Cd Accumulation In Roots Research Articles

The regulatory role of root in cadmium accumulation in a high cadmium-accumulating rice line (Oryza sativa L.).

There are some key processes that regulate cadmium (Cd) accumulation in rice. Understanding the characteristics and mechanisms of Cd accumulation in high Cd-accumulating rice lines benefits for excavating relevant genes. Cd accumulation and distribution in roots of Lu527-8, a high Cd-accumulating rice line, were investigated by a hydroponic experiment, with a control of a normal rice line (Lu527-4). Lu527-8 showed significantly higher Cd concentrations in roots than Lu527-4. More than 81% of Cd in roots of two rice lines is distributed in soluble fraction and cell wall. In soluble fraction, there were more organic acids, amino acids, and phytochelatins in Lu527-8, benefiting Cd accumulation. Pectin and hemicellulose 1 (HC1), especially pectin, were main polysaccharides in cell wall. Lu527-8 showed more pectin and HC1 along with higher pectin methylesterase (PME) activity compared with Lu527-4, promoting Cd accumulation. Besides, Lu527-8 showed higher Cd translocation from root to shoot due to more amounts of ethanol-extractable Cd in roots than Lu527-4. In conclusion, specific characteristics of Cd chemical forms and subcellular distribution in roots of high Cd-accumulating rice line are important for Cd accumulation and translocation.

Cadmium accumulation, subcellular distribution and chemical fractionation in hydroponically grown Sesuvium portulacastrum [Aizoaceae

Sesuvium portulacastrum is a well-known halophyte with considerable Cd accumulation and tolerance under high Cd stress. This species is also considered as a good candidate of Cd phytoremediation in the polluted soils. However, the mechanism of Cd accumulation, distribution and fractionation in different body parts still remain unknown. Seedlings of Sesuvium portulacastrum were studied hydroponically under exposure to a range of Cd concentrations (50 μM or μmol/L to 600 μM or μmol/L) for 28 days to investigate the potential accumulation capability and tolerance mechanisms of this species. Cd accumulation in roots showed that the bio-concentration factor was > 10, suggesting a strong ability to absorb and accumulate Cd. Cd fractionation in the aboveground parts showed the following order of distribution: soluble fraction > cell wall > organelle > cell membrane. In roots, soluble fraction was mostly predominant than other fractions. Cd speciation in leaves and stems was mainly contained of sodium chloride and deionised water extracted forms, suggesting a strong binding ability with pectin and protein as well as with organic acids. In the roots, inorganic form of Cd was dominant than other forms of Cd. It could be suggested that sodium chloride, deionised water and inorganic contained form of Cd are mainly responsible for the adaption of this plant in the Cd stress environment and alleviating Cd toxicity.

Rhizospheric pore-water content predicts the biochar-attenuated accumulation, translocation, and toxicity of cadmium to lettuce

Metal bioavailability controls its behaviors in soil-plant system, especially involved in biochar amendment. This study compared a rhizospheric pore-water extraction against a BCR sequential extraction method to understand cadmium (Cd) bioavailability in two typical Chinese soils. Soils were spiked with five levels of Cd (CdCl2) and remediated with 3% corn-straw derived biochar. After 60 days of lettuce growth, Cd accumulation and enzyme activities in tissues were analyzed. Results showed that biochar increased soil properties (pH, CEC and SOM) compared to un-amended soils, but decreased contents of bioavailable Cd in soil pore-water (Cdpore-water) and BCR extracted Cd (CdFi+Fii). Contents of Cdpore-water were lower in yellow-brown soils than that in red soils. Pearson analysis showed that bioavailable Cd is negatively correlated with soil pH and CEC (p < 0.05). Cd accumulation in lettuce roots and leaves both were decreased by biochar addition, and the established linear equations proved that soil Cdpore-water is the best predictor for Cd accumulation in lettuce roots (r2 = 0.964) and in leaves (r2 = 0.953), followed by CdFi+Fii. Transfer factor (TF) values of Cd from roots to leaves were lower than 1, and slightly better correlated with soil Cdpore-water (r = −0.674, p < 0.01) than CdFi+Fii (r = −0.615, p < 0.01). Aggregated boosted tree (ABT) analyses indicated that soil properties together with Cdpore-water contribute more than 50% to root enzyme activities. Collectively, soil Cdpore-water is a promising predictor of Cd bioavailability, accumulation and toxicity in soil-plant system with biochar addition.

