Identification of low-Cd cultivars of sweet potato (Ipomoea batatas (L.) Lam.) after growing on Cd-contaminated soil: uptake and partitioning to the edible roots

Changes in the Biology and Susceptibility of Weevil (Cylas formicarius) to the Insecticide Spinetoram as a Response to Cadmium Contamination

The sweet potato weevil Cylas formicarius is a notorious underground pest in sweet potato (Ipomoea batatas L.). However, little is known about the effects of cadmium (Cd) stress on weevil biology and resistance to pesticides and biotic agents. Therefore, we fed sweet potato weevils with Cd-contaminated sweet potato and assessed adult food intake and survival and larval developmental duration and mortality rates, as well as resistance to the insecticide spinetoram and susceptibility to the entomopathogenic fungus Beauveria bassiana. With increasing Cd concentration, the number of adult weevil feeding holes, adult survival and life span, and larval developmental duration decreased significantly, whereas larval mortality rates increased significantly. However, at the lowest Cd concentration (30 mg/L), adult feeding was stimulated. Resistance of adult sweet potato weevils to spinetoram increased at low Cd concentration, whereas Cd contamination did not affect sensitivity to B. bassiana. Thus, Cd contamination affected sweet potato weevil biology and resistance, and further studies will investigate weevil Cd accumulation and detoxification mechanisms.

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.

Direct absorption of atmospheric lead by rapeseed siliques is the leading cause of seed lead pollution

Rapeseed cultivation is a novel approach to safely utilizing lead (Pb) contaminated farmland. However, the mechanism of Pb absorption in seeds remains uncertain. A field experiment was conducted to explore this mechanism with two contrasting treatments: rapeseed exposed to atmospheric deposition and non-exposed treatment. Non-exposed treatment ultimately decreased Pb content in leaf, silique, and seed by 46.7%, 53.7%, and 53.6%, respectively. Sub-microstructure analysis further confirmed that rapeseed leaves and siliques could directly absorb atmospheric Pb. In addition, Pb isotope analysis indicates that atmospheric deposition is the primary source of silique and seed Pb. The root and silique organs had relative Pb contributions of 28.0% and 72.0%, respectively, to seed. Thus, the direct absorption of atmospheric Pb by siliques during the filling stage was found to be the leading cause of seed Pb pollution.

Intercropping of Pinellia ternata (herbal plant) with Sedum alfredii (Cd-hyperaccumulator) to reduce soil cadmium (Cd) absorption and improve yield

Soil contamination by cadmium (Cd) is of global concern, threatening not only crop production, but also supply of herbal medicine. Research studies usually grow crops with Sedum alfredii (a Cd-hyperaccumulator). However, intercropping herbal plants with S. alfredii and their interactions with hydro-chemical properties of soil are rarely considered. This study examines the growth of a herbal plant, Pinellia ternata, intercropped with S. alfredii in Cd-contaminated soil. Plant characteristics were assessed, especially biomass and Cd content of bulbil (yield and quality of P. ternata). Soil hydro-chemical properties including water retention, Cd content and organic matter were determined with statistical analyses. At low soil-Cd contamination (0.6 μg/g), bulbil biomass of intercropped P. ternata (PSL) was almost double compared with monoculture of P. ternata (PL), which is barely significant (p ≈ 0.05). The corm biomass of PSL was also significantly greater than that of PL (p < 0.05). Although soil-Cd contamination became more severe by increasing to 3 μg/g, the bulbil biomass in the intercrop was not significantly different from PL (p > 0.05). That said, it is evidenced that the yield of intercropped P. ternata was improved in Cd-contaminated soil. Such improvement was mainly attributed to reduced soil-Cd content and enhanced soil-water retention which was governed by plant roots and soil organic matters. The soil-water retention was first identified as a critical parameter in promoting plant growth under intercropping. More importantly, the bulbil-Cd content of P. ternata in PSL was significantly reduced (p < 0.05). This study demonstrates that the newly proposed intercrop is feasible to improve yield of herbal plants, and at the same time reduce heavy metal absorption and accumulation in medicinal organs, especially for P. ternata. This is anticipated to reduce the human health risk imposed by ingestion of Chinese herbal plants.

