Cadmium adsorption, chelation and compartmentalization limit root-to-shoot translocation of cadmium in rice (Oryza sativa L.)

Rice Responses to Abiotic Stress: Key Proteins and Molecular Mechanisms

The intensification of global climate change and industrialization has exacerbated abiotic stresses on crops, particularly rice, posing significant threats to food security and human health. The mechanisms by which rice responds to these stresses are complex and interrelated. This review aims to provide a comprehensive understanding of the molecular mechanisms underlying rice's response to various abiotic stresses, including drought, salinity, extreme temperatures, and heavy metal pollution. We emphasize the molecular mechanisms and structural roles of key proteins involved in these stress responses, such as the roles of SLAC1 and QUAC1 in stomatal regulation, HKT and SOS proteins in salinity stress, heat shock proteins (HSPs) and heat stress transcription factors (HSFs) in temperature stress, and Nramp and ZIP transport proteins in response to heavy metal stress. This review elucidates the complex response networks of rice to various abiotic stresses, highlighting the key proteins and their related molecular mechanisms, which may further help to improve the strategies of molecular breeding.

A holistic field experimental inquiry into cadmium's migration and translocation dynamics across the entire growth spectrum of five Japonica rice cultivars

The contamination of farmland soils with cadmium (Cd) poses a substantial threat to agricultural productivity, food security and safety, and ultimately human health. However, little research has been done on the Cd transport mechanisms in highly Cd polluted soil via field experiment. This study, from a field-scale perspective, examines the migration and transformation features of Cd throughout the growth cycle of five (C1, C2, C3, C4, H1) Japonica rice cultivars in Jiangsu Province, China. Analysis of pH, SOM, total Cd, DTPA-Cd, and microbial communities were conducted. C1 ~ C3 were classified as High Cd-accumulating rice (HC), while C4 and H1 were considered as low Cd-accumulating rice (LC) based on the Cd levels in their brown rice. Phloem was confirmed as the main pathway for Cd into rice grains in high-Cd soil. For the HC group, the Cd concentration in brown and polished rice was positively correlated with the Cd concentration in the leaves and spikes; while for the LC group, they were significantly positively correlated with the Cd concentration in both stem and spike (p < 0.05). The husks of the LC group were more effective in intercepting and sequestering Cd. It was revealed that 6 % ~ 9.09 % of the Cd content detected in the rice grains could be attributed to the internal translocation processes occurring within the plant itself, and approximately 90.91 % ~ 93.84 % of the Cd was traced back to the roots' absorption during grouting. Rice polishing decreased the Cd content from the level in the brown rice by 18 % ~ 47 %. Distinct microbial profiles separated rice rhizosphere from bulk soil, with the former favouring copiotrophs in nutrient-rich zones and the latter oligotrophs in lean conditions. This study delivers crucial data support from a field perspective for a deeper understanding and control of Cd migration and transformation processes in highly Cd-contaminated soil.

Cadmium distribution in rice: Understanding the role of plant nodes and growth stages

Cadmium (Cd) contamination in farmland poses a significant threat to food security in staple crops, especially rice. Using a mix of hydroponic and soil culture methods, stable isotope tracers, and advanced analytical techniques, this study elucidated the mechanisms of Cd uptake, translocation, and accumulation in rice throughout different growth stages. Despite a notable linear correlation between soil DTPA (diethylene-triaminepentaacetic acid)-Cd and the total Cd concentration of rice, our findings showed that the influence of soil Cd level on the proportion of Cd in grain was negligible. The study highlighted the dynamic response of Cd distribution within plant nodes to changes in DTPA-extractable Cd. Heading stage (HS) and mature stage (MS) were critical for Cd uptake and upward transport in rice, and the contribution of Cd absorption in brown rice was 28.61% and 40.16%, respectively. Moreover, the distribution of Cd in nodes showed how important nodes are for controlling and redistributing Cd in rice. In the HS, the lower node had a function in re-transporting, whereas in the MS, there was a considerable redistribution of Cd in the upper node. These insights can help us understand rice Cd dynamics and develop agronomic techniques and rice cultivars that minimize Cd accumulation.

Trade-offs of reproductive growth and Cd remobilization regulated Cd accumulation in wheat grains (Triticum aestivum L.)

Minimization of cadmium (Cd) accumulation in wheat grain (Triticum aestivum L.) is an important way to prevent Cd hazards to humans. However, little is known about the mechanisms of varietal variation of Cd accumulation in wheat grain. This study explores the physiological mechanisms of Cd bioaccumulation through field and hydroponic experiments on two wheat varieties of low-Cd-accumulating variety (L-6331) and high-Cd-accumulating variety (H-6049). Field study showed that average Cd accumulative rates in spikes of H-6049 were 1.57-fold of L-6331 after flowering, ultimately grain-Cd of H-6049 was 1.70-fold of L-6331 in Cd-contaminated farmland. The hydroponic experiment further confirmed that more vegetative tissues of L-6331 were involved in the remobilization of Cd, which jointly mitigated the process of Cd loaded to grains when leaf-cutting conducted after Cd stress. Additionally, the L1 and N1 of L-6331 play an especially important role in regulating Cd remobilization, and the larger EVB areas in N1 have the morphological feature that facilitates the transfer of Cd to L1. Overall results implied that low-Cd-accumulating variety initiated more trade-offs of reproductive growth and Cd remobilizatoin under Cd-stress after flowering compared with high-Cd-accumulating variety, and provided new insights into the processes of Cd loaded into wheat grains among different varieties.