Enhanced vacuole compartmentalization of cadmium in root cells contributes to glutathione-induced reduction of cadmium translocation from roots to shoots in pakchoi (Brassica chinensis L.)

Our previous studies showed that exogenous glutathione (GSH) decreased cadmium (Cd) concentration in shoots and alleviated the growth inhibition in pakchoi (Brassica chinensis L.) under Cd stress. Nevertheless, it is largely unknown how GSH decreases Cd accumulation in edible parts of pakchoi. This experiment mainly explored the mechanisms of GSH-induced reduction of Cd accumulation in shoot of pakchoi. The results showed that compared with sole Cd treatment, Cd + GSH treatment remarkably increased the expression of BcIRT1 and BcIRT2, and further enhanced the concentrations of Cd and Fe in root. By contrast, GSH application declined the concentration of Cd in the xylem sap. However, these results were not caused by xylem loading process because the expression of BcHMA2 and BcHMA4 had not significant difference between sole Cd treatment and Cd + GSH treatment. In addition, exogenous GSH significantly enhanced the expression of BcPCS1 and promoted the synthesis of PC2, PC3 and PC4 under Cd stress. At the same time, exogenous GSH also significantly improved the expression of BcABCC1 and BcABCC2 in the roots of seedling under Cd stress, suggesting that more PCs-Cd complexes may be sequestrated into vacuoles by ABCC1 and ABCC2 transporters. The results showed that exogenous GSH could up-regulate the expression of BcIRT1/2 to increase the Cd accumulation in root, and the improvement of PCs contents and the expression of BcABCC1/2 enhanced the compartmentalization of Cd in root vacuole of pakchoi under Cd stress. To sum up, exogenous GSH reduce the concentration of free Cd2+ in the cytoplast of root cells and then dropped the loading of Cd into the xylem, which eventually given rise to the reduction of Cd accumulation in edible portion of pakchoi.

Ascorbate-Mediated Modulation of Cadmium Stress Responses: Reactive Oxygen Species and Redox Status in Brassica napus.

The role of ascorbate (AsA) in antioxidant defense system-associated resistance to cadmium (Cd) in oilseed rape plants has not yet been clearly demonstrated. The present study investigated the critical role of exogenous AsA on the physiological and biochemical responses of reactive oxygen species (ROS) and antioxidant scavenging defense systems in oilseed rape (Brassica napus L. cv. Tammi) seedlings exposed to Cd. Cd (10 μM) treatment led to significant reductions in plant growth; increases in the levels of superoxide anion radical, hydrogen peroxide, and malondialdehyde; and increases in Cd uptake and accumulation by the roots and shoots in hydroponically grown 10-day-old seedlings. Moreover, it reduced AsA content and AsA redox ratios, which have been correlated with reductions in glutathione (GSH) and/or nicotinamide adenine dinucleotide phosphate (NADPH) redox status. However, exogenously applying AsA to Cd-exposed seedlings decreased Cd-induced ROS, improved antioxidant defense systems by increasing AsA, GSH, and NADPH contents, and increased Cd uptake and accumulation in both roots and shoots of the plants. These results provided evidence that the enhancement in AsA redox status can be linked to an increase in the GSH and/or NADPH redox ratios through the induction of the AsA–GSH–NADPH cycle. Thus, these results suggest that exogenous AsA application to oilseed rape seedlings under Cd stress might alleviate the overall Cd toxicity by regulating the homeostasis of the AsA–GSH–NADPH cycle, which reestablishes the steady-state cellular redox status.