Effect of potassium intake on cadmium transporters and root cell wall biosynthesis in sweet potato

Large areas of farmland soil in southern China are deficient in potassium (K) and are contaminated with cadmium (Cd). Previously, we suggested that the K supplementation could reduce Cd accumulation in sweet potatoes (Ipomoea batatas (L.) Lam). In the present study, we investigated the underlying physiological and molecular mechanisms. A hydroponic experiment with different K and Cd treatments was performed to compare the transcriptome profile and the cell wall structure in the roots of sweet potato using RNA sequencing, Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The results showed that K supply inhibits the expressions of IRT1 and YSL3, which are responsible for root Cd uptake under Cd exposure. Furthermore, the expressions of COPT5 and Nramp3 were downregulated by K, which increased Cd retention in the root vacuoles. The upregulation of POD, CAD, INT1 and SUS by K contributed to lignin and cellulose biosynthesis and thickening of root xylem cell wall, which further reduced Cd translocation to the shoot. In addition, K affected the expressions of LHT, ACS, TPS and TPP associated with the production of ethylene and trehalose, which involved in plant resistance to Cd toxicity. In general, K application could decrease the uptake and translocation of Cd in sweet potatoes by regulating the expression of genes associated with Cd transporters and root cell wall components.

Transcriptomic analyses of sweet potato in response to Cd exposure and protective effects of K on Cd-induced physiological alterations

We aimed to understand the molecular mechanism of differential cadmium (Cd) accumulation in two cultivars of sweet potato and to clarify the effects of potassium (K) supply on Cd accumulation. Comparative transcriptomes were employed to identify key genes and pathways using a low-Cd (N88) and a high-Cd cultivar (X16) in a pot experiment. The antioxidant capacity and cell wall components of root tips were analyzed to account for the effect of K regulating Cd accumulation in N88 via a hydroponic experiment. Transcriptome analysis revealed that 29 and 20 genes were differentially expressed in N88 and X16, respectively, when comparing the control with the two Cd treatments. X16 had more differentially expressed genes (DEGs), including 2649 common up-regulated and 3173 common down-regulated than N88 in any treatment. GO and KEGG analyses showed that the DEGs were assigned and enriched in different pathways. Some critical DEGs such as PDR, HMA3, COPT5, CAX3, GAUT, CCR, AUX1, CAT, SOD, GSR, and GST were identified. The DEGs were involved in pathways including heavy metal transport or detoxification, cell wall biosynthesis, plant hormone signal transduction, and glutathione metabolism. Additionally, K supply substantially decreased Cd accumulation and reactive oxygen species production and promoted the production of cellulose, pectin and lignin in the root tips when exposed to Cd. Several critical DEGs associated with heavy metal transport and cell wall biosynthesis were responsible for the difference of Cd accumulation between the two cultivars. Application of K could help decrease Cd accumulation in sweet potato.

Cadmium Uptake and Translocation By Potato in Acid and Calcareous Soils

Cadmium contamination of agricultural soils threatens food safety. The bioaccumulation (BAF) of Cd in potato tubers ranged from 0.69 to 1.50 and 0.39 to 0.82 in acidic yellow and alkaline calcareous soils, respectively, when 0.3 to 4.8 mg Cd kg^-1 was added to the soil. The order of Cd concentration for different organs was root > stem > leaf > tuber. The BAF of Cd decreased with the increase of soil Cd concentration. The effect of pH was important for the transfer and accumulation of Cd for potato. Soil Cd concentration was correlated with the plant Cd concentration and soil pH. Cultivars Hui-2 and Xuanshu 2 accumulated less Cd in six potato cultivars. Logarithmic transformation of the data increased the R² value from 0.725 to 0.941 in the prediction model of soil Cd concentration. The data are useful in assessing the ecological risk of Cd to potato in Karst area.

Improving pesticide uptake modeling and management in potatoes: A simple and approximate phloem-adjusted model

To evaluate the impact of the phloem flux on the pesticide uptake process in potatoes, this study developed a phloem-adjusted model based on the classic model that focuses mainly on the diffusion process. To achieve high-throughput simulations, we introduced an approximate method to convert the phloem flux transport process into a simple specific uptake rate of pesticides. In comparison to the classic model (non-phloem model), the phloem-adjusted model generated higher pesticide concentrations and bioconcentration factors (BCFs) in potatoes, owing to the additional pesticide uptake route introduced to the adjusted model. However, the simulation, which was conducted for 740 pesticides, indicated that for most pesticides, the phloem flux route did not contribute a significant portion of the pesticide uptake to potato tubers compared with the soil diffusion route. This was further characterized, using the differential factor (DF), to evaluate the difference in the simulated results between the proposed model and classic models. The largest DF (~0.11) was obtained for pesticides with moderate lipophilicity (i.e., log K_OW of 3.0), indicating that only a difference of 10% was generated between the two models. The 10% increase in pesticide concentration (or BCFs) in potatoes, simulated by the phloem-adjusted model, was within the acceptable uncertainty interval of the classic model, thus confirming the validity of using the classic model to predict the pesticide uptake process in potato tubers. However, we found that the negligibility of the phloem flux route was not merely due to hydrophobicity (i.e., hypothesis of the classic model), but was related to the i) plant physiology of potatoes, ii) lipophilicity of a pesticide, and iii) the diffusivity of a pesticide in water. Although future studies on pesticide concentrations in phloem sap and the dynamic growth of potatoes need to be undertaken, the model developed in this study reveals a more comprehensive pesticide uptake process in potatoes, which can promote the understanding of the pesticide uptake mechanism in potatoes.

