Expression of the novel wheat gene TM20 confers enhanced cadmium tolerance to bakers' yeast

Safe utilization evaluation of two typical traditional Chinese medicinal materials in Cd-contaminated soil based on the analysis of Cd transfer and AHP model

Due to the increasing scarcity of wild resources, most traditional Chinese medicinal materials (TCMMs) in the market are produced via artificial cultivation. The widespread pollution of cadmium (Cd) in soil limits the safe cultivation and use of TCMMs. This study investigated Cd accumulation, distribution, and the medicinal component content under simulated field conditions to clarify the differences in the Cd absorption, transfer and detoxification mechanisms of Polygonatum cyrtonema Hua and Bletilla striata, and provide the preliminary safe utilization conditions of TCMMs based on the analytic hierarchy process (AHP). The results showed that the Cd content of P. cyrtonema Hua was lower than the safety threshold under a high soil Cd concentration of 0.91 mg/kg (Cd-L), while B. striata was safe only at a low Cd concentration of 0.25 mg/kg (CK). Cd at 0.91 mg/kg induced hormesis affecting the net increase in biomass and medicinal component content for both TCMMs, while P. cyrtonema Hua showed better potential for safe utilization. Additionally, P. cyrtonema Hua had stronger resistance to Cd stress, exhibiting superior characteristics for synergistic absorption of Cd with mineral elements, transfer to nonmedical part and safer fixation forms in subcellular components. In contrast, B. striata showed insufficient Cd tolerance, and Cd was easily accumulated in organelles to inhibit plant growth. Our findings may attract more attention to the safe cultivation of TCMMs and provide insight into guidance for the safe utilization of slightly Cd-contaminated soil.

Supplementing two wheat genotypes with ZnSO4 and ZnO nanoparticles showed differential mitigation of Cd phytotoxicity by reducing Cd absorption, preserving root cellular ultrastructure, and regulating metal-transporter gene expression

Cadmium (Cd) contamination is a serious challenge in agricultural soils worldwide, resulting in Cd entering the food chain mainly through plant-based food and threatening human health. Minimizing Cd bioaccumulation in wheat is an important way to prevent Cd hazards to humans. Hydroponic and pot experiments were conducted to comprehensively evaluate the effects of zinc sulfate (ZnSO4) and zinc oxide nanoparticles (nZnO) on Cd uptake, translocation, subcellular distribution, cellular ultrastructure, and gene expression in two wheat genotypes that differ in grain Zn accumulation. Results showed that high-dose nZnO significantly reduced root Cd concentration (52.44%∼56.85%) in two wheats, in contrast to ZnSO4. The S216 exhibited higher tolerance to Cd compared to Z797. Importantly, Zn supplementation enhanced Cd sequestration into vacuoles and binding to cell walls, which conferred stability to ultracellular structures and photosynthetic apparatus. Down-regulation of influx transporter (TaHMA2 and TaLCT1) and up-regulation of efflux transporters (TaTM20 and TaHMA3) in Z797 might contribute to Zn-dependent alleviation of Cd toxicity and enhance its Cd tolerance. Down-regulation of ZIP transporters (TaZIP3, -5, and -7) might contribute to an increase in root Zn concentration and inhibit Cd absorption. Additionally, soil Zn provided an effective strategy for the reduction of grain Cd concentrations in both wheats, with a reduction of 26%∼32% (high ZnSO4) and 11%∼67% (high nZnO), respectively. Collectively, these findings provide new insights and perspectives on the mechanisms of Cd mitigation in wheats with different Zn fertilizers and demonstrate that the effect of nZnO in mitigating Cd stress is greater than that of ZnSO4 fertilizers.