Identification of a new function of metallothionein-like gene OsMT1e for cadmium detoxification and potential phytoremediation

Cadmium (Cd) is a biologically non-essential and toxic heavy metal leaking to the environment via natural emission or anthropogenic activities, thereby contaminating crops and threatening human health. Metallothioneins (MTs) are a group of metal-binding proteins playing critical roles in metal allocation and homeostasis. In this study, we identified a novel function of OsMT1e from rice plants. OsMT1e was dominantly expressed in roots at all developmental stages and, to less extent, expressed in leaves at vegetative and seed filling stages. OsMT1e was mainly targeted to the nucleus and substantially induced by Cd exposure. Expression of OsMT1e in a yeast Cd-sensitive strain ycf1 conferred cellular tolerance to Cd, even though the ycf1 + OsMT1e cells accumulated more Cd than the control cells (ycf1 + pYES2). Both transgenic rice overexpressing (OX) and repressing OsMT1e by RNA interference (RNAi) were developed. Phenotypic analysis revealed that OsMT1e overexpression enhanced the rice growth concerning the increased shoot or root elongation, dry weight and chlorophyll contents, whereas the RNAi lines displayed a sensitive growth phenotype compared to wild-type. Assessment with 0.5, 2 and 10 μM Cd for two weeks revealed that the RNAi lines accumulated less Cd, while the OX lines had an increased Cd accumulation in root and shoot tissues. The contrasting Cd accumulation phenotypes between the OX and RNAi lines were further confirmed by a long-term study with 0.5 μM Cd for one month. Overall, the study unveiled a new function of OsMT1e in rice, which can be potentially used for engineering genotypes for phytoremediation or minimizing Cd in rice crops.

Typical sources of Cd to paddy fields in different contaminated areas and their impacts on Cd accumulation in topsoil and rice in Changzhutan, China

Heavy metal pollution of soils has been worsening increasingly in China, which brings significant health risk to human, it is critical to investigate the sources of heavy metals in agricultural soils and explore the effects of heavy metal accumulation in crops. In this paper, the sources of cadmium (Cd) and their effects on Cd accumulation in soil and rice grown on urban farmland in Changzhutan were investigated. Among the main Cd sources (irrigation water, commercial fertilizer, and atmospheric deposition), the input flux of atmospheric deposition accounted for 76.36%–98.25% of total input flux, significantly higher than the input fluxes of irrigation water and commercial fertilizer. Manure fertilizer was also an important source of Cd in livestock breeding areas. The accumulation behaviors of Cd in soils and plants presented significant spatial variation among the study areas. Higher Cd input flux from atmospheric deposition resulted in higher Cd bioavailability in soil and more Cd accumulation in rice, and the newly deposited Cd contributed 7.35–41.23% in rice tissues. The use of manure fertilizer increased the soil pH and amount of available Cd in soil, as well as the accumulation of Cd in rice roots. Based on sequential extraction, acid-extractable Cd accounted for approximately 52.54%–61.88% of total Cd in atmospherically deposited particles in the study area, resulting in a high proportion of acid-extractable Cd in soil. This study provides useful reference data on the sources of Cd and its bioavailability in soil and rice.

Stabilization of cadmium in polluted soil using palygorskite-coated nanoscale zero-valent iron