Enhancing remediation efficiency of hyperaccumulators through earthworm addition: Evidence from a pot study on cadmium-contaminated soil

Soil cadmium (Cd) contamination is an urgent environmental problem, which endangers human health through the food chain. Bioremediation attracted extensive attention around the world due to the high cost-efficiency. However, the remediation efficiency of different plant and earthworm species of soil Cd pollution is still unclear, it is thus of great significance to explore the combined effects of different remediation plants and earthworm species to improve the bioremediation capacity. In the present study, we consequently selected three species of Cd hyperaccumulator plants (vetiver, P. vittata and S. emarginatum) and three species of earthworms (E. fetida P1, E. fetida P2, and P. guillelmi) to compare the differences in Cd accumulation among various earthworm-plant combinations. Results indicated that the changes of soil pH and SOM in plant-animal combined application induced the higher soil Cd removal efficiency. The Cd removal efficiency showed highest in combination groups P. vittata-E. fetida P2 and P. vittata-P. guillelmi. Meanwhile, the improvements of biomass of plants and animals also were consistent with the increasing of Cd concentration in both plants and earthworms after combined application. It showed that the Cd concentrations in P. vittata were the highest while the TFs of Cd in S. emarginatum displays significantly more than that in others. In conclusion, the recommended combined system of earthworm-plant (P. vittata-E. fetida P2 and P. vittata-P. guillelmi) to provide reference for soil Cd bioremediation system in practice.

Integrated morphological, physiological and transcriptomic analyses reveal response mechanisms of rice under different cadmium exposure routes

Rice (Oryza sativa) is one of the major cereal crops and takes up cadmium (Cd) more readily than other crops. Understanding the mechanism of Cd uptake and defense in rice can help us avoid Cd in the food chain. However, studies comparing Cd uptake, toxicity, and detoxification mechanisms of leaf and root Cd exposure at the morphological, physiological, and transcriptional levels are still lacking. Therefore, experiments were conducted in this study and found that root Cd exposure resulted in more severe oxidative and photosynthetic damage, lower plant biomass, higher Cd accumulation, and transcriptional changes in rice than leaf Cd exposure. The activation of phenylpropanoids biosynthesis in both root and leaf tissues under different Cd exposure routes suggests that increased lignin is the response mechanism of rice under Cd stress. Moreover, the roots of rice are more sensitive to Cd stress and their adaptation responses are more pronounced than those of leaves. Quantitative PCR revealed that OsPOX, OsCAD, OsPAL and OsCCR play important roles in the response to Cd stress, which further emphasize the importance of lignin. Therefore, this study provides theoretical evidence for future chemical and genetic regulation of lignin biosynthesis in crop plants to reduce Cd accumulation.

Zinc induced regulation of PCR1 gene for cadmium stress resistance in rice roots

In this study, the interaction between zinc (Zn) and cadmium (Cd) was investigated in rice roots to evaluate how Zn can protect the plants from Cd stress. Rice seedlings were treated with Cd (100 μM) and Zn (100 μM) in different combinations (Cd alone, Zn alone, Zn+ Cd, Zn+ Cd+ L-NAME, Zn+ Cd+ L-NAME+ SNP). Rice roots treated with only Zn also displayed similar toxic effects, however when combined with Cd exhibited improved growth. Treating the plant with Zn along with Cd distinctly reduced Cd concentration in roots while increasing its own accumulation due to modulation in expression of Zinc-Regulated Transporter (ZRT)-/IRT-Like Protein (OsZIP1) and Plant Cadmium Resistance1 (OsPCR1). Cd reduced plant biomass, cell viability, pigments, photosynthesis and causing oxidative stress due to inhibition in ascorbate-glutathione cycle. L-NAME (NG-nitro L-arginine methyl ester), prominently suppressed the beneficial impacts of Zn against Cd stress, whereas the presence of a NO donor, sodium nitroprusside (SNP), significantly reversed this effect of L-NAME. Collectively, results point that NO signalling is essential for Zn- mediated cross-tolerance against Cd stress via by modulating uptake of Cd and Zn and expression of OsZIP1 and OsPCR1, and ROS homeostasis due to fine tuning of ascorbate-glutathione cycle which finally lessened oxidative stress in rice roots. The results of this study can be utilized to develop new varieties of rice through genetic modifications which will be of great significance for maintaining crop productivity in Cd-contaminated areas throughout the world.

Transcriptomics and metabolomics association analysis revealed the responses of Gynostemma pentaphyllum to cadmium

Gynostemma pentaphyllum an important medicinal herb, can absorb high amounts of cadmium (Cd) which can lead to excessive Cd contamination during the production of medicines and tea. Hence, it is crucial to investigate the response mechanism of G. pentaphyllum under Cd stress to develop varieties with low Cd accumulation and high tolerance. Physiological response analysis, transcriptomics and metabolomics were performed on G. pentaphyllum seedlings exposed to Cd stress. Herein, G. pentaphyllum seedlings could significantly enhance antioxidant enzyme activities (POD, CAT and APX), proline and polysaccharide content subject to Cd stress. Transcriptomics analysis identified the secondary metabolites, carbohydrate metabolism, amino acid metabolism, lipid metabolism, and signal transduction pathways associated with Cd stress, which mainly involved the XTH, EXP and GST genes. Metabolomics analysis identified 126 differentially expressed metabolites, including citric acid, flavonoid and amino acids metabolites, which were accumulated under Cd stress. Multi-omics integrative analysis unraveled that the phenylpropanoid biosynthesis, starch, and sucrose metabolism, alpha-linolenic acid metabolism, and ABC transporter were significantly enriched at the gene and metabolic levels in response to Cd stress in G. pentaphyllum. In conclusion, the genetic regulatory network sheds light on Cd response mechanisms in G. pentaphyllum.

Clonal integration can promote the growth and spread of Alternanthera philoxeroides in cadmium-contaminated environments

Clonal plants are able to support the growth of their ramets in stressful environments via clonal integration between the ramets. However, it remains unclear whether the developmental status of stressed ramets affects the role of clonal integration. Here, we explored the effects of clonal integration at both the ramet level and the whole clonal fragment level when the apical ramets (younger) and basal ramets (older) were subjected to different concentrations of cadmium contamination. We grew pairs of ramets of Alternanthera philoxeroides, which were connected or disconnected by stolon between them. The apical and basal ramets were either uncontaminated or individually subjected to Cd contamination at concentrations of 5 mg kg-1 and 50 mg kg-1, respectively. Our results showed that clonal integration significantly promoted the growth of apical ramets subjected to Cd contamination. More importantly, under high Cd treatment, clonal integration also had a significant positive effect on the fitness of the whole clonal fragments. However, clonal integration did not affect plant growth when basal ramets were subjected to Cd contamination. Our study reveals the influence of the developmental status of stressed ramets on the role of clonal integration in heterogeneous heavy metal stress environments, suggesting that clonal integration may facilitate the spread of A. philoxeroides in Cd-contaminated habitats.