Characterization of genes involved in micronutrients and toxic metals detoxification in Brassica napus by genome-wide cDNA library screening

Stresses caused by deficiency/excess of mineral nutrients or of pollution of toxic metals have already become a primary factor in limiting crop production worldwide. Genes involved in minerals and toxic metals accumulation/tolerance could be potential candidates for improving crop plants with enhanced nutritional efficiency and environmental adaptability. In this study, we first generated a high-quality yeast expression cDNA library of Brassica napus (Westar), and 46 genes mediating excess micronutrients and toxic metals detoxification were screened using the yeast genetic complementation system, including 11, 5, 6, 14, 6, and 5 genes involved in cadmium (Cd), zinc (Zn), iron (Fe), manganese (Mn), boron (B), and copper (Cu) tolerance, respectively. Characterization of genes mediating excess ions stress resistance in this study is beneficial for us to further understand ions homeostasis in B. napus.

A Genome-Wide Identification and Expression Analysis of the Casparian Strip Membrane Domain Protein-like Gene Family in Pogostemon cablin in Response to p-HBA-Induced Continuous Cropping Obstacles

Casparian strip membrane domain protein-like (CASPL) genes are key genes for the formation and regulation of the Casparian strip and play an important role in plant abiotic stress. However, little research has focused on the members, characteristics, and biological functions of the patchouli PatCASPL gene family. In this study, 156 PatCASPL genes were identified at the whole-genome level. Subcellular localization predicted that 75.6% of PatCASPL proteins reside on the cell membrane. A phylogenetic analysis categorized PatCASPL genes into five subclusters alongside Arabidopsis CASPL genes. In a cis-acting element analysis, a total of 16 different cis-elements were identified, among which the photo-responsive element was the most common in the CASPL gene family. A transcriptome analysis showed that p-hydroxybenzoic acid, an allelopathic autotoxic substance, affected the expression pattern of PatCASPLs, including a total of 27 upregulated genes and 30 down-regulated genes, suggesting that these PatCASPLs may play an important role in the regulation of patchouli continuous cropping obstacles by affecting the formation and integrity of Casparian strip bands. These results provided a theoretical basis for exploring and verifying the function of the patchouli PatCASPL gene family and its role in continuous cropping obstacles.

Foliar zinc reduced Cd accumulation in grains by inhibiting Cd mobility in the xylem and increasing Cd retention ability in roots1

Cadmium (Cd) pollution endangers the safe utilization of paddy soils, and foliar zinc (Zn) can reduce the toxic effects of Cd. However, little is known about the effects of foliar Zn application on the transport and immobilization of Cd in key rice tissues and the physiological state of rice plants. A pot experiment was conducted to explore the effects of spraying 0.2% and 0.4% Zn (ZnSO4) during the early grain-filling stage on Cd transport in rice, photosynthesis, glutathione (GSH) levels, Cd concentrations in xylem sap, and the expression of Zn transporter genes. The results showed that grain Cd concentrations in the 0.2% Zn and 0.4% Zn treatments were 24% and 31% lower, respectively, than those of the control treatments at maturity. Compared with the control treatments, the 0.4% Zn treatment increased Cd by 60%, 69%, 23%, and 22% in husks, rachises, first internodes, and roots, respectively. Application of Zn reduced xylem Cd content by up to 26% and downregulated transporter genes (OSZIP12, OSZIP4, and OSZIP7a) in flag leaves. Foliar Zn increased Cd bioaccumulation in roots while decreasing Cd bioaccumulation in grains. Zn reduced GSH concentration in flag leaves and stems, inhibiting photosynthesis (intercellular CO2 concentration, transpiration rate). Taken together, foliar Zn can reduce the expression of Zn transporter genes and the mobility of Cd in the xylem, promoting the fixation of Cd in husks, rachises, first internodes, and roots, ultimately reducing Cd concentration in rice grains.

Cadmium Transport in Maize Root Segments Using a Classical Physiological Approach: Evidence of Influx Largely Exceeding Efflux in Subapical Regions

The bidirectional fluxes of cadmium and calcium across the plasma membrane were assessed and compared in subapical maize root segments. This homogeneous material provides a simplified system for investigating ion fluxes in whole organs. The kinetic profile of cadmium influx was characterized by a combination of a saturable rectangular hyperbola (Km = 30.15) and a straight line (k = 0.0013 L h-1 g-1 fresh weight), indicating the presence of multiple transport systems. In contrast, the influx of calcium was described by a simple Michaelis-Menten function (Km = 26.57 µM). The addition of calcium to the medium reduced cadmium influx into the root segments, suggesting a competition between the two ions for the same transport system(s). The efflux of calcium from the root segments was found to be significantly higher than that of cadmium, which was extremely low under the experimental conditions used. This was further confirmed by comparing cadmium and calcium fluxes across the plasma membrane of inside-out vesicles purified from maize root cortical cells. The inability of the root cortical cells to extrude cadmium may have driven the evolution of metal chelators for detoxifying intracellular cadmium ions.