The palygorskite-coated nanoscale zero-valent iron (PAL-nZVI) was synthesized and used in the remediation of Cd-contaminated soils. The function of PAL-nZVI on fraction of Cd in soils, the growth, Cd translocation, and accumulation in corn (Zea mays L.) were investigated using a pot experiment. Cd-contaminated soils were amended with palygorskite, nanoscale zero-valent iron (nZVI), and PAL-nZVI (with varying mass ratios between PAL and nZVI) and with no amendment as control. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the changes in both the surface and the structure of stabilizers. CaCl2-extractable and toxicity characteristic leaching procedure (TCLP)-extractable Cd were measured in order to estimate the bioavailability and the stabilization efficiency. The Community Bureau of Reference (BCR) sequential extraction method was applied to evaluate the speciation change of Cd in soils stabilized by nZVI, PAL, and PAL-nZVI with different coating mass ratios of PAL to nZVI. The Cd accumulation in root and shoot fresh weight was measured to assess the stabilization efficiency. SEM and XRD indicated that nZVI particles were distributed uniformly and well fixed on the surface of palygorskite. After adding PAL-nZVI, the pH value of soil increased by 1 pH unit; CaCl2-extractable Cd decreased from 2.24 to 0.95 mg/kg; the residual Cd increased by 23.06% after 30-day incubation. The fresh weight of corn reached 43.02 g/plant, and the Cd contents in roots and shoots achieved minimum values of 1.50 and 0.73 mg/kg, respectively, when applying PAL-nZVI composite with a coating mass ratio of 2:1. The application of PAL-nZVI decreased the bioavailability and mobility of Cd in soils effectively. The stabilization showed a better effect when applying PAL-nZVI in Cd-contaminated soils than applying nZVI alone. Hence, it was a feasible and potential material that could be applied to amend the Cd-contaminated soil effectively, which would have a high and valuable utilization in field applications.

Phosphate and methionine affect cadmium uptake in valerian (Valeriana officinalis L.)

This study investigated the effects of exogenous methionine (Met) and different phosphate (PO4) concentrations on Cd uptake and translocation in Valeriana officinalis L. Seedlings were grown in nutrient solutions with three different concentrations of phosphate (900, 1200, and 1500 μM) for two weeks, then exposed for 4 days to 10 μM Cd, either in presence or absence of 400 μM methionine. The Met treatment decreased root Cd accumulation by up to 40%, while it enhanced Cd uptake into the shoots by 50%. In absence of Met, shoot Cd uptake was not affected by the level of phosphate application, although root Cd contents increased. The latter effect was entirely due to increased apoplastic Cd binding. In presence of Met, the Cd accumulation of both plant parts showed trends to increase with increasing phosphate level. In contrast to the treatments without Met, however, the phosphate effect on root Cd was due to increased symplastic root Cd allocation. The results suggest that the effects of Met on Cd uptake were due to the formation of mobile Cd-Met complexes, reducing phosphate-promoted Cd-retention in the apoplast and enhancing Cd transfer into the root symplast. Irrespective of the treatment, shoot Cd accumulation showed a close linear relationship to shoot mass, suggesting that convective transport with the transpirational water stream was the rate-governing uptake process. The results indicate that methionine supplementation could reduce Cd accumulation in valerian roots, which are the parts of this plant harvested for medicinal purposes, in Cd-contaminated soil, while phosphate would enhance it.

The E3 ubiquitin ligase gene SlRING1 is essential for plant tolerance to cadmium stress in Solanum lycopersicum

The E3 ubiquitin ligases participate in the degradation of plant proteins and play a regulatory role in stress response. However, the role of tomato E3 ubiquitin ligase genes in plant response to heavy metal stress remains elusive. Here, we identified 17 tomato E3 ubiquitin ligase genes using blast analysis of highly expressed E3 ubiquitin ligase genes of Arabidopsis thaliana. Through organ expression analysis, three E3 ubiquitin ligase genes with higher expression levels in roots were further screened out, and they were named Sl1, SlRHE1, and SlRING1. Among these three genes, SlRING1 expression was the highest in response to cadmium (Cd) stress. Silencing SlRING1 significantly decreased chlorophyll content, Fv/Fm, photosynthetic rate, and biomass accumulation under Cd stress. The levels of H2O2, electrolyte leakage, and malondialdehyde significantly increased in SlRING1–silenced plants under Cd stress compared with that in non-silenced tomato plants. Cd stress-induced increases in the transcript levels of antioxidant and detoxification genes such as CAT, DHAR, MDHAR, GSH, and PCS were compromised by SlRING1 silencing. Moreover, Cd accumulation in shoots and roots significantly increased in SlRING1-silenced plants compared with non-silenced tomato plants. These findings suggest that SlRING1 plays a positive role in plant tolerance to Cd stress in tomato.

