concentrations in the root, stem, and leaves under 32-days of Cd stress. Cd stress shortened and thickened the rice root, and decreased the proportion of root diameters in the 0-0.2 mm range. Cd stress remarkably increased the Cd and Fe concentration in dithionite-citrate-bicarbonate (DCB) extracts, and the DCB-Cd and DCB-Fe concentrations were the highest in sea rice. The subcellular distribution of Cd in the rice roots indicated that Cd accumulated the most in the soluble fraction and cell wall. The contents of pectin and hemicellulose 2 in the root cell wall of the low-Cd accumulative rice variety CL755 were higher than those in MXZ and sea rice. Collectively, this work provides a general understanding of the Cd effects on sea rice growth and indicates that sea rice has a relatively high Cd accumulation compared with the other two rice cultivars. However, the specifically-related mechanism remains to be further studied.

Facets of iron in arsenic exposed Oryza sativa varieties: A manifestation of plant's adjustment at morpho-biochemical and enzymatic levels☆

Rice consumption is one of the primary sources of arsenic (As) exposure as the grains contain relatively higher concentration of inorganic As. Abundant studies on the ability of iron (Fe) plaque in hampering As uptake by plants has been reported earlier. However, little is known about its role in the mitigation of As mediated oxidative damage in rice plants. The present study highlights the effect of As and Fe co-supplementation on growth response, oxidative stress, Fe uptake related enzymes and nutrient status in rice varieties. Eight different Indica rice varieties were screened and finally four varieties (Varsha, Jaya, PB-1 and IR-64) were selected for detailed investigations. Improved germination and chlorophyll/protein levels during As+Fe co-exposure indicate healthier plants than As(III) treated ones. Interestingly Fe was found act both as an antagonist and also as a synergist of As treatments. It acted by reducing As translocation and improving the nutritional levels and enhancing the oxidative stress. Fe uptake related enzymes (nitrite reductase and ferric chelate reductase) and phytosiderophores analysis revealed that Fe supplementation can reduce its deficiency in rice plants. Morpho-biochemical, oxidative stress and nutrient analysis symbolizes higher tolerance of PB-1 towards As, while Varsha being most sensitive, efficiently combated the As(III) stress in the presence of Fe.

Root adaptation in Echinodorus osiris Rataj plant under cadmium stress

Cadmium tolerant plant, Echinodorus osiris Rataj, was selected to study its root adaptive mechanism under Cd stress. The change of root porosity, radial oxygen loss (ROL), and iron plaque formation was investigated. Results suggested that Cd treatment decreased 28.6-49.9% of ROL and reduced 13.5-23.3% of root porosity but increased 63.4-147.2% of iron plaque after 21 days, respectively. Under different Cd treatments, the uptake of Cd in root presented quick and mild models while it showed relatively consistent increase in shoot. Correlation analysis demonstrated that Cd concentrations in plant were related negatively with root porosity but had no significant correlation with ROL. There was significant positive correlation between root porosity and ROL; however, they both related negatively with root iron plaque. Moreover, the scanning electron microscopy indicates a barrier to the movement of Cd in endodermis layers.

Photosynthetic light reactions in Oryza sativa L. under Cd stress: Influence of iron, calcium, and zinc supplements

Some mineral nutrients may help to alleviate cadmium stress in plants. Therefore, influence of Fe, Ca, and Zn supplements on photosynthesis light reactions under Cd stress studied in two Indian rice cultivars namely, MO-16 and MTU-7029 respectively. Exogenous application of both Fe and Ca ions helped to uphold quantum efficiency and linear electron transport during Cd stress. Also, recovery of biomass noticed during Cd treatment with Fe and Ca supplements. It was found that accumulation of carotenoids as well as non photochemical quenching enhances with Fe, Ca, and Zn supplements. Chlorophyll a/b ratio increased with Cd accumulation as a strategy to increase light harvest. Lipid peroxidation level was ascertained the highest during Cd plus Zn treatments. Above results point that both Fe and Ca ions supplements help to alleviate Cd stress on photosynthesis light reactions of rice plants.

