Identification of wheat (Triticum aestivum L.) genotypes for food safety on two different cadmium contaminated soils

Multiomics and biotechnologies for understanding and influencing cadmium accumulation and stress response in plants

Cadmium (Cd) is one of the most toxic heavy metals faced by plants and, additionally, via the food chain, threatens human health. It is principally dispersed through agro-ecosystems via anthropogenic activities and geogenic sources. Given its high mobility and persistence, Cd, although not required, can be readily assimilated by plants thereby posing a threat to plant growth and productivity as well as animal and human health. Thus, breeding crop plants in which the edible parts contain low to zero Cd as safe food stuffs and harvesting shoots of high Cd-containing plants as a route for decontaminating soils are vital strategies to cope with this problem. Recently, multiomics approaches have been employed to considerably enhance our understanding of the mechanisms underlying (i) Cd toxicity, (ii) Cd accumulation, (iii) Cd detoxification and (iv) Cd acquisition tolerance in plants. This information can be deployed in the development of the biotechnological tools for developing plants with modulated Cd tolerance and detoxification to safeguard cellular and genetic integrity as well as to minimize food chain contamination. The aim of this review is to provide a current update about the mechanisms involved in Cd uptake by plants and the recent developments in the area of multiomics approach in terms of Cd stress responses, as well as in the development of Cd tolerant and low Cd accumulating crops.

Variations in Cadmium and Lead Bioaccessibility in Wheat Cultivars and Their Correlations with Nutrient Components

To reduce the health risks of exposure to Cd and Pb in wheat, a field experiment was conducted to investigate the differences in Cd and Pb bioaccessibility among the grains of 11 wheat cultivars and their relationships with the nutrient compositions of grains. The grain concentrations (Cd: 0.14-0.56 mg kg-1, Pb: 0.08-0.39 mg kg-1) and bioaccessibility (5.28-57.43% and 0.72-7.72% for Cd and Pb in the intestinal phase, respectively) of Cd and Pb differed significantly among the 11 cultivars. A safe wheat cultivar (Shannong16) with a relatively low health risk and the lowest grain Cd and Pb concentrations was selected. Ca, Mg, phytate, and methionine played key roles in affecting Cd and Pb bioaccessibility in wheat, with Ca and phytate significantly negatively correlated with Cd and Pb bioaccessibility. These findings can be used to optimize the selection strategy for safe wheat cultivars for healthy grain production in Cd-polluted farmland.

The Effect of Exogenous Cadmium and Zinc Applications on Cadmium, Zinc and Essential Mineral Bioaccessibility in Three Lines of Rice That Differ in Grain Cadmium Accumulation

Millions of people around the world rely on rice (Oryza sativa) for a significant portion of daily calories, but rice is a relatively poor source of essential micronutrients like iron and zinc. Rice has been shown to accumulate alarmingly high concentrations of toxic elements, such as cadmium. Cadmium in foods can lead to renal failure, bone mineral density loss, cancer, and significant neurotoxicological effects. Several strategies to limit cadmium and increase micronutrient density in staple food crops like rice have been explored, but even when cadmium concentrations are reduced by a management strategy, total cadmium levels in rice grain are an unreliable means of estimating human health risk because only a fraction of the minerals in grains are bioaccessible. The goal of this work was to assess the influence of cadmium and zinc supplied to plant roots on the bioaccessibility of cadmium and essential minerals from grains of three rice lines (GSOR 310546/low grain Cd, GSOR 311667/medium grain Cd, and GSOR 310428/high grain Cd) that differed in grain cadmium accumulation. Treatments consisted of 0 μM Cd + 2 μM Zn (c0z2), 1 μM Cd + 2 μM Zn (c1z2), or 1 μM Cd + 10 μM Zn (c1z10). Our results revealed that an increased grain cadmium concentration does not always correlate with increased cadmium bioaccessibility. Among the three rice lines tested, Cd bioaccessibility increased from 2.5% in grains from the c1z2 treatment to 17.7% in grains from the c1z10 treatment. Furthermore, Cd bioccessibility in the low-Cd-accumulating line was significantly higher than the high line in c1z10 treatment. Zinc bioaccessibility increased in the high-cadmium-accumulating line when cadmium was elevated in grains, and in the low-cadmium line when both cadmium and zinc were increased in the rice grains. Our results showed that both exogenous cadmium and elevated zinc treatments increased the bioaccessibility of other minerals from grains of the low- or high-grain cadmium lines of rice. Differences in mineral bioaccessibility were dependent on rice line. Calculations also showed that increased cadmium bioaccessibility correlated with increased risk of dietary exposure to consumers. Furthermore, our results suggest that zinc fertilization increased dietary exposure to cadmium in both high and low lines. This information can inform future experiments to analyze genotypic effects of mineral bioavailability from rice, with the goal of reducing cadmium absorption while simultaneously increasing zinc absorption from rice grains.