Wheat-associated Pseudomonas taiwanensis WRS8 reduces cadmium uptake by increasing root surface cadmium adsorption and decreasing cadmium uptake and transport related gene expression in wheat.

Metal-resistant bacteria can reduce Cd accumulation in plants, but mechanisms underlying this effect are poorly understood. In this study, a highly effective Cd-resistant WRS8 strain was obtained from the rhizoshere soil of Triticum aestivum L. Yangmai-13 and identified as Pseudomonas taiwanensis based on 16S rRNA gene sequence analysis. Strain WRS8 was investigated for its effects on Cd availability and wheat tissue Cd contents and the related mechanisms using a hydroponic culture experiment. In strain WRS8-inoculated solution, the Cd concentration reduced and the pH and cell-adsorbed Cd increased with time. Strain WRS8 increased the wheat root and above-ground tissue dry weights by 11-36% compared to the controls. In strain WRS8-inoculated wheat plants, the Cd contents of the roots and above-ground tissues decreased by 78-85% and 88-94% and the Cd bioconcentration and translocation factors decreased by 78-85% and 46-58% at days 3 and 10, respectively, compared with the controls. The root surface-adsorbed Cd contents increased by 99-121% in the WRS8 strain-inoculated wheat plants at days 3 and 10 compared to the controls. Furthermore, strain WRS8 colonized the wheat root surfaces and interiors and reduced the expression levels of the LCT1 and HMA2 genes involved in Cd accumulation and transport in wheat roots by 46% and 30%, respectively, compared to the controls. In the Cd-contaminated soils, strain WRS8 significantly reduced the available Cd content by 20-24% and increased the pH compared to the controls. These findings showed the important role of strain WRS8 in reducing solution and soil Cd availability and suggested that strain WRS8 reduced the wheat tissue Cd accumulation by increasing root surface Cd adsorption and decreasing wheat root Cd uptake and transport-related gene expression and may provide a new and effective wheat rhizobacteria-enhanced approach for reducing wheat Cd uptake in Cd-polluted environments.

Transcriptomic and physiological comparsion of the short-term responses of two Oryza sativa L. varieties to cadmium

Though responses to Cd in rice (Oryza sativa L.) have been well established, there are few reports about the different effects of short-term exposure to Cd between the subspecies indica (‘Kasalath’ variety) and japonica (‘Nipponbare’ variety). We observed similar Cd accumulation and root damage in roots of two varieties under 6 h Cd stress, but KEGG pathway analysis of transcriptome data revealed that pathways involved in different metabolic processes were affected. Among them, antioxidant enzyme system was involved in Cd detoxification in ‘Nipponbare’ variety, whereas GSH metabolism played important roles in ‘Kasalath’ variety. From the results of physiological experiments, extremely higher activity of SOD, POD, and CAT was observed in ‘Nipponbare’ variety after a short-term exposure to Cd. At the same time, the immediate Cd-induced stimulation of Cys and PCs were observed in ‘Kasalath’ variety along with the enlargement of vacuoles. This study provides an overview of main responses to short-term Cd stress and identifies potential target genes that may contribute to the different mechanisms of Cd stress-resistance in roots of japonica subspecies and indica subspecies.