Cocultivation with Solanum nigrum and inoculation with Rhizophagus intraradices can improve plant photosynthesis and antioxidant defense to alleviate cadmium toxicity to soybean

High Cd pollution can damage plant physiology and seriously threaten ecological security and human health. Therefore, we designed a cropping system, arbuscular mycorrhizal fungi (AMF) - soybean - Solanum nigrum L., to solve the high Cd pollution problem in an environmentally and economically friendly way. The results showed that AMF were able to break free from the constraints of cocultivation and still promote plant photosynthesis and growth in combined treatments to resist Cd stress. In addition, cocultivation combined with AMF improved the antioxidant defense to scavenge reactive oxygen species by promoting the production of antioxidant enzymes and nonenzyme substances in host plants. The glutathione content in soybean and the catalase activity in nightshade were recorded at the highest values under cocultivation combined with AMF treatment, which were 23.68% and 129.12% higher than those of monoculture without AMF treatments. The improvement in antioxidant defense alleviated oxidative stress, which was manifested by the reduction in Cd dense electronic particles in the ultrastructure and a 26.38% decrease in MDA content. Furthermore, this cropping mode combined the advantages of cocultivation to improve the Cd extraction efficiency and Rhizophagus intraradices to limit Cd accumulation and transport so that Cd was more accumulated and restricted in the roots of the cocultivated Solanum nigrum L., and the Cd concentration in soybean beans was reduced by 56% compared with the soybean monoculture without AMF treatment. Therefore, we suggest that this cropping system is a comprehensive and mild remediation technology suitable for highly Cd-contaminated soil.

Mechanism of calcium signal response to cadmium stress in duckweed

Cadmium (Cd) causes serious damage to plants. Although calcium (Ca) signal has been found to respond to certain stress, the localization of Ca and molecular mechanisms underlying Ca signal in plants during Cd stress are largely unknown. In this study, Ca²⁺-sensing fluorescent reporter (GCaMP3) transgenic duckweed showed the Ca²⁺ signal response in Lemna turionifera 5511 (duckweed) during Cd stress. Subsequently, the subcellular localization of Ca²⁺ has been studied during Cd stress by transmission electron microscopy, showing the accumulation of Ca²⁺ in vacuoles. Also, Ca²⁺ flow during Cd stress has been measured. At the same time, the effects of exogenous glutamic acid (Glu) and γ-aminobutyric (GABA) on duckweed can better clarify the signal operation mechanism of plants to Cd stress. The molecular mechanism of Ca²⁺ signal responsed during Cd stress showed that Cd treatment promotes the positive response of Ca signaling channels in plant cells, and thus affects the intracellular Ca content. These novel signal studies provided an important Ca²⁺ signal molecular mechanism during Cd stress.

Silicon Fertilization: A Step towards Cadmium-Free Fragrant Rice

Soil contamination with toxic cadmium (Cd) is becoming a serious global problem and poses a key hazard to environments and the health of human beings worldwide. The present study investigated the effects of foliar applications of three forms of silicate chemicals (calcium silicate, sodium silicate, and potassium silicate) at four rates (0.25%, 0.5%, 0.75%, and 1.0%) at tillering stage on rice growth and the accumulation of Cd under Cd stress (30 mg kg^-1). The results showed that Cd stress reduced the yield-related traits and enlarged Cd contents in different rice organs. The leaf gas exchange attributes and yield traits were enhanced, and the Cd accumulation and bioaccumulation factor in rice organs were reduced, especially in grains, through silicon application. In shoots, roots, and grains, foliar spray of Si reduced Cd contents by 40.3%, 50.7%, and 47.9%, respectively. The effectiveness of silicate compounds in reducing Cd toxicity varied with the kind of chemicals and doses of foliar applications. Foliar application of potassium silicate, at a rate of 0.5%, at tillering stage, showed the best effectiveness in improving grain yield, while mitigating Cd accumulation in rice grains. The outcome of this study provides a promising practicable approach in alleviating Cd toxicity in rice and preventing the entrance of Cd into the food chain.

Effect of Isosteviol on Wheat Seed Germination and Seedling Growth under Cadmium Stress

Cadmium (Cd) contamination of agricultural soil has become a serious threat to global food security. The present study highlights the effect of added isosteviol in modulating growth physiology and antioxidant defense systems conferring tolerance against cadmium (Cd) stress in wheat. Wheat growth, chlorophyll content, malondialdehyde (MDA) content of leaves, dehydrogenase activity of root, and antioxidant enzyme activity were determined to get an overview of cellular response in conquering Cd-induced oxidative stress damages. The results indicated that wheat germination was inhibited under Cd2+ concentration at 10 µM. The presence of isosteviol and gibberellic acid (GA) significantly alleviated the inhibitory effect on the growth of wheat seedling under 10 µM Cd2+ stress. Moreover, different concentrations of isosteviol and GA regulated the physiological changes of wheat under Cd stress: more chlorophyll a + b content; less MDA content; and higher dehydrogenase activity of root and antioxidant enzyme activity of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), as compared to Cd alone in wheat seedling. The present study thus suggests a possible role of isosteviol in amelioration of Cd stress by increasing chlorophyll content and root dehydrogenase activity, which also could reduce oxidative damage of the cell membrane by regulating the activities of antioxidant enzymes in wheat seedling.