Molecular-Assisted Breeding of Cadmium Pollution-Safe Cultivars

Cadmium (Cd) contamination in edible agricultural products, especially in crops intended for consumption, has raised worldwide concerns regarding food safety. Breeding of Cd pollution-safe cultivars (Cd-PSCs) is an effective solution to preventing the entry of Cd into the food chain from contaminated agricultural soil. Molecular-assisted breeding methods, based on molecular mechanisms for cultivar-dependent Cd accumulation and bioinformatic tools, have been developed to accelerate and facilitate the breeding of Cd-PSCs. This review summarizes the recent progress in the research of the low Cd accumulation traits of Cd-PSCs in different crops. Furthermore, the application of molecular-assisted breeding methods, including transgenic approaches, genome editing, marker-assisted selection, whole genome-wide association analysis, and transcriptome, has been highlighted to outline the breeding of Cd-PSCs by identifying critical genes and molecular biomarkers. This review provides a comprehensive overview of the development of Cd-PSCs and the potential future for breeding Cd-PSC using modern molecular technologies.

Influence of cadmium ion on denaturation kinetics of hen egg white-lysozyme under thermal and acidic conditions

To study the influence of Cd(II) ions on denaturation kinetics of hen egg white lysozyme (HEWL) under thermal and acidic conditions, spontaneous Raman spectroscopy in conjunction with Thioflavin-T fluorescence, AFM imaging, far-UV circular dichroism spectroscopy, and transmittance assays was conducted. Four distinctive Raman spectral markers for protein tertiary and secondary structures were recorded to follow the kinetics of conformational transformation. Through comparing variations of these markers in the presence or absence of Cd(II) ions, Cd(II) ions show an ability to efficiently accelerate the disruption of tertiary structure, and meanwhile, to promote the direct formation of organized β-sheets from the uncoiling of α-helices by skipping intermediate random coils. More significantly, with the action of Cd(II) ions, the initially resulting oligomers with disordered structures tend to assemble into aggregates with random structures like gels more than amyloid fibrils, along with a so-called "off-pathway" denaturation pathway. Our results advance the in-depth understanding of corresponding ion-specific effects.

Agronomic and ionomics indicators of high-yield, mineral-dense, and low-Cd grains of wheat (Triticum aestivum L.) cultivars

The accumulation of toxic and essential nutrient elements in wheat grain influences wheat yield, grain nutritional quality, and human health. Here, we assessed the potential for breeding wheat cultivars to combine high yield with low cadmium and high iron and/or zinc concentrations in grains, and we screened appropriate cultivars. A pot experiment was conducted to explore differences in grain cadmium, iron, and zinc concentrations among 68 wheat cultivars, as well as their relationships with other nutrient elements and agronomic characters. The results showed 2.04-, 1.71-, and 1.64-fold differences in grain cadmium, iron, and zinc concentrations, respectively, among the 68 cultivars. Grain cadmium concentration was positively correlated with grain zinc, iron, magnesium, phosphorus, and manganese concentrations. Grain copper concentration was positively correlated with grain zinc and iron concentrations, but not with grain cadmium concentration. Therefore, copper has a potential role in regulating grain iron and zinc accumulation without influencing cadmium concentration in wheat grain. There were no significant relationships between grain cadmium concentration and four important wheat agronomic characters (i.e., grain yield, straw yield, thousand kernel weight, and plant height), indicating that the breeding of low-cadmium-accumulating cultivars with dwarfism and high yield characteristics is possible. On cluster analysis, four cultivars (Ningmai11, Xumai35, Baomai6, and Aikang58) exhibited low-cadmium and high-yield characteristics. Among them, Aikang58 contained moderate iron and zinc concentrations, while Ningmai11 had relatively high iron but low zinc concentrations in the grain. These results imply that it is feasible to breed high-yield dwarf wheat with low cadmium and moderate iron and zinc concentrations in the grain.