Cadmium Tolerance in Alfalfa is Related to the Up-regulation of Iron and Sulfur Transporter Genes along with Phytochelatin Accumulation

Cadmium (Cd) toxicity is a serious limitation for agricultural production. In this study, we explored tolerance mechanism associated with Cd toxicity tolerance in alfalfa plants. We used three distinct alfalfa cultivars M. sativa cv. Vernal, M. sativa cv. Zhung Mu, and M. sativa cv. Xing Jiang Daye in this study. Cd showed declined chlorophyll score in Xing Jiang Daye compared with Zhung Mu and Vernal. No significant change observed among the cultivars for root and shoot length. Atomic absorption spectroscopy analysis demonstrated a significant accumulation of Cd, Fe, S and PC in distinct alfalfa cultivars. However, Zhung Mu and Xing Jiang Daye declined Cd accumulation in root, where Fe, S and PC incremented only in Zhung Mu. It suggests that excess Cd in Zhung Mu possibly inhibited in root by the increased accumulation of Fe, S and PC. This was further confirmed by the response of Fe (MsIRT1) and S transporters (MsSULTR1;2 and MsSULTR1;3), and MsPCS1 genes associated with Fe, S and PC availability and translocation in roots and shoots. It suggests that specially the transcript signal inducing the responses to adjust Cd especially in Zhung Mu. This finding provides the essential background for further molecular breeding program for forage crops.

Effect of biochar on the accumulation and distribution of cadmium in tobacco (Yunyan 87) at different developmental stages

Cadmium (Cd) easily accumulates in tobacco, which endangers public health through Cd exposure from smoking. However, its uptake, translocation, and distribution in tobacco plants during plant development or its response to biochar application are poorly understood. A pot experiment was conducted with tobacco (Yunyan 87) grown in soil severely contaminated with Cd (30 mg kg−1) amended with 0, 1, and 2% (w/w) tobacco stem-derived biochar (BC). The absorption and accumulation of Cd in all parts of the tobacco plants were most active from the rosette stage to the fast growing stage, during which approximately 90% of the Cd deposited in the tobacco leaves occurred, especially in the lower leaves. The Cd concentrations in most plant parts without added biochar decreased significantly by 52.61–78.30% due to the rapid increase in biomass (dilution effect), although the Cd concentration in the lower leaves increased by 48.89% (P < 0.05). However, with the slowdown of the growth rate of tobacco at the maturity stage, the proportion of Cd accumulation in roots and stems without biochar addition increased by 29.01%, resulting in an increased Cd concentration in roots and stems by 63.29–86.80% (P < 0.05). In the different growth stages, the application of biochar reduced the contents of DTPA-extractable and exchangeable Cd in the soil by 5.11–35.14% and 9.20–54.05%, respectively, thus reducing the absorption, accumulation and concentration of Cd in all parts of the tobacco plant. In addition, the inhibitive effect of biochar on the Cd concentration in the leaves was weak at the rosette stage (22.17–53.72%) compared with the other stages (46.14–78.88%), and the degree of inhibition of biochar on the Cd concentration in the middle leaves (37.94–59.24%) was lower than that in the upper and lower leaves (49.04–73.54%) at all developmental stages. However, the long-term remediation effect of biochar on soil Cd contamination needs to be further verified, and the combination of biochar and other technologies should receive additional attention.

The predominant role of pectin in binding Cd in the root cell wall of a high Cd accumulating rice line (Oryza sativa L.)

Cell wall (CW) plays an important role in Cd accumulation in roots of metal-tolerant plants, including rice. The role of CW polysaccharides, especially pectin, in binding Cd in roots of a high Cd accumulating (HA) rice line of Lu527-8 and a non-high Cd accumulating (NHA) rice line of Lu527-4 was investigated in this study. About 59%–63% of Cd in roots of the two rice lines was bound to CWs, indicating that CW was the main site for Cd accumulation in roots of the two rice lines. Cd adsorbed on the root CWs of the HA was 1.1–1.2 times more than that of the NHA, demonstrating the root CWs of the HA showed greater Cd binding ability. Cd exposure induced more Cd accumulation in pectin and hemicellulose in the HA. In particular, up to 65% of Cd accumulation in root CWs of the HA was observed in pectin. The removal of pectin lead to a 50% decrease for the amounts of Cd adsorption on root CWs of the HA, indicating that pectin was the major binding site for Cd in root CWs of the HA. The HA showed greater pectin methylesterase activities, resulting in lower degree of pectin methylesterification along with more low-methylesterified pectins in root CWs than the NHA. The more accumulation of low-methylesterified pectins in CWs induced by Cd contributed greatly to the high Cd accumulation in roots of the HA rice line of Lu527-8.