Growth and nutritional responses of wild and domesticated cacao genotypes to soil Cd stress

Cadmium (Cd), a toxic non-essential metal, is easily accumulated in cacao tissues. This represents a risk for cacao exportation, and consequently it affects the economic well-being of the resource-poor-small-producers in Latin America. A greenhouse experiment was conducted with 53 wild and domesticated cacao genotypes to determine their response to Cd in terms of growth and Cd and essential nutrients accumulation. Cacao seedlings were grown for 6 months in an acidic soil with or without added Cd. The total concentration of macro (Ca, K, Mg, N and P) and micronutrients (B, Cu, Fe, Mn and Zn) as well as Cd were measured in shoots along with growth (biometric) parameters after harvest. The results revealed that even if there was a wide range of Cd concentrations among genotypes, there was a reduction in the concentration of essential nutrients in genotypes grown in Cd spiked soils, however these concentrations were not significantly different from the control. In the case of growth parameters, the effects of Cd were diverse across all genotypes some of them being more tolerant to Cd stress than others. Thus, different growth responses to Cd stress are related to a genotype effect. Based on their lower Cd concentration, a total of 11 cacao genotypes (AYP-22, PAS-105, UGU-126, ICT-1026, ICT-1087, ICT-1189, ICT-1292, PH-17, CCN-51, ICS-39 and TSH-565) are proposed here as low Cd-accumulating genotypes. Therefore, these genotypes are potentially useful as rootstock to reduce uptake and transport of Cd, especially in economically important cacao cultivars.

Synergistic effect of silicon and selenium on the alleviation of cadmium toxicity in rice plants

Silicon (Si) and selenium (Se), two beneficial elements that alleviate cadmium (Cd) toxicity, are important for agricultural production and human health. However, the effects and related mechanisms of Si-Se interaction on Cd toxicity alleviation are still poorly understood. Herein, a hydroponic experiment was employed to evaluate the effects of Si and Se alone and together, on the growth, Cd content, and biochemical parameters of Cd-treated rice plants. The results revealed that both Si and Se can effectively alleviate Cd toxicity, and a strong synergistic effect of Si and Se was observed. Simultaneous use of Si and Se significantly promoted rice plant growth, decreased malondialdehyde (MDA) content in both the roots and shoots, and reduced Cd translocation factor leading to a significant 73.2 % decrease in shoot Cd content. Additionally, Si-Se interaction increased glutathione (GSH) content, phytochelatin (PC) content and Cd distribution in root cell walls and organelles. Furthermore, the relative expression of OsHMA2 was down-regulated, while those of OsNramp1 and OsMHA3 were up-regulated. The above findings suggest that synergistic effect of Si and Se on Cd toxicity amelioration occurs mainly via regulating gene expression, sequestering Cd in the root cell walls and organelles, and reducing Cd transfer to the shoots.

Ameliorative role of Bacillus subtilis FBL-10 and silicon against lead induced stress in Solanum melongena

The continuous deterioration of arable lands by metal pollution compels finding suitable strategies to increase plant tolerance under contaminated regimes. Current study was designed to examine the synergistic role of Bacillus subtilis FBL-10 and silicon (Si) with respect to mitigation of lead (Pb) induced phytotoxicity in Solanum melongena L. Lead stress (75 mg kg-1) reduced chlorophyll (Chl) content, photosynthetic rate and gas exchange characteristics of S. melongena plants. The Si and B. subtilis FBL-10 individually upgraded all the above-mentioned growth attributes. However, co-application of Si (50 mg kg-1) and B. subtilis FBL-10 significantly improved biochemical and growth attributes of Pb challenged plants. The abridged levels of oxidative markers including hydrogen peroxide (H2O2), and malondialdehyde (MDA) besides reduced Pb accumulation in foliage tissues, were recorded in Si and microbe assisted plants. Furthermore, plants inoculated with B. subtilis FBL-10 alone or in combination with Si showed increment in total soluble proteins, photosynthetic rate and gas exchange attributes. The inoculated plants treated with Si exhibited higher level of auxins and improved activity of antioxidant enzymes under Pb stress. Present research elucidates interactive role of B. subtilis FBL-10 and Si in reduction of Pb toxicity in S. melongena plants. Alone application of Si or B. subtilis FBL-10 was less effective for attenuation of Pb stress; however, synergism between both phyto-protectants demonstrated fabulous ability for Pb stress assuagement. Consequently, executions of field studies become indispensable to comprehend the efficacy of Si applied alone or in combination with plant growth promoting bacteria (PGPB) like B. subtilis FBL-10. From current research, it is concluded that the interaction of Si and PGPB seems an auspicious technique and eco-friendly approach to enhance metal tolerance in crop plants.

Responses of glutathione and phytochelatins biosysthesis in a cadmium accumulator of Perilla frutescens (L.) Britt. under cadmium contaminated conditions

Screening new accumulators of heavy metal and identifying their tolerance, enrichment capacity of heavy metals are currently hot issues in phytoremediation research. A series of hydroponic experiments were conducted to analyze the effects of glutathione and phytochelatins in roots, stems, and leaves of Perilla frutescens under cadmium stress. The results showed that the non-protein thiols in roots and stems mainly existed in the form of GSH, PC₂, PC₃, and PC₄ under Cd stress condition, while in leaves they existed in the form of GSH, PC₂, and PC₃. Furthermore, the contents of GSH and PCs positively correlated with Cd, but negatively correlated with root vigor and chlorophyll content under Cd stress conditions. After 21 days of treatments, the contents of Cd in different parts of the plant were 1465.2-3092.9 mg· kg^-1 in the roots, 199.6-478.4 mg·kg^-1 in the stems and 61.3-96.9 mg· kg^-1 in the leaves at 2, 5, 10 mg·L^-1 Cd levels respectively, and the amount of Cd uptakes were up to 3547.7-5701.7 μg·plant^-1. Therefore, P. frutescens performed high capacity in Cd accumulation, and PCs played a key role in Cd tolerance. The application prospect of the plant in phytoremediation Cd polluted soil was also discussed.