Soil properties drive the bacterial community to cadmium contamination in the rhizosphere of two contrasting wheat (Triticum aestivum L.) genotypes

Cadmium (Cd) bioavailability in the rhizosphere makes an important difference in grain Cd accumulation in wheat. Here, pot experiments combined with 16S rRNA gene sequencing were conducted to compare the Cd bioavailability and bacterial community in the rhizosphere of two wheat (Triticum aestivum L.) genotypes, a low-Cd-accumulating genotype in grains (LT) and a high-Cd-accumulating genotype in grains (HT), grown on four different soils with Cd contamination. Results showed that there was non-significant difference in total Cd concentration among four soils. However, except for black soil, DTPA-Cd concentrations in HT rhizospheres were higher than those of LT in fluvisol, paddy soil and purple soil. Results of 16S rRNA gene sequencing showed that soil type (52.7%) was the strongest determinant of root-associated community, while there were still some differences in rhizosphere bacterial community composition between two wheat genotypes. Taxa specifically colonized in HT rhizosphere (Acidobacteria, Gemmatimonadetes, Bacteroidetes and Deltaproteobacteria) could participate in metal activation, whereas LT rhizosphere was highly enriched by plant growth-promoting taxa. In addition, PICRUSt2 analysis also predicted high relative abundances of imputed functional profiles related to membrane transport and amino acid metabolism in HT rhizosphere. These results revealed that the rhizosphere bacterial community may be an important factor regulating Cd uptake and accumulation in wheat and indicated that the high Cd-accumulating cultivar might improve Cd bioavailability in the rhizosphere by recruiting taxa related to Cd activation, thus promoting Cd uptake and accumulation.

Cadmium mobility and health risk assessment in the soil-rice-human system using in vitro biaccessibility and in vivo bioavailability assay: Two year field experiment

Humans are mainly exposed to cadmium (Cd) due to the rice consumption, however there exist considerable differences across rice cultivars in terms of Cd absorption and accumulation in the grains, and subsequent release after digestion (bioaccessibility), as well as uptake by Caco-2 cells of humans (bioavailability). This study comprised of field and lab simulation trials where in the field, firstly 39 mid-rice cultivars were screened for their phytoremediation potential coupled with safe production in relation to uptake and translocation of Cd. Lower Cd concentrations (˂0.2 mg kg^-1) in polished rice of 74 % cultivars were ascribed to the increased root to straw translocation indicating that straw may acquire higher accumulation of Cd. Furthermore, the ionomic profile demonstrated that the spatial distribution of metals in different rice organs corresponds to the plant growth morphology. In the second year, in vitro-in vivo assay model was employed to assess the bioaccessibility and bioavailability of Cd in polished rice and to further estimate the daily Cd intake by humans through rice grains. The results of bioaccessibility and bioavailability assays and daily estimated Cd intake presented the corresponding values of 39.02-59.76 %, 8.69-24.26 %, and 0.0185-0.9713 μg kg^-1 body weight day^-1, respectively. There exists a strong connection between total Cd and bioaccessible Cd to humans (R² = 0.94, P < 0.01). Polynomial fitting (R² = 0.91, P < 0.01) showed a better statistically significant correlation between total Cd contents and bioavailable levels, suggesting that in vitro-in vivo assays should be considered in future studies. The results of field experiments and in vitro-in vivo assays recommended the Tianyouhuazhan (MR-29), Heliangyou1hao (MR-17), and Yongyou15 (MR-1) as suitable mid-rice cultivars for the phytoremediation of slightly Cd contaminated soils coupled with rice agro-production due to their high nutritional value and low total and bioavailable Cd for human.