Sorghum Ionomics Reveals the Functional SbHMA3a Allele that Limits Excess Cadmium Accumulation in Grains

Understanding uptake and redistribution of essential minerals or sequestering of toxic elements is important for optimized crop production. Although the mechanisms controlling mineral transport have been elucidated in rice and other species, little is understood in sorghum-an important C4 cereal crop. Here, we assessed the genetic factors that govern grain ionome profiles in sorghum using recombinant inbred lines (RILs) derived from a cross between BTx623 and NOG (Takakibi). Pairwise correlation and clustering analysis of 22 elements, measured in sorghum grains harvested under greenhouse conditions, indicated that the parental lines, as well as the RILs, show different ionomes. In particular, BTx623 accumulated significantly higher levels of cadmium (Cd) than NOG, because of differential root-to-shoot translocation factors between the two lines. Quantitative trait locus (QTL) analysis revealed a prominent QTL for grain Cd concentration on chromosome 2. Detailed analysis identified SbHMA3a, encoding a P1B-type ATPase heavy metal transporter, as responsible for low Cd accumulation in grains; the NOG allele encoded a functional HMA3 transporter (SbHMA3a-NOG) whose Cd-transporting activity was confirmed by heterologous expression in yeast. BTx623 possessed a truncated, loss-of-function SbHMA3a allele. The functionality of SbHMA3a in NOG was confirmed by Cd concentrations of F2 grains derived from the reciprocal cross, in which the NOG allele behaved in a dominant manner. We concluded that SbHMA3a-NOG is a Cd transporter that sequesters excess Cd in root tissues, as shown in other HMA3s. Our findings will facilitate the isolation of breeding cultivars with low Cd in grains or in exploiting high-Cd cultivars for phytoremediation.

Soil and foliar applications of silicon and selenium effects on cadmium accumulation and plant growth by modulation of antioxidant system and Cd translocation: Comparison of soft vs. durum wheat varieties

Minimization of Cd accumulation in wheat is an effective strategy to prevent Cd hazard to human. This study compared and highlighted the roles of soil and foliar applications of Se and Si effects on Cd accumulation and toxicity in soft and durum wheat. Soil Se (0.5-1.0 mg kg^-1) and Si (3-6 mg kg^-1) applications provided an effective strategy to reduce wheat grain Cd concentrations of both wheat varieties by 59-61 % and 16-30 %, but foliar Se (0.125-0.25 mM) and Si (2.5-5 mM) application reduced grain Cd of soft wheat by 20-36 %. Both soil and foliar Se and Si applications significantly alleviated Cd toxicity by regulation of Cd transport genes, as reflected by increased the grain yield and antioxidant enzymes activities, and reduced MDA in wheat tissues. Selenium applications were more effective than Si on the reduction of Cd-induced toxicity and concentrations in soft wheat, but not in durum wheat due to more tolerant to Cd. Downregulation of influx transporter (TaNramp5) and upregulation of efflux transporter (TaTM20 and TaHMA3) in soft wheat may contribute to the Si/Se-dependent Cd mitigation and enhance the tolerance to toxic Cd. Overall, Se/Si applications, especially soil Se, can be efficiently used for reducing grain Cd uptake from Cd-contaminated soils.

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.