Transcriptome sequencing reveals the effects of cadmium toxicity on the cold tolerance of the wolf spider Pirata subpiraticus

As the predominant predator of pests in rice fields, spiders have been exposed to cadmium (Cd) pollution for a long time. The livability of spiders during the overwintering period is closely related to population growth in spring, but the effects of Cd on spider's survival of cold hardness and the underlining mechanism remain unclear. In the present study, we found that some growth parameters (body length, width, mass and livability) in the wolf spider Pirata subpiraticus were altered distinctively under Cd stress. To investigate the effects of Cd toxicity on the spider at molecular levels, RNA-sequencing was performed on the spiderlings undergoing ambient temperature alterations. Transcriptome data showed that a total of 807 differentially expressed genes (DEGs) were yielded in the comparison. The obtained DEGs were mainly linked with metabolism-related process, including oxidoreductase activity and lipid transport, and 25 DEGs were associated with the reported cryoprotectants, including glycerol, arginine, cysteine, heat shock protein, glucose and mannose. Growth factors (insulin growth factor, platelet-derived growth factor and transforming growth factor) and cytochrome P450 encoding genes were dramatically expressed in the spider. Furthermore, transcriptional factors (TFs) family were characterized according to the transcriptomic profile, and ZBTB TFs were represented the most distinctive alterations in the characterized genes. Collectively, our study illustrated that Cd poses disadvantageous effects on the growth of P. subpiraticus at cold ambient temperature, and the spiders are capable of responding to the adverse Cd stress by expressing the genes involved in the metabolism of energy substances, cryoprotectants and immune-related components.

Combined application of silicon and nitric oxide jointly alleviated cadmium accumulation and toxicity in maize

Cadmium (Cd) contamination is a serious threat to plants and humans. Application of silicon (Si) or nitric oxide (NO) could alleviate Cd accumulation and toxicity in plants, but whether they have joint effects on alleviating of Cd accumulation and toxicity are not known. Therefore, the combined effect of Si and NO application on maize growth, Cd uptake, Cd transports and Cd accumulation were investigated in a pot experiment. Here, we reported that Cd stress decreased growth, caused Cd accumulation in plants. The combined application of Si and NO triggered a significant response in maize, increasing plant growth and reducing Cd uptake, accumulation, translocation and bioaccumulation factors under Cd stress. The grain Cd concentration was decreased by 66 % in the Si and NO combined treatment than Cd treatment. Moreover, the combined application of Si and NO reduced Cd health risk index in maize more effectively than either treatment alone. This study provided new evidence that Si and NO have a strong joint effect on alleviating the adverse effects of Cd toxicity by decreasing Cd uptake and accumulation. We advocate for supplement of Cd-contaminated soil with Si fertilizers and treatment of crops with NO as a practical approach to alleviating Cd toxicity.

The hybridization between Sphagneticola trilobata (L.) Pruski and Sphagneticola calendulacea (L.) Pruski improved the tolerance of hybrid to cadmium stress

Hybridization is common between invasive and native species and may be accompanied by invasive evolution. The hybrid of Sphagneticola trilobata (alien invasive species) and Sphagneticola calendulacea (indigenous congener) was found in South China. According to previous studies, the hybrid performed weak environmental adaptability in comparison with parents. However, based on the results from this study, the hybridization significantly improved the tolerance of the hybrid to cadmium (Cd) stress (200 μmol L^-1). Under Cd stress, the hybrid lines showed lowest level of oxidative damage and the highest level of photosynthetic efficiency. Compared with the parents, the hybrid utilized more active detoxification strategies, such as the cell walls of the leaves and roots adsorbed 88% and 95% Cd, respectively, reducing the amount of Cd entering cells; moreover, most of the Cd that entered cells was transformed into less toxic chemical forms through the reduction of the highly toxic chemical forms; furthermore, it accumulated a large number of phytochelatins to bind Cd²⁺ and reduced the damage of organelles by Cd²⁺. The results demonstrate that hybridization between S. trilobata and S. calendulacea improved the adaptability of the new hybrid species to Cd stress and may pose a greater threat to the survival of the native parent species in the presence of serious water and soil pollution.

Silicon application improved the yield and nutritional quality while reduced cadmium concentration in rice

Silicon (Si) is an essential nutrient for rice, but its effects on the yield and quality of rice under heavy metal stress remain uncertain. In this study, two typical paddy soils (acidic and calcareous purple soils) in the western region of Chongqing were selected for field plot experiment, with the purpose of understanding the effects of Si implementation methods on grain yields and cadmium (Cd) uptake, transport, and accumulation in the grain of a hybrid rice (Oryza sativa L, Changliangyou 772). Four treatments were set for the purposes including soil-based Si application, foliar spray of Si alone, foliar spray of selenium (Se)-containing Si fertilizer, and a control without Si application, respectively. The results indicated that the Si applications reduced Cd contents in brown rice by 11.45~51.85% in the slightly Cd-contaminated acidic purple soil (pH = 4.77, soil total Cd content 0.413 mg kg^-1) and 26.93~43.77% in the purple calcareous paddy soil (pH = 7.77) with similar Cd-polluting levels. It is worth noting that the Cd content of conventional fertilized rice exceeds the Chinese National Food Safety Standard limit (0.2 mg kg^-1, GB2762-2017) in the slightly Cd-contaminated acidic purple soil, and foliar spray treatments showed most effective effects that meets the safety threshold standard. Soil-based Si application reduced Cd accumulation in rice grains mainly by inhibiting the translocation of Cd from stem to the rice grain or root to stem, while foliar sprays of Si mainly by inhibiting the translocation of Cd from stem to brown rice. Si applications increased the rice yield by 17.15 to 25.45% in calcareous paddy soil with foliar spray being the best, while no significant yield increase was found in acidic paddy soil. Si and Se-containing Si fertilizer improved the nutritional quality of rice grain as indicated by the increases of Se, Si, and protein contents and the significant decreases of Cd contents in the rice grains. The comprehensive effects in improving the rice quality follow the order of foliar spray of Se-containing Si fertilizer > foliar spray of Si alone > soil-based Si application. Thus, foliar spray Si-containing fertilizer could be helpful in increasing rice yield while reducing the Cd uptake in rice grains, which might be a feasible approach in controlling Cd entry into the human body via crops.