Phosphate addition diminishes the efficacy of wollastonite in decreasing Cd uptake by rice (Oryza sativa L.) in paddy soil

Cadmium (Cd) contamination in paddy soils poses food security risks and public health concerns. Exploring effective strategies to reduce rice grain Cd is an urgent need. In this study, field plot experiments were conducted to evaluate the effects of wollastonite application with or without phosphate (P) addition on Cd accumulation in rice (Oryza sativa L.). Co-application of P and wollastonite showed greater efficiency than wollastonite amendments alone in raising soil pH and CEC and decreasing soil Cd availability. Cd concentration in brown rice was decreased by 71% under the wollastonite treatment alone, but was decreased by only 29-39% when wollastonite was coupled with different P amendments. This seeming contradiction could be ascribed to the dramatic decline in the phytoavailability of manganese (Mn) and the increase in molar ratio of iron (Fe) to Mn (Fe/Mn) in Fe plaques on root surfaces in the presence of P additions. Significant negative correlations between Mn and Cd in rice plants and positive correlations between Fe/Mn in Fe plaque and Cd in rice plants indicated that P-induced soil Mn deficiency and reduced Mn in Fe plaque impeded the alleviation of Cd accumulation in rice. Application of wollastonite in Si-deficient paddy soils was effective in reducing rice Cd accumulation while boosting rice yield, but co-application of P and wollastonite was counterproductive and should be avoided. This work emphasized that a better understanding of the relationships between Cd and related mineral nutrient uptake would be helpful in developing more efficient measures to reduce rice grain Cd.

Enterobacter asburiae Reduces Cadmium Toxicity in Maize Plants by Repressing Iron Uptake-Associated Pathways

Soil microbes have recently been utilized to improve cadmium (Cd) tolerance and lower its accumulation in plants. Nevertheless, whether rhizobacteria can prevent Cd uptake by graminaceous plants and the underlying mechanisms remain elusive. In this study, inoculation with Enterobacter asburiae NC16 reduced transpiration rates and the expression of some iron (Fe) uptake-related genes including ZmFer, ZmYS1, ZmZIP, and ZmNAS2 in maize (Zea mays) plants, which contributed to mitigation of Cd toxicity. However, the inoculation with NC16 failed to suppress the transpiration rates and transcription of these Fe uptake-related genes in plants treated with fluridone, an abscisic acid (ABA) biosynthetic inhibitor, indicating that the impacts of NC16-inoculation observed were dependent on the actions of ABA. We found that NC16 increased the host ABA levels by mediating the metabolism of ABA rather than its synthesis. Moreover, the capacity of NC16 to inhibit plant uptake of Cd was greatly weakened in plants overexpressing ZmZIP, encoding a zinc/iron transporter. Collectively, our findings indicated that E. asburiae NC16 reduced Cd toxicity in maize plants at least partially by hampering the Fe uptake-associated pathways.

Calcium acetate-induced reduction of cadmium accumulation in Oryza sativa: Expression of auto-inhibited calcium-ATPase and cadmium transporters

Calcium (Ca) signalling has an essential role in regulating plant responses to various abiotic stresses. This study applied Ca in various forms (Ca acetate and CaCl₂ ) and concentrations to reduce cadmium (Cd) concentration in rice and propose a possible mechanism through which Ca acts to control the Cd concentration in rice. The results showed that supplementation of Cd-contaminated soil with Ca acetate reduced the Cd concentration in rice after exposure for 7 days in both hydroponic and soil conditions. The possible involvement of the auto-inhibited Ca²⁺ -ATPase gene (ACA) might act to control the primary signal of the Cd stress response. The messages from ACA3 and ACA13 tended to up-regulate the low-affinity cation transporter (OsLCT1) and down-regulate Cd uptake and the Cd translocation transporter, including the genes, natural resistance-associated macrophage protein 5 (Nramp5) and Zn/Cd-transporting ATPase 2 (HMA2), which resulted in a reduction in the Cd concentration in rice. After cultivation for 120 days, the application of Ca acetate into Cd-contaminated soil inhibited Cd uptake of rice. Increasing the Ca acetate concentration in the soil lowered the Cd concentration in rice shoots and grains. Moreover, Ca acetate maintained rice productivity and quality whereas both aspects decreased under Cd stress.