Effects of zinc application on cadmium (Cd) accumulation and plant growth through modulation of the antioxidant system and translocation of Cd in low- and high-Cd wheat cultivars

Cadmium (Cd) contamination is a big challenge for managing food supply and safety around the world. Reduction of the bioaccumulation of cadmium (Cd) in wheat is an important way to minimize Cd hazards to human health. This study compared and highlighted the effects of soil and foliar applications of Zn on Cd accumulation and toxicity in cultivars with high Cd accumulation (high-Cd wheat) and low Cd accumulation (low-Cd wheat). Both foliar and soil Zn applications provided effective strategies for reducing wheat grain Cd concentrations in the high-Cd wheat by 26-49% and 25-52%, respectively, and these also significantly reduced the concentrations in wheat stems and leaves. Foliar and soil Zn applications significantly reduced Cd in leaves and stems of the low-Cd wheat but had no effects on grain Cd. Both soil and foliar Zn applications significantly alleviated Cd toxicity by regulation of Cd transport genes, as reflected by the increased grain yield and antioxidant enzyme activity in the wheat tissues. Gene expression in response to zinc application differed in the two wheat cultivars. Down-regulation of the influx transporter (TaNramp5) and upregulation of the efflux transporters (TaTM20 and TaHMA3) in the high-Cd wheat may have contributed to the Zn-dependent Cd alleviation and enhanced its tolerance to Cd toxicity. Additionally, foliar Zn applications down-regulated the leaf TaHMA2 expression that reduced root Cd translocation to shoots, while soil Zn applications down-regulated the root TaLCT1 expression, which contributed to the reduction of root Cd concentrations. Soil (99 kg ZnSO₄·7H₂O ha^-1) and foliar (0.36 kg ZnSO₄·7H₂O ha^-1) Zn applications can effectively decrease the Cd in grains and guarantee food safety and yield, simultaneously. The presented results provide a new insight into the mechanisms of, and strategies for, using Zn for the Cd reduction in wheat.

Effects of node restriction on cadmium accumulation in eight Chinese wheat (Triticum turgidum) cultivars

Minimization of cadmium (Cd) accumulation in wheat is an effective method to prevent Cd-related health risks to humans. To understand the underlying mechanisms of restricting Cd transport, the role of nodes in Cd restriction was studied in eight Chinese wheat cultivars. The Cd accumulation differed significantly among the cultivars. The grain Cd concentrations were mainly dependent on the Cd concentrations in the roots and shoots. The Cd transport in the shoots controlled the wheat grain Cd accumulations. Nodes in the wheat stem have distinct functions in the transfer, distribution, and restriction of Cd. The node connected to the panicle showed the lowest translocation factors. The area of the vascular bundles, especially the diffuse vascular bundles, in the junctional node with the flag leaf was the key factor in restricting Cd transfer to the wheat grain. There was a significant relation between these areas and the grain Cd concentrations. The conclusion of this study is that screening or breeding cultivars with low Cd concentrations in the roots or with smaller areas of diffuse vascular bundles in the junctional nodes with the flag leaf is an effective strategy to decrease Cd concentration in wheat grains.

Modulation of Plant and Fungal Gene Expression Upon Cd Exposure and Symbiosis in Ericoid Mycorrhizal Vaccinium myrtillus

The success of Ericaceae in stressful habitats enriched in heavy metals has been ascribed to the distinctive abilities of their mycorrhizal fungal partners to withstand heavy metal stress and to enhance metal tolerance in the host plant. Whereas heavy metal tolerance has been extensively investigated in some ericoid mycorrhizal (ERM) fungi, the molecular and cellular mechanisms that extend tolerance to the host plant are currently unknown. Here, we show a reduced Cd content in Cd-exposed mycorrhizal roots of Vaccinium myrtillus colonized by a metal tolerant isolate of the fungus Oidiodendron maius as compared to non-mycorrhizal roots. To better understand this phenotype, we applied Next Generation Sequencing technologies to analyze gene expression in V. myrtillus and O. maius Zn grown under normal and Cd-stressed conditions, in the free living and in the mycorrhizal status. The results clearly showed that Cd had a stronger impact on plant gene expression than symbiosis, whereas fungal gene expression was mainly regulated by symbiosis. The higher abundance of transcripts coding for stress related proteins in non-mycorrhizal roots may be related to the higher Cd content. Regulated plant metal transporters have been identified that may play a role in reducing Cd content in mycorrhizal roots exposed to this metal.