The role of auxins and auxin-producing bacteria in the tolerance and accumulation of cadmium by plants

Cadmium (Cd) is one of the most toxic heavy metals for plant physiology and development. This review discusses Cd effects on auxin biosynthesis and homeostasis, and the strategies for restoring plant growth based on exogenous auxin application. First, the two well-characterized auxin biosynthesis pathways in plants are described, as well as the effect of exogenous auxin application on plant growth. Then, review describes the impacts of Cd on the content, biosynthesis, conjugation, and oxidation of endogenous auxins, which are related to a decrease in root development, photosynthesis, and biomass production. Finally, compelling evidence of the beneficial effects of auxin-producing rhizobacteria in plants exposed to Cd is showed, focusing on photosynthesis, oxidative stress, and production of antioxidant compounds and osmolytes that counteract Cd toxicity, favoring plant growth and improve phytoremediation efficiency. Expanding our understanding of the positive effects of exogenous auxins application and the interactions between bacteria and plants growing in Cd-polluted environments will allow us to propose phytoremediation strategies for restoring environments contaminated with this metal.

Screening of low-Cd accumulating early rice cultivars coupled with phytoremediation and agro-production: Bioavailability and bioaccessibility tests

Previous studies have focused on total cadmium (Cd) accumulation in rice or its transformation in soil, but only a few have examined the entire soil-rice-human system. This study investigated the Cd bioaccessibility and bioavailability for humans from grains of early rice cultivars grown in a Cd-polluted field and further combined with multi-traits to discover and evaluate the optimum safe production and phytoremediation potential cultivars. The results revealed that Cd concentration in polished rice was <0.20 mg kg^-1 in 79 % of early rice cultivars, implying that Cd levels in rice might be reduced by cultivar selection. Furthermore, the higher values of root to straw translocation factor indicates the maximal accumulation of Cd in straw and with highest soil to straw accumulation factor (>1.0) in 66.67 % of cultivars. However, bioaccessibility and bioavailability varied greatly among cultivars with corresponding values ranging from 5.68 to 7.67 % and 1.87 to 5.71 ng g^-1, respectively. Despite the fact that polynomial fitting revealed a statistically significant relationship between Cd content in polished rice and bioavailable Cd in humans (R² = 0.718, P = 0.025), poor goodness of fit for bioaccessibility, bioavailability, and toxicity varied even within low-Cd accumulating cultivars. As a result of multi trait analysis and bioavailability, Zhuliangyou4024 (ER-9), Lingliangyou211 (ER-3), and Yonxian15 (ER-28) were found to be the three best early rice cultivars with higher essential nutrients, less total and bioavailable Cd, and relative high phytoremediation potential and are suitable for healthy rice production and soil remediation.

WRKY74 regulates cadmium tolerance through glutathione-dependent pathway in wheat