Nutrient uptake by constructed floating wetland plants during the construction phase of an urban residential development

This study investigated plant growth, nutrient partitioning and total nutrient uptake by tall sedge (Carex appressa) plants in two large-scale Constructed Floating Wetlands (CFW1 and CFW2). These CFWs were installed to treat stormwater runoff discharging into a newly-constructed 2.6-ha lake during the construction phase of a 45-ha residential development. Nutrient concentrations of C. appressa shoot above the mat, biomass within the mat, and roots below the mat were analysed 0, 12 and 16 months after planting. Extensive root growth was evident after 12 and 16 months. Some nutrients (nitrogen, phosphorus, sulphur) were distributed almost evenly among the above-, within-, and below-mat components, while others (aluminium, copper, iron, manganese) were concentrated in or on the roots. Given the low concentrations of nutrients within the water column, large amounts of nutrients were removed from stormwater by the plants. Total nitrogen uptake was 20.20 ± 2.88 kg in CFW1 and 15.00 ± 2.07 kg in CFW2 over the 16-month study period. Total potassium uptake was 12.59 ± 1.64 kg in CFW1 and 7.20 ± 1.56 kg in CFW2. Phosphorus uptake was low as a consequence of low phosphorus availability in the water. High aluminium, iron and manganese concentrations were found in the roots, demonstrating that C. appressa removed and sequestered large quantities of these water pollutants from urban stormwater runoff. For example, total aluminium uptake was 7.82 ± 1.73 kg in CFW1 and 5.62 ± 0.75 kg in CFW2. This study demonstrated multiple benefits of CFWs for stormwater treatment in the early stages of an urban development.

Iron toxicity resistance strategies in tropical grasses: The role of apoplastic radicular barriers

The revegetation of mined areas poses a great challenge to the iron ore mining industry. The initial recovery process in degraded areas might rely on the use of Fe-resistant grasses. Tropical grasses, such as Paspalum densum and Echinochloa crus-galli, show different resistance strategies to iron toxicity; however, these mechanisms are poorly understood. The Fe-resistance mechanisms and direct iron toxicity as a function of root apex removal were investigated. To achieve this purpose, both grass species were grown for up to 480 hr in a nutrient solution containing 0.019 or 7 mmol/L Fe-EDTA after the root apices had been removed or maintained. Cultivation in the presence of excess iron-induced leaf bronzing and the formation of iron plaque on the root surfaces of both grass species, but was more significant on those plants whose root apex had been removed. Iron accumulation was higher in the roots, but reached phytotoxic levels in the aerial parts as well. It did not hinder the biosynthesis of chloroplastidic pigments. No significant changes in gas exchange and chlorophyll a fluorescence occurred in either grass when their roots were kept intact; the contrary was true for plants with excised root apices. In both studied grasses, the root apoplastic barriers had an important function in the restriction of iron translocation from the root to the aerial plant parts, especially in E. crus-galli. Root apex removal negatively influenced the iron toxicity resistance mechanisms (tolerance in P. densum and avoidance in E. crus-galli).