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

Over the last decade, human population has been facing great challenges in ensuring appropriate supply of food free from cadmium (Cd) contamination. Selection of genetically low-Cd wheat (Triticum aestivum L.) genotypes, with a large biomass and high accumulation of Cd in straw but low-Cd concentration in grains, is an inventive approach of phytoremediation while keeping agricultural production in moderately contaminated soils. In this study, variations in Cd uptake and translocation among the 30 wheat genotypes in two different sites were investigated in field experiments. Significant differences in grain Cd concentration were observed between the two sites, with averaged values of 0.048 and 0.053 mg kg^-1 DW, respectively. Based on straw Cd accumulation, grain Cd concentration, and TF_rs, Bainong207 and Aikang58 for site A and Huaimai23 and Yannong21 for site B are promising candidates of low-Cd genotypes, which have considerable potential in achieving phytoremediation while keeping agricultural production on moderately or slightly Cd-polluted soil. The results indicate that it is possible to select the optimal low-Cd genotypes of wheat for different soil types by taking consideration of the effect of soil-wheat genotype interaction on grain Cd concentration.

Functional characterisation of two phytochelatin synthases in rice (Oryza sativa cv. Milyang 117) that respond to cadmium stress

Cadmium (Cd) is one of the most toxic heavy metals and a non-essential element to all organisms, including plants; however, the genes involved in Cd resistance in plants remain poorly characterised. To identify Cd resistance genes in rice, we screened a rice cDNA expression library treated with CdCl₂ using a yeast (Saccharomyces cerevisiae) mutant ycf1 strain (DTY167) and isolated two rice phytochelatin synthases (OsPCS5 and OsPCS15). The genes were strongly induced by Cd treatment and conferred increased resistance to Cd when expressed in the ycf1 mutant strain. In addition, the Cd concentration was twofold higher in yeast expressing OsPCS5 and OsPCS15 than in vector-transformed yeast, and OsPCS5 and OsPCS15 localised in the cytoplasm. Arabidopsis thaliana plants overexpressing OsPCS5/-15 paradoxically exhibited increased sensitivity to Cd, suggesting that overexpression of OsPCS5/-15 resulted in toxicity due to excess phytochelatin production in A. thaliana. These data indicate that OsPCS5 and OsPCS15 are involved in Cd tolerance, which may be related to the relative abundances of phytochelatins synthesised by these phytochelatin synthases.

Silicon decreases cadmium concentrations by modulating root endodermal suberin development in wheat plants

Silicon (Si) can alleviate cadmium (Cd) toxicity in many plants, but mechanisms underlying this beneficial effect are still lacking. In this study, the roles of Si in time-dependent apoplastic and symplastic Cd absorption by roots of wheat plants were investigated. Results showed that, during short-term Cd exposure, the symplastic pathway of Cd in roots was not significantly affected by Si. Cell wall properties and cell wall-bound Cd regarding the apoplastic pathway were unaffected by Si either. Nevertheless, Cd concentrations in the apoplastic fluid of roots were decreased by Si. The reason could be that Si delayed endodermal suberization of roots resulting in promoted apoplastic Cd translocation to shoots, thus decreasing Cd in the apoplastic fluid of roots after short-term Cd stress. By contrast, after long-term Cd stress, cell wall properties and the expression of genes related to Cd influx and transport were unaffected. Intriguingly, Si up-regulated the expression of the Cd efflux-related gene TaTM20 and repressed apoplastic Cd translocation to shoots, which might contribute to decrease of Cd after long-term Cd exposure. Taken together, these results indicate that Si-dependent decrease in root Cd concentrations during short-term Cd exposure helps plants to mitigate Cd toxicity in the long-term.