Phosphorus deficiency induces root proliferation and Cd absorption but inhibits Cd tolerance and Cd translocation in roots of Populus × euramericana

To disclose how phosphorus deficiency influence phytoremediation of Cd contamination using poplars, root architecture, Cd absorption, Cd translocation and antioxidant defense in poplar roots were investigated using a clone of Populus × euramericana. Root growth was unaltered by Cd exposure regardless of P conditions, while the degree of root proliferation upon P deficiency was changed by high level of Cd exposure. The concentration and content of Cd accumulation in roots were increased by P deficiency. This can be partially explained by the increased expression of genes encoding PM H + -ATPase under the combined conditions of P deficiency and high Cd exposure, which enhanced Cd2+-H+ exchanges and led to an increment of Cd uptake under P deficiency. Despite of the increasing Cd accumulation in roots, the translocation of Cd from roots to aerial tissues sharply decreased upon P deficiency. The relative expression of genes responsible for Cd translocation (HMA4) decreased upon P deficiency and thus inhibited Cd translocation via xylem. GR activity was decreased by P deficiency, which can inhibit the form of GSH and GSH–Cd complexes and decrease Cd translocation via GSH–Cd complexes. The transportation of PC–Cd complexes into vacuole decreased under P deficiency as a result of the low expression of PCS and ABCC1, and thus suppressed Cd tolerance and Cd detoxification in roots. Moreover, P deficiency decreased the levels of antioxidase (GR and CAT) and phytohormones including JA, ABA and GA3, which synchronously reduced antioxidant capacity in roots.

Mechanisms of low cadmium accumulation in storage root of sweetpotato (Ipomoea batatas L.)

Sweetpotato (Ipomoea batatas L.) possess great application prospects due to their low cadmium (Cd) concentration within their storage roots despite growth on Cd-polluted fields. The mechanisms of low Cd accumulation in storage root is not entirely clear. We found that the blocking effect of Cd uptake in the root absorption system and the characteristics of Cd distribution in storage root play a decisive role in the regulation of low Cd accumulation in storage root. Cd absorbed from the rhizosphere mainly accumulated in feeder roots in Cd dose-dependent accumulation analyses. Meanwhile, we found that Cd absorbed by the peels of storage root was mainly transported from peels to shoots, rather than directly into the fleshed storage root. Further analysis indicated that Cd uptake, transport, and distribution in sweetpotato hinges on whether Cd enters the plant plasma membrane by either the symplast or apoplast pathway. The Cd concentration in feeder root decreased after respiratory inhibitors CCCP and DNP treatment and increased after the culture temperature was raised from 28 ℃ to 35 ℃. Non-invasive microelectrode Cd flux measurements further revealed that Cd uptake in feeder root was affected greatly by the Cd concentration of the solution and was markedly reduced by respiratory inhibitor CCCP. Relative to the elongation zone and mature zone, the meristematic zone was the main site of Cd uptake in the root absorption system. This study suggests that inhibition of Cd uptake by the root absorption system and the characteristics of Cd distribution in storage root are the main reasons for low cadmium accumulation in storage root.