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.

Differences in absorption of cadmium and lead among fourteen sweet potato cultivars and health risk assessment

Planting sweet potato (Ipomoea batatas (L.) Lam.) instead of rice in the area contaminated with heavy metals is one of the measures to ensure people's health and agricultural economy. Therefore, it is important to screen the low accumulation cultivars of sweet potato and to find out the concentration rule of cadmium (Cd) and lead (Pb) in edible parts along with the associated health risks to humans. A field experiment was performed with fourteen of three main types (starch, purple, and edible-type) of sweet potato cultivars grown on farmland polluted with Cd and Pb in eastern Hunan Province, China. The Cd and Pb concentrations in the sweet potato tissues as well as the yield were measured. The yield of the shoot and tuberous root of the fourteen sweet potato cultivars ranged from 14.59 to 68.57 and 26.35-50.76 t ha^-1 with mean values of 33.09 and 33.46 t ha^-1, respectively. Compared with purple and edible-type cultivars, the starch-type cultivar had lower Cd and Pb concentrations in the flesh, but higher in the shoot. The Cd and Pb concentrations in the flesh of cultivars Shangshu 19, Sushu 24, Yushu 98, and Xiangshu 98 were lower than MCL provided in Chinese National Food Safety Standards GB2762-2017. Based on the hazard index (HI), the consumption of sweet potato flesh is lower health risk, while shoots pose a greater health risk to local people and Cd is the main cause of the risk. As a result, sweet potato cultivars Shangshu 19, Sushu 24, Yushu 98 and Xiangshu 98 can be plant in serve Cd and Pb contaminated soils with the advantages of easy cultivation, high yield and economic benefits without stopping agricultural production.

Comparative assessment of Brassica pekinensis L. genotypes for phytoavoidation of nitrate, cadmium and lead in multi-pollutant field

Information is needed for comparative assessment and agronomic practices for phytoavoidation in multi-pollutant field. A field study was conducted to explore 97 Brassica pekinensis L. genotypes with permissible limit of contaminants growing in a severely Cd, moderately nitrate and slightly Pb multi-polluted field. Thirteen genotypes, i.e. KGZY, CXQW, CAIB, JINL, JQIN, JFEN, WMQF, XLSH, TAIK, BJXS, JUKA, XYJQ and GQBW, were identified with permissible limit for nitrate, Cd and Pb based on their resistance to heavy metal and nitrate accumulation in leaves when grown in co-contaminated soils. Furthermore, the correlation between essential and toxic elements concentrations in plant of B. pekinensis were inconsistent. Generally speaking, application of increasing Ca, K and S fertilizers in appropriate forms and dosages tended to increase the yield and quality of B. pekinensis cultivated in multi-pollutant field.

Fractionation and chemical structure of dissolved organic matter in the rhizosphere associated with cadmium accumulation in tobacco lines (Nicotiana tabacum L.)

Reducing cadmium (Cd) accumulation in flue-cured tobacco is the main degree to reduce the harm of Cd to human health. In this study, a rhizobag experiment was conducted to investigate the characteristics of dissolved organic matter (DOM) in the rhizosphere of a low-Cd-accumulating tobacco line (RG11) and its role in the processes of Cd accumulation by plants. Cd concentrations in the roots and leaves of RG11 were 24.09-25.30 and 31.08-34.41% lower, respectively, than those of Yuyan5 under Cd stress. Cd exposure promoted DOM accumulation in the rhizosphere soils of the two tobacco lines. DOM concentrations in the rhizosphere soils of RG11 were 8.29-14.31% lower than those of Yuyan5 under Cd stress. RG11 presented less hydrophilic acid and hydrophilic base fractions, along with more hydrophobic acid and hydrophobic base fractions of DOM in the rhizosphere than those of Yuyan5 under Cd exposure. Fourier transform infrared spectroscopy results showed that RG11 exhibited less O-H, C-H, C=C, COO-, and C-O functional groups in rhizosphere DOM than those of Yuyan5 under Cd stress. Thus, the DOM in the rhizosphere of RG11 showed lower ability to solubilize Cd in soils, resulting in less Cd uptake by roots. This could be considered to be one of the important mechanisms of low Cd accumulation in leaves of RG11.