Breeding Low-Cadmium Wheat: Progress and Perspectives

Farmland cadmium (Cd) contamination has adverse impacts on both wheat grain yield and people’s well-being through food consumption. Safe farming using low-Cd cultivars has been proposed as a promising approach to address the farmland Cd pollution problem. To date, several dozen low-Cd wheat cultivars have been screened worldwide based on a Cd inhibition test, representing candidates for wheat Cd minimization. Unfortunately, the breeding of low-Cd wheat cultivars with desired traits or enhanced Cd exclusion has not been extensively explored. Moreover, the wheat Cd inhibition test for variety screening and conventional breeding is expensive and time-consuming. As an alternative, low-Cd wheat cultivars that were developed with molecular genetics and breeding approaches can be promising, typically by the association of marker-assisted selection (MAS) with conventional breeding practices. In this review, we provide a synthetics view of the background and knowledge basis for the breeding of low-Cd wheat cultivars.

Overexpression of heat stress-responsive TaMBF1c, a wheat (Triticum aestivum L.) Multiprotein Bridging Factor, confers heat tolerance in both yeast and rice

Previously, we found an ethylene-responsive transcriptional co-activator, which was significantly induced by heat stress (HS) in both thermo-sensitive and thermo-tolerant wheat. The corresponding ORF was isolated from wheat, and named TaMBF1c (Multiprotein Bridging Factor1c). The deduced amino acid sequence revealed the presence of conserved MBF1 and helix-turn-helix domains at the N- and C-terminus, respectively, which were highly similar to rice ERTCA (Ethylene Response Transcriptional Co-Activator) and Arabidopsis MBF1c. The promoter region of TaMBF1c contained three heat shock elements (HSEs) and other stress-responsive elements. There was no detectable mRNA of TaMBF1c under control conditions, but the transcript was rapidly and significantly induced by heat stress not only at the seedling stage, but also at the flowering stage. It was also slightly induced by drought and H2O2 stresses, as well as by application of the ethylene synthesis precursor ACC, but not, however, by circadian rhythm, salt, ABA or MeJA treatments. Under normal temperatures, TaMBF1c-eGFP protein showed predominant nuclear localization with some levels of cytosol localization in the bombarded onion epidermal cells, but it was mainly detected in the nucleus with almost no eGFP signals in cytosol when the bombarded onion cells were cultured under high temperature conditions. Overexpression of TaMBF1c in yeast imparted tolerance to heat stress compared to cells expressing the vector alone. Most importantly, transgenic rice plants engineered to overexpress TaMBF1c showed higher thermotolerance than control plants at both seedling and reproductive stages. In addition, transcript levels of six Heat Shock Protein and two Trehalose Phosphate Synthase genes were higher in TaMBF1c transgenic lines than in wild-type rice upon heat treatment. Collectively, the present data suggest that TaMBF1c plays a pivotal role in plant thermotolerance and holds promising possibilities for improving heat tolerance in crops.

Rice DEP1, encoding a highly cysteine-rich G protein γ subunit, confers cadmium tolerance on yeast cells and plants

A rice cDNA, OsDEP1, encoding a highly cysteine (Cys)-rich G protein γ subunit, was initially identified as it conferred cadmium (Cd) tolerance on yeast cells. Of the 426 aa constituting OsDEP1, 120 are Cys residues (28.2%), of which 88 are clustered in the C-terminal half region (aa 170-426). To evaluate the independent effects of these two regions, two truncated versions of the OsDEP1-expressing plasmids pOsDEP1(1-169) and pOsDEP1(170-426) were used to examine their effects on yeast Cd tolerance. Although OsDEP1(170-426) conferred a similar level of Cd tolerance as the intact OsDEP1, OsDEP1(1-169) provided no such tolerance, indicating that the tolerance effect is localized to the aa 170-426 C-terminal peptide region. The Cd responses of transgenic Arabidopsis plants constitutively expressing OsDEP1, OsDEP1(1-169) or OsDEP1(170-426), were similar to the observations in yeast cells, with OsDEP1 and OsDEP1(170-426) transgenic plants displaying Cd tolerance but OsDEP1(1-169) plants showing no such tolerance. In addition, a positive correlation between the transcript levels of OsDEP1 or OsDEP1(170-426) in the transgenics and the Cd content of these plants upon Cd application was observed. As several Arabidopsis loss-of-function heterotrimeric G protein β and γ subunit gene mutants did not show differences in their Cd sensitivity compared with wild-type plants, we propose that the Cys-rich region of OsDEP1 may function directly as a trap for Cd ions.