Cadmium (Cd) is a toxic heavy metal to plants and human health. Ascorbate (ASA)-glutathione (GSH) synthesis pathway plays key roles in Cd detoxification, while its molecular regulatory mechanism remains largely unknown, especially in wheat. Here, we found a WRKY transcription factor-TaWRKY74, and its function in wheat Cd stress is not clear in previous studies. The expression levels of TaWRKY74 were significantly induced by Cd stress. Compared to control, the activities of GST, GR, or APX were significantly increased by 1.55-, 1.43-, or 1.75-fold and 1.63-, 2.65-, or 2.30-fold in shoots and roots of transiently TaWRKY74-silenced wheat plants under Cd stress. Similarly, the contents of hydrogen peroxide (H₂O₂), malondialdehyde (MDA), GSH, or Cd were also significantly increased by 2.39- or 1.25-fold, 1.54- or 1.20-fold, and 1.34- or 5.94-fold in shoots or roots in transiently TaWRKY74-silenced wheat plants, while ASA content was decreased by 47.4 or 43.3% in shoots, 10.7 or 6.5% in roots in these silenced wheat plants, respectively. Moreover, the expression levels of GSH, GPX, GR, DHAR, MDHAR, and APX genes, which are involved in ASA-GSH synthesis, were separately induced by 2.42-, 2.16-, 3.28-, 2.08-, 1.92-, and 2.23-fold in shoots, or by 10.69-, 3.33-, 3.26-, 1.81-, 16.53-, and 3.57-fold in roots of the BSMV-VIGS-TaWRKY74-inoculated wheat plants, respectively. However, the expression levels of TaNramp1, TaNramp5, TaHMA2, TaHMA3, TaLCT1, and TaIRT1 metal transporters genes were decreased by 21.2-76.3% (56.6%, 59.2%, 76.3%, 53.6%, 35.8%, and 21.2%) in roots of the BSMV-VIGS-TaWRKY74-inoculated wheat plants. Taken together, our results suggested that TaWRKY74 alleviated Cd toxicity in wheat by affecting the expression of ASA-GSH synthesis genes and suppressing the expression of Cd transporter genes, and further affecting Cd uptake and translocation in wheat plants.

Selection of low-cadmium and high-micronutrient wheat cultivars and exploration of the relationship between agronomic traits and grain cadmium

The cadmium (Cd) and micronutrient contents in grains were used as screening indicators through a pot experiment, and the hierarchical cluster analysis was used to select wheat cultivars with low Cd and high micronutrient contents. The potential human health risks caused by wheat intake and the relationship between the Cd concentration in wheat grains and 12 agronomic traits were also investigated using the risk assessment model and logistic equation fitting, respectively. Yannong-23, Zhongmmai-175, and Luyuan-502, the main wheat cultivars promoted in the Huang-Huai-Hai region of China, were screened for low Cd accumulation and high micronutrient. Health risk assessment results demonstrated that children showed a high noncarcinogenic risk and that adults posed a high carcinogenic risk. The results of the agronomic trait analysis showed that low-Cd accumulation wheat cultivars had high spikelet number and fresh and dry weights of root, stem, and leaf (p < 0.05). Logistic curve fitting results showed that among all agronomic traits, the root dry weight was the most suitable factor with remarkable goodness of fit and showed a significant negative correlation. The Cd concentration in wheat grains could be predicted by the logistic curve equation obtained by fitting this trait. Results provided theoretical support for the safe use of slightly to moderately contaminated farmland, formulation of health risk management policies for different populations, and breeding of high-quality wheat.

Recent Advances in Minimizing Cadmium Accumulation in Wheat

Cadmium (Cd), a toxic heavy metal, affects the yield and quality of crops. Wheat (Triticum aestivum L.) can accumulate high Cd content in the grain, which poses a major worldwide hazard to human health. Advances in our understanding of Cd toxicity for plants and humans, different parameters influencing Cd uptake and accumulation, as well as phytoremediation technologies to relieve Cd pollution in wheat have been made very recently. In particular, the molecular mechanisms of wheat under Cd stress have been increasingly recognized. In this review, we focus on the recently described omics and functional genes uncovering Cd stress, as well as different mitigation strategies to reduce Cd toxicity in wheat.

Evaluation of nickel toxicity in wheat as function of diverse type of fertilizers: implications for public health

Many studies have described the physiological, biochemical, and molecular responses to heavy metal toxicity and deficiencies individually in plants. The present study assess nickel (Ni) concentration in amended soil, plant vegetative parts, and wheat grains, grown under diverse types of fertilizers in Sargodha, Punjab, Pakistan. Different varieties of wheat were grown in pot and fields. Different treatments (municipal solid waste, poultry waste, press mud, farm yard manure) of fertilizers were applied in order to study the metal level increased in different parts (root, shoot, grain) of wheat due to fertilization. Results indicated that metal level was found highest in roots followed by shoot and grain. The highest level of nickel in root was present in V1 (2.35 mg/kg) due to T2 (2.60 mg/kg) treatment. Higher nickel levels in wheat shoot and grains were observed in V5 (2.36 mg/kg) and V8 (2.29 mg/kg), respectively, due to applied treatment T2 (2.57 mg/kg). This study concluded that treatment T9 was proven safe in view of the observed Ni concentration, while treatment T2 (municipal solid waste) resulted in higher accumulation of nickel in wheat grains which showed that municipal solid waste should be treated before their application in agriculture fields to secure the public health. This study recommended that although application of fertilizers increased the plant growth and nutritional value, it also enhanced metal accumulation in the wheat grains which could be harmful for consumers especially human being. Government should take actions to prevent metal toxicity in human food chain.