A comparative study of root cadmium radial transport in seedlings of two wheat (Triticum aestivum L.) genotypes differing in grain cadmium accumulation

The radial transport of cadmium (Cd) is essential for Cd influx in roots. The role of radial transport pathway on the Cd translocation from root to shoot among wheat genotypes are still poorly understood. This study explored the role of apoplastic and symplastic pathway on root Cd uptake and root-to-shoot translocation in Zhenmai 10 (ZM10, high Cd in grains) and Aikang 58 (AK58, low Cd in grains). Under Cd treatment, the deposition of Casparian strips (CSs) and suberin lamellae (SL) initiated closer to the root apex in ZM10 than that in AK58, which resulted in the lower Cd concentration in apoplastic fluid of ZM10. Simultaneously, Cd-induced expression levels of genes related to Cd uptake in roots were significantly higher in AK58 by contrast with ZM10, contributing to the symplastic Cd accumulation in AK58 root. Moreover, the addition of metabolic inhibitor CCCP noticeably decreased the Cd accumulation in root of both genotypes. Intriguingly, compared to ZM10, greater amounts of Cd were sequestrated in the cell walls and vacuoles in roots of AK58, limiting the translocation of Cd from root to shoot. Furthermore, the elevated TaHMA2 expression in ZM10 indicates that ZM10 had a higher capacity of xylem loading Cd than AK58. All of these results herein suggest that the radial transport is significant for Cd accumulation in roots, but it cannot explain the difference in root-to-shoot translocation of Cd in wheat genotypes with contrast Cd accumulation in grains.

Reduction of Cd accumulation in Se-biofortified rice by using fermented manure and fly ash.

Large areas of soils in China are contaminated with Cd and are deficient in Se. Therefore, here, we aimed to reduce Cd accumulation while increasing Se content in rice grain, and to elucidate the mechanisms associated. A greenhouse pot experiment was conducted to determine grain concentrations of Se and Cd upon foliar spraying of Se combined with the application of horse manure and/or fly ash to different contaminated soils containing Cd 0.51 (T1), 1.46 (T2), and 4.59mg Cd kg-1 (T3). The amount of Fe, Si, and Cd in root iron plaque, and concentrations of Cd and Si in rice tissues were also determined. Foliar spray of Se increased Se concentration in brown rice from approximately 0.04 to 0.15mgkg-1. Fly ash significantly reduced Cd concentration in brown rice from 0.07 to 0.05, 0.15 to 0.09, and 1.00 to 0.55mgkg-1 at the T1, T2, and T3 treatment levels, respectively, and soil Cd bioavailability (by at least 33.3%), while it increased Si content in rice roots and shoots by at least 34%. The increase of Si concentration in rice tissues inhibited Cd translocation to brown rice by at least 17%. Horse manure increased the formation of root Fe plaque by approximately 2.3-fold, which resulted in the significant reduction of Cd accumulation in brown rice, shoots, and roots by 36-56%. Thus, foliar spray of Se in combination with the application of fly ash and horse manure proved an effective method to produce Cd-low and Se-rich rice.

A field study to predict Cd bioaccumulation in a soil-wheat system: Application of a geochemical model

An accurate model to predict Cd accumulation in crops based on soil properties would facilitate evaluations of soil quality and the potential risk posed by metals. However, given the heterogeneity of soil, such models are difficult to establish on large regional scales. This study for the first time examined the applicability of a multi-surface speciation model (MSM) in predicting Cd accumulation in wheat at a regional field scale, based on 140 soil-wheat paired samples collected from a 205-km2 field. The MSM resulted in a better correlation between Cd accumulation in wheat grain (R2 = 0.75) and roots (R2 = 0.74) than obtained with chemical extraction methods (total Cd in soil, 0.01 M CaCl2, and 0.43 M HNO3). In addition, while the performance of the MSM was comparable to that of a traditional multiple regression model, a parameter-fitting process was not required. The predictive ability of the MSM was further used to assess and predict the soil Cd risk and to develop a soil Cd sensitivity map to better localize areas of greatest sensitivity to Cd contamination. The results showed that the MSM can serve as a useful tool for regional soil risk assessments and thus in the development of soil protection measures.