Response of Maize Seedlings to Cadmium Application after Different Time Intervals

Present study was conducted to appraise the inhibitory effects of cadmium applied at different time intervals on various growth and biochemical parameters in two maize lines, Maize-TargetedMutagenesis 1 and 2 (MTM-1 and MTM-2). Twenty-day-old seedlings were exposed to 0, 3, 6, 9, and 12 mg CdCl2 kg−1 sand. Both maize lines exhibited significant perturbations in important biochemical attributes being employed for screening the crops for cadmium tolerance. The results showed that a higher concentration of cadmium (12 mg CdCl2 kg−1) considerably reduced the plant growth in line MTM-1 on the 5th, 10th, and 15th day after the treatment. In contrast, irrespective of exposure time, the plant biomass and leaf area did not show inhibitory effects of cadmium, specifically at 3 mg CdCl2 kg−1 in line MTM-2. In addition, MTM-2 was found to be more tolerant than line MTM-1 in terms of lower levels of hydrogen peroxide (H2O2), malondialdehyde (MDA) contents, and relative membrane permeability (RMP). Moreover, H2O2, MDA, RMP, and anthocyanin increased at all levels of cadmium in both lines, but a significant decline was observed in photosynthetic pigments, total free amino acids, and proline contents in all treatments particularly on the 10th and 15th day after treatment.

Soil metatranscriptomics for mining eukaryotic heavy metal resistance genes

Heavy metals are pollutants which affect all organisms. Since a small number of eukaryotes have been investigated with respect to metal resistance, we hypothesize that many genes that control this phenomenon remain to be identified. This was tested by screening soil eukaryotic metatranscriptomes which encompass RNA from organisms belonging to the main eukaryotic phyla. Soil-extracted polyadenylated mRNAs were converted into cDNAs and 35 of them were selected for their ability to rescue the metal (Cd or Zn) sensitive phenotype of yeast mutants. Few of the genes belonged to families known to confer metal resistance when overexpressed in yeast. Several of them were homologous to genes that had not been studied in the context of metal resistance. For instance, the BOLA ones, which conferred cross metal (Zn, Co, Cd, Mn) resistance may act by interfering with Fe homeostasis. Other genes, such as those encoding 110- to 130-amino-acid-long, cysteine-rich polypeptides, had no homologues in databases. This study confirms that functional metatranscriptomics represents a powerful approach to address basic biological processes in eukaryotes. The selected genes can be used to probe new pathways involved in metal homeostasis and to manipulate the resistance level of selected organisms.

Orthologs of the class A4 heat shock transcription factor HsfA4a confer cadmium tolerance in wheat and rice

Cadmium (Cd) is a widespread soil pollutant; thus, the underlying molecular controls of plant Cd tolerance are of substantial interest. A screen for wheat (Triticum aestivum) genes that confer Cd tolerance to a Cd hypersensitive yeast strain identified Heat shock transcription factor A4a (HsfA4a). Ta HsfA4a is most similar to the class A4 Hsfs from monocots. The most closely related rice (Oryza sativa) homolog, Os HsfA4a, conferred Cd tolerance in yeast, as did Ta HsfA4a, but the second most closely related rice homolog, Os HsfA4d, did not. Cd tolerance was enhanced in rice plants expressing Ta HsfA4a and decreased in rice plants with knocked-down expression of Os HsfA4a. An analysis of the functional domain using chimeric proteins constructed from Ta HsfA4a and Os HsfA4d revealed that the DNA binding domain (DBD) of HsfA4a is critical for Cd tolerance, and within the DBD, Ala-31 and Leu-42 are important for Cd tolerance. Moreover, Ta HsfA4a-mediated Cd resistance in yeast requires metallothionein (MT). In the roots of wheat and rice, Cd stress caused increases in HsfA4a expression, together the MT genes. Our findings thus suggest that HsfA4a of wheat and rice confers Cd tolerance by upregulating MT gene expression in planta.