Cadmium-zinc cross-talk delineates toxicity tolerance in rice via differential genes expression and physiological / ultrastructural adjustments

Understanding the physiological and molecular response of crop genotypes could be useful in eco-toxicological evaluation with cadmium (Cd) and could be a strategy to solve heavy metal contamination in agriculture. This study corroborates unique patterns of Cd accumulation and molecular mechanisms adopted by plants to acquire Cd tolerance and counteractive effects of zinc (Zn) against Cd toxicity. Two rice (Oryza sativa) genotypes (Heizhan 43 and Yinni 801) differing in cadmium tolerance and its accumulation in plant tissues were investigated hydroponically using two Cd levels [Cd₁₀ (10 μM L^-1) and Cd₁₅ (15 μM L^-1)] and two Zn levels [Zn₂₅ (25 μM L^-2) and Zn₅₀ (50 μM L^-1)] and their combinations. Cadmium toxicity rendered substantial reduction in plant height, biomass, chlorophyll contents and photosynthesis as compared to the control plants after 15 days of treatment. Supplementation of Zn evidently ameliorated Cd toxicity by minimizing the reduction in plant growth, chlorophyll contents and photosynthetic attributes (Pn, gs, Ci, and Tr). Comparatively, lower accumulation of Cd in Yinni 801 under combined treatments revealed a preferential uptake of Zn in this genotype. A cross-talk among Cd, Zn, Fe, Ca and K correlated with fluctuating gs, Ci and Tr. Both genotypes also differed in morphological alterations of cell membrane, chloroplasts and appearance of enlarged plastoglobuli along with distorted mitochondria. An increased ascorbate peroxidase activity in roots of Yinni 801 presented a defensive strategy. Relative expression of Cd and Zn ion transporter genes also confirmed the genotypic background of phenotypic divergence. The OsLCT1 and OsHMA2 expression was significant in Heizhan 43, indicating possible translocation of Cd from shoot to grains contrary to Yinni 801, which accumulated Cd in shoot and showed stunted growth. Zn supplementation promises tolerance to Cd in Yinni 801 by differential expression of putative genes for Cd translocation with minimum ultrastructural modifications by maintaining physiological functions in contrast to Heizhan 43.

Cadmium allocation to grains in durum wheat exposed to low Cd concentrations in hydroponics

This study aims to characterize the response of durum wheat to different concentrations of Cd found in agricultural soils. One French durum wheat cultivar (i.e. Sculptur) was exposed to low concentrations of Cd (5 nM or 100 nM) in hydroponics. After anthesis, the plants were fed with a solution enriched with the stable isotope ¹¹¹Cd to trace the newly absorbed Cd. Plants were sampled at anthesis and grain maturity to assess how plant growth, Cd uptake and partitioning among organs, as well as Cd remobilization, differed between the two Cd exposure levels. Durum wheat did not show any visual symptoms of Cd toxicity, regardless of which Cd treatment was applied. However, post-anthesis durum wheat growth was 14% penalized at 100 nM due to the large transpiration-based accumulation of Cd in leaves at this stage. The allocation of Cd to the grains was not restricted but enhanced at 100 nM compared to 5 nM. Both the root-to-shoot Cd translocation and the fraction of aboveground Cd allocated to grains were higher in plants exposed to 100 nM. Cadmium was remobilized exclusively from roots and stems, and remobilized Cd contributed on average to 40-45% of the Cd accumulated in mature grains, regardless of which Cd treatment was applied. The relevance of these results to decreasing the concentration of Cd in durum wheat grains is discussed.

Inhibition of Cd accumulation in grains of wheat and rice under rotation mode using composite silicate amendment

The accumulation of heavy metals in soils and crops jeopardizes human health, and thus remedying soil and ensuring food safety have attracted wide concern. In this study, composite silicate was employed as an amendment to inhibit cadmium (Cd) accumulation in the grains of wheat and rice in an upland/paddy rotation mode in field-scale remediation. The composite silicate amendment (CSA) at a dosage of 0.2-0.8% decreased the Cd concentration in wheat grains in the first growing season of upland mode by 7.5-58.3% compared with CK, and decreased the Cd concentration in brown rice by 38.7-58.1% in the second season of paddy mode. The minimum values satisfy the Chinese National and International Standards. The results confirmed the inhibitory effect of CSA on the accumulation of Cd in crop grains. CSA increased the soil pH obviously and enhanced the sorption of Cd on soil particles by 14.6-56.2%, and declined the DTPA- and HCl-extractable Cd concentrations in the soil by 16.2-49.5% and 23.8-75.6%, respectively. Furthermore, CSA decreased the exchangeable Cd fraction by 21.5-41.6% in the sequential extraction. The immobilization effect was retained in both growing seasons in terms of Cd concentration in the crop grains and extractable Cd concentration in the soil. CSA had a negligible effect on the normal growth of wheat and rice and the available Zn and Cu concentration in the soil, indicating its environmental friendliness. Considering its low cost and abundant reserves, CSA can be recommended as an immobilization amendment for Cd-polluted paddy soil in wheat/rice rotation mode.