Accumulation and distribution of cadmium and lead in 28 oilseed rape cultivars grown in a contaminated field

Heavy metal pollution in soils has become an important concern for human health. Therefore, it is vital to develop suitable remediation strategies for contaminated soils. Oilseed rape tolerates high concentrations of heavy metals and is a promising candidate for the phytoextraction of cadmium (Cd) and lead (Pb) from metal-contaminated soils. A field experiment was conducted to evaluate 28 oilseed rape cultivars including Brassica napus L. and Brassica juncea L. for their ability to accumulate Cd and Pb. These cultivars were grown in a field co-contaminated with Cd (0.78 mg kg^-1) and Pb (330 mg kg^-1). The results showed that concentrations in shoots ranged from 1.22 to 3.01 mg kg^-1 for Cd and from 10.8 to 29.5 mg kg^-1 for Pb. Cadmium and Pb accumulations in shoots could reach 83.4 and 799 μg plant^-1, respectively. The majority of translocation factors (TFs) for Cd (> 1.0) were higher than for Pb (≤ 1.0). However, concentrations of Cd and Pb in seeds were much lower, in the range of 0.04 to 0.21 mg kg^-1 and 0.04 to 0.51 mg kg^-1, respectively. The seed yields of oilseed rape varied from 1238 to 2904 kg ha^-1, with a mean value of 2289 kg ha^-1. Among the cultivars, three (OS-9, OS-12, and OS-15) were selected as Cd and Pb potential accumulators, with Cd accumulation in shoots being 2.74-3.70 times higher and Pb accumulation in shoots being 3.37-5.23 times higher as compared with the lowest accumulating cultivar. These selected cultivars (B. napus) have application potential for phytoextraction of Cd and Pb from polluted soils without stopping agricultural activities and accompanying food safety issues.

Characterization of fava bean (Vicia faba L.) genotypes for phytoremediation of cadmium and lead co-contaminated soils coupled with agro-production

The identification of high yield genotypes that are capable of accumulating multiple heavy metals in the non-edible parts (roots and shoots), but not in the edible parts (seeds) and have desired nutritional value is necessary for accomplishing phytoremediation coupled with agro-production. In this study, 17 fava bean genotypes were screened in two different field conditions to examine their phytoremediation potential in terms of uptake and translocation of Cd and Pb. Ten genotypes, LBAO, JNJX, DCAN, QXCJ, QIKM, LXYC, YDL6, RBCD, QPID and ZHW6 were found as the best accumulators for Cd and Pb with permissible limit of metals in seeds. The concentration of plant nutrients were genotype and soil type dependent and there was a significant correlation between these two factors. Furthermore, the three genotypes DCAN, LBAO and LXYC showed best performance in alluvial soil type while QPID, RBCD and LXYC were the best in red soil type. Genotype LXYC was similar for both soil types and appeared to be the best fit for phytoremediation coupled with agro-production for slightly or moderately Cd and Pb co-contaminated soil. Therefore, fava bean LXYC genotype is suggested as a potential candidate for phytoremediation of Cd/Pb co-contaminated soils coupled with agro-production.

Cultivar-Dependent Accumulation and Translocation of Perfluorooctanesulfonate among Lettuce ( Lactuca sativa L.) Cultivars Grown on Perfluorooctanesulfonate-Contaminated Soil

Perfluorooctanesulfonate (PFOS) is a toxic and persistent organic pollutant that can be widely detected in agricultural soils. In this study, two lettuce cultivars with low PFOS accumulation were screened out to reduce the exposure of PFOS to the human body via vegetable consumption. The screened low-PFOS cultivars may help to ensure food safety, despite planting in highly PFOS-polluted soils (1.0 mg/kg), due to their high tolerance to PFOS and 4.4-5.7 times lower shoot PFOS concentration than the high-PFOS cultivars. Protein content and protein-mediated transpiration played key roles in regulating PFOS accumulation in the lettuce cultivars tested. Lower protein content, lower stomatal conductance, and lower transpiration rate resulted in low PFOS accumulation. This study reveals the mechanism of forming low-PFOS accumulation of lettuce cultivars at physiological and biochemical levels and lays a foundation for developing a cost-effective and safe approach to grow vegetables in PFOS-polluted soils.