Safe utilization of cadmium- and lead-contaminated farmland by cultivating a winter rapeseed/maize rotation compared with two phytoextraction approaches

Compared with phytoextraction, growing suitable crops may be a more profitable and practical approach for managing contaminated farmland, especially when there are multiple pollutants. In this 5-year field study, the phytoaccumulator Hylotelephium spectabile, the high-biomass species amaranth (Amaranthus hypochondriacus), and a winter rapeseed/maize rotation crop were cultivated on farmland contaminated with cadmium (Cd) and lead (Pb). Over 4 consecutive years, the annual Cd uptake and extraction efficiency of H. spectabile was 117.6 g hm^-2 and 2.36%, respectively. The Cd extraction efficiency of amaranth was equivalent to that of H. spectabile because of its high biomass, and it extracted more Pb (660-2210 g hm^-2) from the soil than did H. spectabile. However, neither of these species was able to remediate contaminated farmland rapidly and inexpensively, even with enhancing strategies such as variety screening and the addition of fertilizers and a chelating agent. A safe utilization approach to cultivate rapeseed instead of wheat significantly reduced the carcinogenic and noncarcinogenic risks. The concentrations of heavy metals in rapeseed oil were below the limits specified in the Chinese national food standard, and the heavy metal concentrations in the byproducts (rapeseed meal and straw) were below the limits specified in Chinese national standards for organic fertilizer and feed. The cost of safe utilization was one-quarter that of phytoextraction, and the net economic benefit was 33.5%-123.5% higher than that of wheat crops. Therefore, the rapeseed/maize rotation is a profitable and feasible approach for the safe utilization of Cd- and Pb-contaminated farmland on the northern plains of China.

Cadmium accumulation in rice straws and derived biochars as affected by metal exposure, soil types and rice genotypes

Straw residues, as one of the conservation farming practices, are being strongly encouraged in China, including some cadmium-polluted areas. Nowadays, a portion of this plant residue is promoted to be removed for reducing excess metal(loid) in the soil and to be used for bioenergy production. Nevertheless, the possible influences of contaminated straw or the burial of its derived biochars on Cd accumulation in soil and data based on health risk assessment associated with different status and extent of soil contamination were relatively unknown. Thus it is important to provide a more systematical understanding of contaminated straw burial at specific contamination zones, which may provide useful guidance for straw utilization. In this study, we harvested two genotypes of rice straw from 6 contaminated levels among three soil types to comprehensively study the total Cd contents in straws and its derived biochars and correlate the sets of straw characteristics and Cd contents in three different contamination zones. The total Cd concentration in straws grew at a steady rate relatively with increasing soil Cd contamination levels, compared to those in biochars which performed more fluctuate due to the strong burning. According to correlational analyses, three-way ANOVA showed that the moisture, ash, volatile and fixed carbon were all significantly affected by straw Cd_Total contents (p < 0.001). Such relationships were attributed to guide straw removal portions for gasification. Meanwhile, there was a significant correlation between straw Cd concentration and soil types (p < 0.001), confirming that it might be worth determining soil remediation by straw removal according to site-specific farmland conditions. This work will help to assess efforts toward predicting Cd concentration in the paddy soils related to kinds of contamination status and would also give useful guidance to make sustainable management strategies for crop straws in polluted regions.Novelty statement This work provided data on how much rice straw is needed to remove to ensure the minimal amount to control soil contamination and reduce costs according to site-specific conditions and soil Cd contamination status. It also explains the correlations between straw characteristics related to bioenergy use and soil conditions which would give guidance to balance using crop straw for increased bioenergy production and the need to also protect, preserve, and enhance soil resources.

