OsZIP1 functions as a metal efflux transporter limiting excess zinc, copper and cadmium accumulation in rice

Metal tolerance protein CsMTP4 has dual functions in maintaining zinc homeostasis in tea plant

Plants have evolved a series of zinc (Zn) homeostasis mechanisms to cope with the fluctuating Zn in the environment. How Zn is taken up, translocated and tolerate by tea plant remains unknown. In this study, on the basis of RNA-Sequencing, we isolated a plasma membrane-localized Metal Tolerance Protein (MTP) family member CsMTP4 from Zn-deficient tea plant roots and investigated its role in regulation of Zn homeostasis in tea plant. Heterologous expression of CsMTP4 specifically enhanced the tolerance of transgenic yeast to Zn excess. Moreover, overexpression of CsMTP4 in tea plant hairy roots stimulated Zn uptake under Zn deficiency. In addition, CsMTP4 promoted the growth of transgenic Arabidopsis plants by translocating Zn from roots to shoots under Zn deficiency and conferred the tolerance to Zn excess by enhancing the efflux of Zn from root cells. Transcriptome analysis of the CsMTP4 transgenic Arabidopsis found that the expression of Zn metabolism-related genes were differentially regulated compared with wild-type plants when exposed to Zn deficiency and excess conditions. This study provides a mechanistic understanding of Zn uptake and translocation in plants and a new strategy to improve phytoremediation efficiency.

The OsZIP2 transporter is involved in root-to-shoot translocation and intervascular transfer of cadmium in rice

Cadmium (Cd) is a toxic metal that poses serious threats to human health. Rice is a major source of dietary Cd but how rice plants transport Cd to the grain is not fully understood. Here, we characterize the function of the ZIP (ZRT, IRT-like protein) family protein, OsZIP2, in the root-to-shoot translocation of Cd and intervascular transfer of Cd in nodes. OsZIP2 is localized at the plasma membrane and exhibited Cd2+ transport activity when heterologously expressed in yeast. OsZIP2 is strongly expressed in xylem parenchyma cells in roots and in enlarged vascular bundles in nodes. Knockout of OsZIP2 significantly enhanced root-to-shoot translocation of Cd and alleviated the inhibition of root elongation by excess Cd stress; whereas overexpression of OsZIP2 decreased Cd translocation to shoots and resulted in Cd sensitivity. Knockout of OsZIP2 increased Cd allocation to the flag leaf but decreased Cd allocation to the panicle and grain. We further reveal that the variation of OsZIP2 expression level contributes to grain Cd concentration among rice germplasms. Our results demonstrate that OsZIP2 functions in root-to-shoot translocation of Cd in roots and intervascular transfer of Cd in nodes, which can be used for breeding low Cd rice varieties.

Transporters and phytohormones analysis reveals differential regulation of ryegrass (Lolium perenne L.) in response to cadmium and arsenic stresses

Cadmium (Cd) and arsenic (As) are two highly toxic heavy metals and metalloids that coexist in many situations posing severe threats to plants. Our investigation was conducted to explore the different regulatory mechanisms of ryegrass (Lolium perenne L.) responding to individual and combined Cd and As stresses in hydroponics. Results showed that the ryegrass well-growth phenotype was not affected by Cd stress of 10 mg·L-1. However, As of 10 mg·L-1 caused rapid water loss, proline surge, and chlorosis in shoots, suggesting that ryegrass was highly sensitive to As. Transcriptomic analysis revealed that the transcription factor LpIRO2 mediated the upregulation of ZIP1 and YSL6 that played an important role in Cd tolerance. We found that the presence of As caused the overexpression of LpSWT12, a process potentially regulated by bHLH14, to mitigate hyperosmolarity. Indoleacetic acid (IAA) and abscisic acid (ABA) contents and expression of their signaling-related genes were significantly affected by As stress rather than Cd. We predict a regulatory network to illustrate the interaction between transporters, transcription factors, and signaling transduction, and explain the antagonism of Cd and As toxicity. This present work provides a research basis for plant protection from Cd and As pollution.

Regulation of metal homoeostasis by two F-group bZIP transcription factors bZIP48 and bZIP50 in rice

Zinc (Zn) deficiency not only impairs plant growth and development but also has negative effects on human health. Rice (Oryza Sativa L.) is a staple food for over half of the global population, yet the regulation of Zn deficiency response in rice remains largely unknown. In this study, we provide evidence that two F-group bZIP transcription factors, OsbZIP48/50, play a crucial role in Zn deficiency response. Mutations in OsbZIP48/50 result in impaired growth and reduced Zn/Fe/Cu content under Zn deficiency conditions. The N-terminus of OsbZIP48/OsbZIP50 contains two Zn sensor motifs (ZSMs), deletion or mutation of these ZSMs leads to increased nuclear localization. Both OsbZIP48 and OsbZIP50 exhibit transcriptional activation activity, and the upregulation of 1117 genes involved in metal uptake and other processes by Zn deficiency is diminished in the OsbZIP48/50 double mutant. Both OsbZIP48 and OsbZIP50 bind to the promoter of OsZIP10 and activate the ZDRE cis-element. Amino acid substitution mutation of the ZSM domain of OsbZIP48 in OsbZIP50 mutant background increases the content of Zn/Fe/Cu in brown rice seeds and leaves. Therefore, this study demonstrates that OsbZIP48/50 play a crucial role in regulating metal homoeostasis and identifies their downstream genes involved in the Zn deficiency response in rice.