Iron plaque formation in the roots of Pistia stratiotes L.: importance in phytoremediation of cadmium

Aquatic macrophytes play an important role in the removal of toxic metals from wastewater. Therefore, the induction of Fe plaque on the roots, and its consequences on Cd tolerance investigated in an aquatic macrophyte Pistia stratiotes L. The presence of Fe²⁺ ion but not Fe³⁺ resulted in Fe plaque formation. Induction of Fe plaque decreased Ca and increased K and Fe accumulations in the root. Plaque formed plants had accumulated less Cd until 50.0 µM CdCl₂ treatments because plaque acted as a barrier to Cd exposure. However, at higher concentrations (500.0 µM CdCl₂), plaque formed plants contained more Cd in the roots. Cadmium inducible ion leakage in the root and lowering of the photosynthetic pigment content were less in plants with a plaque. Stretching of aromatic carbonyl groups and alkyl groups among plaque formed plants upon Cd treatments indicated the putative role of phenolics in Cd detoxification.

Selenite mitigates cadmium-induced oxidative stress and affects Cd uptake in rice seedlings under different water management systems

Cadmium (Cd) is toxic throughout the food chain. Selenium (Se) can mitigate Cd accumulation in rice plants, although it is unclear why it has such effect. The objectives of this study were to investigate the effects of Se application on Cd-induced oxidative stress and antioxidant activities, and the combined effects of Se application and water management on the formation of iron plaque on the rice surface and Fe, Cd, and Se accumulation in rice plants. Rice seedlings were grown in Cd-contaminated soil with or without the addition of Se, and in aerobic or flooded conditions. Exogenous Se reduced Cd-induced oxidative stress. In the flooded treatment, exogenous selenite significantly decreased Cd concentrations in rice tissues, whereas it noticeably enhanced Cd concentrations in rice tissues in the aerobic treatment. Furthermore, selenite addition and flooding promote the formation of iron plaque and increase Fe concentrations in rice tissues. Pearson correlation analysis shows that plant Cd was significantly negatively correlated with Fe concentrations in rice tissues, and plant Fe was significantly positively correlated with Se concentrations in rice tissues, but no significant correlation was found between Cd and Se concentrations Thus, exogenous selenite may indirectly affect Cd uptake by influencing the formation of iron plaque on rice root surface, Fe uptake and Fe levels in rice.

Formation of iron plaque in the roots of Spartina alterniflora and its effect on the immobilization of wastewater-borne pollutants

Iron plaque (IP) plays an important role in the absorption of heavy metals (HMs) and nutrients in wetland plants. The present study aims to investigate the effect of IP in Spartina alterniflora on the immobilization of wastewater borne HMs and nutrients. The physiological responses and effect of IP formation on the uptake of HMs, nitrogen (N), and phosphorus (P) were studied in S. alterniflora subjected to different synthetic wastewater (SW) levels and waterlogging durations. Results showed that IP formed in roots of S. alterniflora increased significantly with increasing SW concentration but decreased under prolonged waterlogging. Increasing the waterlogging time enhanced the alcohol dehydrogenase activity and the ethylene content in the roots of S. alterniflora. HMs including Cu, Pb, and Cr, did not significantly accumulate in the IP, despite that the IP content increased with the increasing of SW levels. The SEM-EDX analysis revealed that IP formed on the surface of S. alterniflora did absorb HMs such as Cu, Zn, and Cr. At a fixed level of SW, the amount of HMs that accumulated in the DCB extract was substantially proportional to the IP concentration in the root. Increasing of the SW level enhanced the accumulation of P in the leaves and roots of S. alterniflora. In conclusion, IP formed on S. alterniflora helped immobilize SW pollutants, including HMs and P, and the formation of IP and its effect on pollutant immobilization were influenced by the waterlogging conditions.

Influence of iron plaque on the uptake and accumulation of chromium by rice (Oryza sativa L.) seedlings: Insights from hydroponic and soil cultivation