Phytoavailability, translocation and soil thresholds derivation of cadmium for food safety through soil-wheat (Triticum aestivum L.) system

Cadmium (Cd) pollution in cultivated soils has posed a great risk to human health through the soil-plant-human pathway. Therefore, it is important to derive soil thresholds for the low-Cd accumulating genotype of wheat (Triticum aestivum L.) to promote its application in agricultural production on Cd-contaminated sites. Here, a pot experiment was performed to explore the transfer characteristics of Cd in two contrasting wheat genotypes at three different soils and the effect of soil parameters together with soil safety Cd thresholds derivation. Generally, grain Cd highly accumulating wheat genotype (Zhenmai10, HT) showed higher Cd accumulation in grains than grain Cd weakly accumulating wheat genotype (Aikang58, LT). Stepwise multiple linear regression (SMLR) analysis (log-transformed Freundlich-type) indicated that Cd accumulation in wheat grains was strongly related to soil total Cd concentration and pH for both genotypes (R2 = 0.907*** for HT; R2 = 0.910*** for LT). Combining the simple regression model of soil-plant transfer system with the risk assessment method based on human health, soil total Cd thresholds for three soils were calculated with the values of 0.62, 0.82, and 0.62 mg kg-1 in LT genotype and 0.31, 0.77, and 0.49 mg kg-1 in HT genotype. Therefore, we suggested that when deducing soil thresholds, the ability of wheat genotypes to accumulate Cd and soil properties should be considered because of the large differences in soil thresholds between different genotypes and types of soils. We believe our results will promote the application of low-Cd wheat genotypes to agricultural production, thereby ensuring the safety of their products.

Physiological and metabolomics responses of two wheat (Triticum aestivum L.) genotypes differing in grain cadmium accumulation

To reduce cadmium (Cd) pollution of food chains, screening and breeding of low-Cd-accumulating genotypes have received increasing attention. However, the mechanisms involving Cd tolerance and accumulation are not fully understood. Here, we investigated the physiological responses and metabolomics profiling on two wheat (Triticum aestivum L.) genotypes, a low-Cd-accumulating genotype in grains (Aikang58, AK58) and a high-Cd-accumulating genotype in grains (Zhenmai10, ZM10), in hydroponic culture treated without/with Cd for 7 days. The results showed that AK58 was a Cd tolerant genotype with higher capacity of antioxidant systems in root. In addition, the concentrations of Cd bound to root cell walls were higher in AK58 than ZM10, of which pectin and hemicellulose played important roles in Cd binding. Moreover, subcellular distribution manifested that Cd sequestrated in the vacuoles was another tolerance mechanism in AK58. Simultaneously, metabolomics profiling showed that, in AK58, phenylalanine metabolism, alanine, aspartate and glutamate metabolism, isoquinoline alkaloid biosynthesis, arginine and proline metabolism, arginine biosynthesis and glyoxylate and dicarboxylate metabolism are highly related to antioxidant defense system, cell wall biosynthesis and metabolisms of phytochelatins together with other organic ligands, playing crucial roles in Cd tolerance and Cd fixation mechanisms in roots. These novel findings should be useful for molecular assisted screening and breeding of low Cd-accumulating genotypes for wheat crop.

Modulation of growth and key physiobiochemical attributes after foliar application of zinc sulphate (ZnSO4) on wheat (Triticum aestivum L.) under cadmium (Cd) stress