Variations in cadmium and nitrate co-accumulation among water spinach genotypes and implications for screening safe genotypes for human consumption

Vegetables are important constituents of the human diet. Heavy metals and nitrate are among the major contaminants of vegetables. Consumption of vegetables and fruits with accumulated heavy metals and nitrate has the potential to damage different body organs leading to unwanted effects. Breeding vegetables with low heavy metal and nitrate contaminants is a cost-effective approach. We investigated 38 water spinach genotypes for low Cd and nitrate co-accumulation. Four genotypes, i.e. JXDY, GZQL, XGDB, and B888, were found to have low co-accumulation of Cd (<0.71 mg/kg dry weight) and nitrate (<3100 mg/kg fresh weight) in the edible parts when grown in soils with moderate contamination of both Cd (1.10 mg/kg) and nitrate (235.2 mg/kg). These genotypes should be appropriate with minimized risk to humans who consume them. The Cd levels in the edible parts of water spinach were positively correlated with the concentration of Pb or Zn, but Cd, Pb, or Zn was negatively correlated with P concentration. These results indicate that these three heavy metals may be absorbed into the plant in similar proportions or in combination, minimizing the influx to aerial parts. Increasing P fertilizer application rates appears to prevent heavy metal and nitrate translocation to shoot tissues and the edible parts of water spinach on co-contaminated soils.

The Response of Duckweed (Lemna minor L.) Roots to Cd and Its Chemical Forms

The response of duckweed (Lemna minor L.) roots to Cd and its chemical forms was investigated. The relative root growth rate and concentrations of Cd and its different chemical forms in the root, that is, ethanol-extractable (FE-Cd), HCl-extractable (FHCl-Cd), and residual fractions (Fr-Cd), were quantified. Weibull model was used to unravel the regression between the relative root elongation (RRL) with chemical forms of Cd. Parameters assessed catalase (CAT), peroxidases (POD), and superoxide dismutase (SOD), as well as malondialdehyde (MDA) and total antioxidant capacity (A-TOC). Our results show that both the relative root growth rate and relative frond number were affected by Cd concentrations. The chemical forms of Cd were influenced by Cd content in the medium. Relative root elongation (RRL) showed a significant correlation with chemical forms of Cd. Additionally, POD and SOD increased at lower Cd concentrations followed by a decrease at higher Cd concentrations (at more than 5 μM Cd). Moreover, MDA and A-TOC increased and CAT decreased with increasing Cd exposure. Furthermore, CAT showed a significant correlation with FHCl-Cd. Taken together, it can be concluded that the chemical forms of Cd are statistically significant predictors of Cd toxicity to duckweed and to the other similar aquatic plants.

Effects of phosphorus on chemical forms of Cd in plants of four spinach (Spinacia oleracea L.) cultivars differing in Cd accumulation

In order to clarify how cadmium (Cd) chemical forms in planta relate to the genotype difference in Cd accumulation of spinach (Spinacia oleracea L.), two low-Cd and two high-Cd cultivars were compared under a hydroponic experiment with two concentrations of Cd (8.98 or 44.71 μmol Cd L(-1)). The concentrations of phosphorus in the hydroponic system were also adjusted to two levels (0.5 and 1.0 mmol L(-1)) to investigate the influence of phosphorus on the forms and accumulation of Cd in the tested cultivars. Average Cd concentrations in shoots were 8.50-10.06 mg kg(-1) for high-Cd cultivars and 6.11-6.64 mg kg(-1) for low-Cd cultivars a under lower Cd treatment and were as high as 24.41-31.35 mg kg(-1) and 19.65-25.76 mg kg(-1), respectively, under a higher treatment. Phosphorus significantly decreased Cd accumulation in the tested cultivars, and the effect had superiority over the cultivar alternation under higher Cd stress. Cadmium in the NaCl-extractable fraction of the plant tissues showed the greatest relationship to genotype difference of Cd accumulation. The difference in the capacity to binding Cd into F HAc, F HCl, or F Residue was another important mechanism involving in the genotype difference in Cd accumulation of spinach. Among them, average proportion of Cd in F HAc in low-Cd cultivars was higher than that in high-Cd cultivars in association with the effect of phosphorus.