Split application of silicon in cadmium (Cd) spiked alkaline soil plays a vital role in decreasing Cd accumulation in rice (Oryza sativa L.) grains

Food contamination with cadmium (Cd) is a serious health threat to humans worldwide and Cd accumulation by rice is a major source of Cd entrance to the food chain. Silicon (Si) application decreases the Cd content in rice but the timing of Si application may need further investigation. The present study investigated the effect of split application of Si in the soil (600 kg/ha of Si) at different growth stages of rice on the growth and Cd accumulation by rice under Cd stress. Rice plants were grown in the presence and absence of Cd and Si was applied in the soil at different growth stages of rice under Cd stress. The results indicated that Cd stress alone reduced the growth and photosynthesis and increased the Cd content in different tissues and grains of rice. Silicon application improved the plant growth and reduced the Cd accumulation, translocation factor, and bioaccumulation factor in rice especially in grains, whereas the response of Si varied with the application of Si at different growth stages. The application of Si in three splits (transplanting (S1), tillering (S2), panicle initiation (S3)) was the best in improving growth and reducing Cd concentrations in plants compared to other combinations of Si application. Silicon application in three splits (S1+S2+S3) reduced the grain Cd concentrations below the threshold level (0.2 mg/kg) and reduced the Cd health risk index under the experimental conditions. Overall, split application of Si at three growth stages may function as remediator and diminishes Cd uptake into rice grains.

Arbuscular mycorrhizal fungi alleviate Cd phytotoxicity by altering Cd subcellular distribution and chemical forms in Zea mays

Arbuscular mycorrhizal fungus (AMF) can relieve Cd phytotoxicity and improve plant growth, but the mechanisms involved in this process have still been not completely known. In the present work, a pot experiment was conducted to examine productions of glutathione (GSH) and phytochelatins (PCs), and absorption, chemical forms and subcellular distribution of Cd in maize (Zea mays) inoculated with or without AMF (Rhizophagus intraradices (Ri) and Glomus versiforme (Gv)) in Cd-amended soils (0, 1 and 5 mg Cd kg^-1 soil). In general, both Ri and Gv inoculation dramatically enhanced biomass production and reduced Cd concentrations in shoots and roots of maize when compared to the non-mycorrhizal treatment. Moreover, both Ri and Gv symbiosis obviously increased contents of GSH and PCs, both in shoots and roots. Subcellular distribution of Cd in maize indicated that most of Cd (more than 90%) was accumulated in cell wall and soluble fraction. In addition, Cd proportions in soluble fractions in shoots of maize inoculated with Gv or Ri were considerably increased, but reduced in cell wall fractions compared to non-mycorrhizal maize, indicating that mycorrhizal symbiosis promoted Cd transfer to vacuoles. Furthermore, proportions of Cd in inorganic and water-soluble forms were declined, but elevated in pectates and proteins-integrated forms in mycorrhizal maize, which suggested that Gv and Ri could convert Cd into inactive forms. These observations could provide a further understanding of potential Cd detoxification mechanism in maize inoculated with AMF.

Cadmium isotope fractionation in the soil – cacao systems of Ecuador: a pilot field study

The often high Cd concentrations of cacao beans are a serious concern for producers in Latin America due to the implementation of stricter Cd limits for cocoa products by the European Union in 2019. This is the first investigation to employ coupled Cd isotope and concentration measurements to study soil – cacao systems. Analyses were carried out for 29 samples of soils, soil amendments and cacao tree organs from organic farms in Ecuador that harvest three distinct cacao cultivars. The majority of soils from 0–80 cm depth have very similar δ^114/110Cd of about −0.1‰ to 0‰. Two 0–5 cm topsoils, however, have high Cd concentrations coupled with heavy Cd isotope compositions of δ^114/110Cd ≈ 0.2%, possibly indicating Cd additions from the tree litter used as organic fertilizer. Whilst cacao leaves, pods and beans are ubiquitously enriched in Cd relative to soils there are distinct Cd isotope signatures. The leaves and pods are isotopically heavier than the soils, with similar Δ^114/110Cd_leaf–soil values of 0.22 ± 0.07‰ to 0.41 ± 0.09‰. In contrast, the data reveal differences in Δ^114/110Cd_bean–leaf that may be linked to distinct cacao cultivars. In detail, Δ^114/110Cd_bean–leaf values of −0.34‰ to −0.40‰ were obtained for Nacional cacao from two farms, whilst CCN-51 hybrid cacao from a third farm showed no fractionation within error (−0.08 ± 0.13‰). As such, further work to investigate whether Cd isotopes are indeed useful for tracing sources of Cd enrichments in soils and to inform genetic efforts to reduce the Cd burden of cocoa is indicated.

Cd isotope composition in cacao seems to be cultivar-specific whereas Cd in soil is probably due to tree litter recycling.

Screening of candidate gene responses to cadmium stress by RNA sequencing in oilseed rape (Brassica napus L.)

Cadmium (Cd) stress is one of the most serious threats to agriculture in the world. Oilseed rape (Brassica napus L.) is an important oil crop; however, Cd can easily accumulate in rapeseed and thus harm human health through the food chain. In the first experiment, our purpose was to measure the Cd accumulation in mature B. napus plants and its influences on fatty acid composition. The results showed that most Cd was accumulated in the root, and the seed fatty acid content was considerably different at different Cd toxicity levels. In the second experiment, 7-day-old B. napus seedlings stressed by Cd (1 mM) for 0 h (CK-0h), 24 h (T-24h), or 72 h (T-72h) were submitted to physiological and biological analyses, RNA-Seq and qRT-PCR. In total, 5469 and 6769 differentially expressed genes (DEGs) were identified in the comparisons of "CK-0h vs T-24h" and "CK-0h vs T-72h", respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses showed that the photosynthetic and glutathione (GSH) pathways were significantly enriched in response to Cd stress. Key factors in the response to Cd stress included BnPCS1, BnGSTU12, BnGSTU5, and BnHMAs. The transcription factors BnWRKY11 (BnaA03g51590D), BnWRKY28 (BnaA03g43640D), BnWRKY33 (BnaA03g17820D), and BnWRKY75 (BnaA03g04160D) were upregulated after Cd exposure. The present study revealed that upregulation of the genes encoding GST and PCS under Cd stress promoted the formation of low-molecular weight complexes (PC-Cd), and upregulation of heavy metal ATPase genes induced PC-Cd transfer to vacuoles. These findings may provide the basis for the molecular mechanism of the response of B. napus to Cd.