A pot experiment was conducted to examine the effect of foliar application of various levels of ZnSO4 on wheat (Triticum aestivum L.) under cadmium (Cd) stress. Seeds of two wheat varieties i.e., Ujala-2016 and Anaj-2017 were sown in sand filled plastic pots. Cadmium (CdCl2) stress i.e., 0 and 0.5 mM CdCl2 was applied in full strength Hoagland's nutrient solution after 4 weeks of seed germination. Foliar spray of varying ZnSO4 levels i.e., 0, 2, 4, 6 and 8 mM was applied after 2 weeks of CdCl2 stress induction (of 6 week old plants). After 3 weeks of foliar treatment leaf samples of 9 week old wheat plants were collected for the determination of changes in various growth and physiobiochemical attributes. Results obtained showed that cadmium stress (0.5 mM CdCl2) significantly decreased shoot and root fresh and dry weights, shoot and root lengths, yield attributes, chlorophyll a contents and total phenolics, while increased hydrogen peroxide (H2O2), total soluble proteins, free proline, glycinebetaine (GB) contents, and activities of antioxidant enzymes i.e., catalase (CAT), ascorbate peroxidase (APX) and peroxidase (POD). Foliar application of varying ZnSO4 levels significantly increased various growth attributes, chlorophyll b contents, H2O2, free proline, GB and activities of antioxidant enzymes i.e., CAT, POD and APX, while decreased relative water contents and total phenolics under Cd stress or non stress conditions. Furthermore, both wheat varieties showed differential response under Cd stress and towards foliar application of ZnSO4 e.g., wheat var. Ujala-2016 was higher in shoot dry weight, root length, root fresh and dry weights, total leaf area per plant, 100 grains weight, number of tillers per plant, chlorophyll b, hydrogen peroxide (H2O2), activities of APX, POD, glycinebetaine and leaf free proline contents, while var. Anaj-2017 exhibited high shoot fresh weight, grain yield per plant, no. of grains per plant, chlorophyll contents, chlorophyll a/b ratio, total phenolics, MDA and total soluble protein contents under cadmium stress or non stress conditions.

Cataloging of Cd Allocation in Late Rice Cultivars Grown in Polluted Gleysol: Implications for Selection of Cultivars with Minimal Risk to Human Health

Cadmium (Cd) is a toxic trace metal that has polluted 20% of agricultural land in China where its concentration exceeds the standards for Chinese farmland. Plants are capable of accumulating Cd and other trace metals, but this capacity varies with species and cultivars within a species. Rice is a staple food consumed by half of the global population. In order to select safe late rice cultivars that are suitable late rice cultivars that can be cultivated in for growing in slightly contaminated soil, a two-year field experiment was conducted with 27 in the first year and 9 late rice cultivars in the second year. The results showed that plant Cd concentrations varied among the cultivars, with high magnitudes of variation occurred in straw and grains. Five genotypes including LR-12, LR-17, LR-24, LR-25 and LR-26 were identified as low accumulators for the first year while LR-15 and LR-17 were identified as promising cultivars based on Cd concentration in the polished rice grains (<0.02 mg kg⁻¹ DW). In addition, these cultivars had favorable traits, including mineral nutrition and grain yield. Therefore, these genotypes should be considered for cultivation in slightly or moderately Cd contaminated soils.

Cadmium Uptake by Wheat (Triticum aestivum L.): An Overview

Cadmium is a toxic heavy metal that may be detected in soils and plants. Wheat, as a food consumed by 60% of the world’s population, may uptake a high quantity of Cd through its roots and translocate Cd to the shoots and grains thus posing risks to human health. Therefore, we tried to explore the journey of Cd in wheat via a review of several papers. Cadmium may reach the root cells by some transporters (such as zinc-regulated transporter/iron-regulated transporter-like protein, low-affinity calcium transporters, and natural resistance-associated macrophages), and some cation channels or Cd chelates via yellow stripe 1-like proteins. In addition, some of the effective factors regarding Cd uptake into wheat, such as pH, organic matter, cation exchange capacity (CEC), Fe and Mn oxide content, and soil texture (clay content), were investigated in this paper. Increasing Fe and Mn oxide content and clay minerals may decrease the Cd uptake by plants, whereas reducing pH and CEC may increase it. In addition, the feasibility of methods to diminish Cd accumulation in wheat was studied. Amongst agronomic approaches for decreasing the uptake of Cd by wheat, using organic amendments is most effective. Using biochar might reduce the Cd accumulation in wheat grains by up to 97.8%.