The effects of iron plaque formation on chromium (Cr) uptake and accumulation by rice seedlings (Oryza sativa L.) were assessed using hydroponic and soil experiments, where each 3 levels of Fe supplementation were added to Hoagland solution (0, 30, and 100 mg Fe²⁺ L^-1) and a typical paddy soil (0, 1, and 2 g Fe²⁺ kg^-1). For each treatment, rice seedlings were exposed to different levels of Cr as chromate at 0, 0.5, 2, 5, 10, and 20 mg L^-1 in solution or 300 mg kg^-1 in soil. Low levels of Cr supply (0.5, 2, and 5 mg L^-1) promoted root biomass, while high levels (10 and 20 mg L^-1) decreased root and shoot biomass and undermined the density and integrity of iron plaque. Iron supply significantly increased the proportion of Cr in iron plaque, but decreased that in rice plants. The results of hydroponic experiment showed that iron plaque formed with Fe supply at 100 mg L^-1 markedly reduced Cr accumulation in shoots of rice seedlings when exposure to 10 and 20 mg L^-1 Cr. The soil culture experiment also demonstrated that exogenous Fe addition significantly decreased Cr concentration in leaf and stem of rice seedlings. These results suggested that iron plaque with appropriate amount was effective to reduce the uptake and accumulation of Cr in rice plants, which have strong implication for taking measures to regulate Cr accumulation in rice grains.

Root iron plaque alleviates cadmium toxicity to rice (Oryza sativa) seedlings

Iron plaque (IP) on root surface can enhance the tolerance of plants to environmental stresses. However, it remains unclear the impact of Fe²⁺ on cadmium (Cd) toxicity to rice (Oryza sativa) seedlings. In this study, the effects of different Fe²⁺ and Cd²⁺ concentration combinations on rice growth were examined hydroponically. Results indicated that Fe²⁺ concentration up to 3.2 mM did not damage rice roots while induced IP formation obviously. Cd²⁺ of 10 μM repressed rice growth significantly, while the addition of 0.2 mM Fe²⁺ to 10 μM Cd²⁺ solution (Cd+Fe) did not damage rice roots, indicating that Fe²⁺ could ameliorate Cd toxicity to rice seedlings. Microstructure analysis showed Cd+Fe treatment induced the formation of IP with dense and intricate network structure, Cd adsorption on the root surface was reduced significantly. Cd concentration of rice roots and shoots and Cd translocation from roots to shoots with Fe+Cd treatment were reduced by 34.1%, 36.0% and 20.1%, respectively, in comparison to a single Cd treatment. Noteworthy, the removal of IP resulted in a larger loss of root biomass under Cd treatment. In addition, Cd+Fe treatment increased the activities of root superoxide dismutase and catalase by 105.5% and 177.4%, and decreased H₂O₂ and O₂·^- accumulation of rice roots by 56.9% and 35.9%, and recovered Cd-triggered electrolyte leakage obviously, when compared with a single Cd treatment. The results from this experiment indicated that the formed dense IP on rice roots decreased Cd absorption and reactive oxygen species accumulation, and Fe²⁺ supply alleviated Cd toxicity to rice seedlings.

Topdressing iron fertilizer coupled with pre-immobilization in acidic paddy fields reduced cadmium uptake by rice (Oryza sativa L.)

Soil cadmium (Cd) contamination has become a serious problem in China. This study was conducted to test the effects of basal application of hydrated lime and iron fertilizer alone or together and topdressing of iron fertilizer at the tillering stage alone or coupled with basal application of hydrated lime, on reducing the accumulation of Cd in brown rice grown in an acidic paddy field slightly contaminated with Cd. The results showed that Cd in brown rice (BR-Cd) was dependent on not only the pH increase and CaCl₂-extractable Cd reduction in the soil due to lime amendment but also Cd sequestration by the iron plaque on root surfaces. However, lime significantly decreased the amounts of Fe and Cd in the iron plaque on the surface of rice root. Topdressing of ferrous sulfate at the tillering stage resulted in the highest Fe and Cd sequestration in the iron plaque. Compared with the control (0.71 mg kg^-1 BR-Cd), the basal application of lime and ferrous sulfate alone or together reduced BR-Cd by 45.8%, 18.3%, and 53.1%, respectively; topdressing of ferrous sulfate alone reduced BR-Cd by 23.6%, and topdressing of ferrous sulfate at the tillering stage coupled with basal application of lime yielded the lowest BR-Cd level with a 74.6% reduction. This result was further confirmed by field experiments at two sites in the following year.