Absorption, translocation, and detoxification of Cd in two different castor bean (Ricinus communis L.) cultivars

Cadmium (Cd) is considered to be the most phytotoxic heavy metal pollutant. The selection of castor bean cultivars with Cd tolerance and the exploration of the physiological mechanisms involved in Cd tolerance are critical steps for improving phytoremediation performance. In this study, a hydroponic experiment was used to investigate variations in Cd transportation, chelation, and subcellular distribution in two different castor bean cultivars, namely JX-22 and ZB-9. Both cultivars had high tolerance index scores, indicating that both cultivars were tolerant to Cd. The findings of the present study indicate that Cd is significantly more mobile in JX-22 than in ZB-9 during xylem and phloem transportation, resulting in the accumulation of Cd in the shoots of JX-22 was 7.67 times that in ZB-9. Subcellular distribution assessment verified that more Cd was bound to the biologically detoxified metal fractions than the metal sensitive fractions in JX-22. The contents of the non-protein thiol pool and glutathione in the leaves were higher in JX-22 than ZB-9 when exposed to Cd. These results indicate that JX-22 has a greater ability to accumulate Cd, and well-coordinated physiological changes in JX-22 afford greater Cd tolerance in comparison to ZB-9 under Cd exposure, indicating that JX-22 is suitable for use in the remediation of Cd-contaminated soils.

A Node-Expressed Transporter OsCCX2 Is Involved in Grain Cadmium Accumulation of Rice

Excessive cadmium (Cd) accumulation in grains of rice (Oryza sativa L.) is a risk to food security. The transporters in the nodes of rice are involved in the distribution of mineral elements including toxic elements to different tissues such as grains. However, the mechanism of Cd accumulation in grains is largely unknown. Here, we report a node-expressed transporter gene, OsCCX2, a putative cation/calcium (Ca) exchanger, mediating Cd accumulation in the grains of rice. Knockout of OsCCX2 caused a remarkable reduction of Cd content in the grains. Further study showed that disruption of this gene led to a reduced root-to-shoot translocation ratio of Cd. Moreover, Cd distribution was also disturbed in different levels of internode and leaf. OsCCX2 is localized to plasma membrane, and OsCCX2 is mainly expressed in xylem region of vascular tissues at the nodes. OsCCX2 might function as an efflux transporter, responsible for Cd loading into xylem vessels. Therefore, our finding revealed a novel Cd transporter involved in grain Cd accumulation, possibly via a Ca transport pathway in the nodes of rice.

Cell metabolomics reveals the neurotoxicity mechanism of cadmium in PC12 cells

The heavy metals such as cadmium (Cd) can induce neurotoxicity. Extensive studies about the effects of Cd on human health have been reported, however, a systematic investigation on the molecular mechanisms of the effects of Cd on central nervous system is still needed. In this paper, the neuronal PC-12 cells were treated with a series of concentrations of CdCl₂ for 48h. Then the cytotoxicity was evaluated by MTT (3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) assay. The IC₁₅ value (15% inhibiting concentration) was selected for further mechanism studies. After PC-12 cells incubated with CdCl₂ at a dose of IC₁₅ for 48h, the intracellular and extracellular metabolites were profiled using ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS)-based cell metabolomics approach. As found, the effects of the heavy metal Cd produced on the PC-12 cell viability were dose-dependent. The metabolic changes were involved in the glycolysis and gluconeogenesis, biopterin metabolism, tryptophan metabolism, tyrosine metabolism, glycerophospholipid metabolism, and fatty acids beta-oxidation. These could cause the perturbation of cell membrane, redox balance, energy supply, cellular detoxification, further affecting the cellular proliferation and apoptosis and other cellular activities.

Remediation mechanisms of mercapto-grafted palygorskite for cadmium pollutant in paddy soil

The immobilization agent was the key factor that determined the success of remediation of heavy metal polluted soil. In this study, mercapto-grafted palygorskite (MP) as a novel and efficient immobilization agent was utilized for the remediation of Cd-polluted paddy soil in pot trials, and the remediation mechanisms were investigated in the aspect of soil chemistry and plant physiology with different rice cultivars as model plants. Mercapto-grafted palygorskite at applied doses of 0.1-0.3% could reduce Cd contents of brown rice and straws of different cultivars significantly. Both reduced DTPA-extractable Cd contents in rhizosphere and non-rhizosphere soil and decreasing Cd contents in iron plaques on rice root surfaces confirmed that MP was an efficient immobilization agent for Cd pollutant in paddy soil. In the aspect of soil chemistry, the pH values of rhizosphere and non-rhizosphere soils had no statistical changes in the MP treatment groups, but their zeta potentials decreased obviously, indicating that MP could enhance the fixation or sorption of Cd on soil compositions. In the aspect of antioxidant system, MP could increase POD activity of rice roots significantly to alleviate the stress of Cd to roots, and resulted in the decrease of T-AOC, SOD, and CAT activities of rice roots of the selected cultivars. MP had no inhabitation or enhancement effects on TSH of rice roots but enhance the contents of MTs and NPT to binding Cd to complete detoxification process. MP as a novel and efficient immobilization agent could complete the remediation effects through soil chemistry and plant physiological mechanisms.

Physiological mechanisms of a wetland plant (Echinodorus osiris Rataj) to cadmium detoxification

Physiological responses of Echinodorus osiris Rataj plant under cadmium (Cd) stress (5 and 15 mg L^-1) were studied by researching the change of non-enzymatic antioxidants and the exudation of root organic acids. There was a significant increase of ascorbic acid, glutathione, and non-protein thiols in the plant, and the increment was much obvious in roots than that in leaves with increased Cd stress. The accumulation of Cd was associated with mitochondrial structural damages in roots, while the organelle structure, such as chloroplast, in leaves remains intact. In exudates collected from the plants in the treatment with 15 mg L^-1 Cd, oxalate, citric, and succinic acids responded intensively than other organic acids.