bHLH104 confers tolerance to cadmium stress in Arabidopsis thaliana

Cd is a non-essential heavy metal that is toxic to both plants and animals. Here, we reveal that the transcription factor bHLH104 positively regulates Cd tolerance in Arabidopsis thaliana. We show that Fe deficiency-responsive genes were induced by Cd treatment, and that their upregulation was suppressed in bhlh104 loss-of-function mutants, but enhanced upon overexpression of bHLH104. Correspondingly, the bhlh104 mutants displayed sensitivity to Cd stress, whereas plants overexpressing bHLH104 exhibited enhanced Cd tolerance. Further analysis suggested that bHLH104 positively regulates four heavy metal detoxification-associated genes, IREG2, MTP3, HMA3 and NAS4, which play roles in Cd sequestration and tolerance. The bHLH104 overexpression plants accumulated high levels of Cd in the root but low levels of Cd in the shoot, which might contribute to the Cd tolerance in those lines. The present study thus points to bHLH104 as a potentially useful tool for genetic engineering of plants with enhanced Cd tolerance.

Cadmium exposure triggers genotype-dependent changes in seed vigor and germination of tomato offspring

Although negative effects on the offspring fitness can be triggered by the mother-plant exposure to environmental stresses, some plants are able to "remember" past incidents and enhance the progeny tolerance. Here, the mineral profile, cytogenetic modifications, and physiological potential of seeds from two tomato cultivars, with contrasting tolerance degrees to cadmium (Cd) toxicity, were evaluated after plant exposure to this metal. Both cultivars exhibited high Cd translocation to the seeds; however, the tolerant tomato accumulated more Cd than did the sensitive one. As a consequence of the Cd accumulation, reductions in the Mn concentration in Cd-challenged plants were detected. Surprisingly, seed germination and vigor were increased in the tolerant tomato cultivar after Cd exposure, despite increases in the chromosomal abnormalities. By contrast, seeds from the sensitive cultivar exhibited no changes in their physiological potential after Cd exposure, despite Cd-induced reductions in the mitotic index. Moreover, bunch position exerted effects on the vigor and type of chromosomal abnormality. The results show that maternal plant exposure to Cd can affect tomato offspring by changing the seed physiological potential, and such effect can be partially explained by alterations in the seed-derived elements (essential and non-essential) and genotype-dependent tolerance mechanisms.

Bacillus amyloliquefaciens SAY09 Increases Cadmium Resistance in Plants by Activation of Auxin-Mediated Signaling Pathways

Without physical contact with plants, certain plant growth-promoting rhizobacteria (PGPR) can release volatile organic compounds (VOCs) to regulate nutrient acquisition and induce systemic immunity in plants. However, whether the PGPR-emitted VOCs can induce cadmium (Cd) tolerance of plants and the underlying mechanisms remain elusive. In this study, we probed the effects of Bacillus amyloliquefaciens (strain SAY09)-emitted VOCs on the growth of Arabidopsis plants under Cd stress. SAY09 exposure alleviates Cd toxicity in plants with increased auxin biosynthesis. RNA-Seq analyses revealed that SAY09 exposure provoked iron (Fe) uptake- and cell wall-associated pathways in the Cd-treated plants. However, SAY09 exposure failed to increase Cd resistance of plants after treatment with 1-naphthylphthalamic acid (NPA) or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO). Under Cd stress, SAY09 exposure markedly promoted Fe absorption in plants with the increased hemicellulose 1 (HC1) content and Cd deposition in root cell wall, whereas these effects were almost abrogated by treatment with NPA or c-PTIO. Moreover, exogenous NPA remarkably repressed the accumulation of nitric oxide (NO) in the SAY09-exposed roots under Cd stress. Taken together, the findings indicated that NO acted as downstream signals of SAY09-induced auxin to regulate Fe acquisition and augment Cd fixation in roots, thereby ameliorating Cd toxicity.