Glomus versiforme and intercropping with Sphagneticola calendulacea decrease Cd accumulation in maize

Little information is available on the influence of the compound use of intercropping (IN) and arbuscular mycorrhizal fungus (AMF) on Cd accumulation and the expression of Cd transporter genes in two intercropped plants. A pot experiment was conducted to study the influences of IN and AMF-Glomus versiforme on growth and Cd uptake of two intercropped plants-maize and Cd hyperaccumulator Sphagneticola calendulacea, and the expression of Cd transporter genes in maize in Cd-polluted soils. IN, AMF and combined treatments of IN and AMF (IN + AMF) obviously improved biomass, photosynthesis and total antioxidant capacities of two plants. Moreover, single and compound treatments of IN and AMF evidently reduced Cd contents in maize, and the greatest decreases appeared in the compound treatment. However, Cd contents of S. calendulacea in IN, AMF and IN + AMF groups were notably improved. Furthermore, the single and compound treatments of IN and AMF significantly downregulated the expression levels of Nramp1, HMA1, ABCC1 and ABCC10 in roots and leaves, and the largest decreases were observed in the combined treatment. Our work first revealed that the combined use of IN and AMF appeared to have a synergistic effect on decreasing Cd content by downregulating the expression of Cd transporter genes in maize.

Co-foliar application of zinc and nano-silicon to rice helps in reducing cadmium exposure risk: Investigations through in-vitro digestion with human cell line bioavailability assay

Foliar application of zinc (Zn) or silicon nanoparticles (Si-NPs) may exert regulatory effects on cadmium (Cd) accumulation in rice grains, however, their impact on Cd bioavailability during human rice consumption remains elusive. This study comprehensively investigated the application of Zn with or without Si-NPs in reducing Cd accumulation in rice grains as well to exactly evaluate the potential risk of Cd exposure resulting from the rice consumption by employing field experiment as well laboratory bioaccessibility and bioavailability assay. Sole Zn (ZnSO4) or in combination with Si (ZnSO4 +Si and ZnO+Si) efficiently lowered the Cd concentration in rice grains. However, the impact of bioaccessible (0.1215-0.1623 mg kg-1) and bioavailable Cd (0.0245-0.0393 mg kg-1) during simulated human rice consumption depicted inconsistent trend. The straw HCl-extractable fraction of Cd (FHCl-Cd) exhibited a significant correlation with total, bioaccessible, and bioavailable Cd in grains, indicating the critical role of FHCl-Cd in Cd accumulation and translocation from grains to human. Additionally, foliar spraying of Zn+Si raised the nutritional value of rice grains, leading to increased protein content and reduced phytic acid concentration. Overall, this study demonstrates the potential of foliar application of ZnSO4 +Si in mitigating the Cd levels in rice grains and associated health risks upon consumption.

Decrypting Molecular Mechanisms Involved in Counteracting Copper and Nickel Toxicity in Jack Pine (Pinus banksiana) Based on Transcriptomic Analysis

The remediation of copper and nickel-afflicted sites is challenged by the different physiological effects imposed by each metal on a given plant system. Pinus banksiana is resilient against copper and nickel, providing an opportunity to build a valuable resource to investigate the responding gene expression toward each metal. The objectives of this study were to (1) extend the analysis of the Pinus banksiana transcriptome exposed to nickel and copper, (2) assess the differential gene expression in nickel-resistant compared to copper-resistant genotypes, and (3) identify mechanisms specific to each metal. The Illumina platform was used to sequence RNA that was extracted from seedlings treated with each of the metals. There were 449 differentially expressed genes (DEGs) between copper-resistant genotypes (RGs) and nickel-resistant genotypes (RGs) at a high stringency cut-off, indicating a distinct pattern of gene expression toward each metal. For biological processes, 19.8% of DEGs were associated with the DNA metabolic process, followed by the response to stress (13.15%) and the response to chemicals (8.59%). For metabolic function, 27.9% of DEGs were associated with nuclease activity, followed by nucleotide binding (27.64%) and kinase activity (10.16%). Overall, 21.49% of DEGs were localized to the plasma membrane, followed by the cytosol (16.26%) and chloroplast (12.43%). Annotation of the top upregulated genes in copper RG compared to nickel RG identified genes and mechanisms that were specific to copper and not to nickel. NtPDR, AtHIPP10, and YSL1 were identified as genes associated with copper resistance. Various genes related to cell wall metabolism were identified, and they included genes encoding for HCT, CslE6, MPG, and polygalacturonase. Annotation of the top downregulated genes in copper RG compared to nickel RG revealed genes and mechanisms that were specific to nickel and not copper. Various regulatory and signaling-related genes associated with the stress response were identified. They included UGT, TIFY, ACC, dirigent protein, peroxidase, and glyoxyalase I. Additional research is needed to determine the specific functions of signaling and stress response mechanisms in nickel-resistant plants.

A long non-coding RNA associated with H3K7me3 methylation negatively regulates OsZIP16 transcription under cadmium stress

Cadmium (Cd) is a toxic environmental pollutant, posing a high risk to crop production and human health. However, the genetic mechanisms for regulation of Cd accumulation in crops are poorly understood. In this study, we functionally identified a novel long non-coding RNA (lncRNA, TCONS_00502780) that repressed a locus encoding an uncharacterized metal transporter ZIP16 (ZRT/IRT-like Protein) in rice. LncRNA-OsZIP16 (L16) is resident in the antisense strand of OsZIP16. Both L16 and OsZIP16 were transcriptionally expressed during the life cycle, but under Cd stress the L16 transcription was repressed, whereas the OsZIP16 expression was upregulated. OsZIP16 is localized to the endoplasmic reticulum. Knocking out OsZIP16 by CRISPR-Cas9 (C16) resulted in Cd sensitivity, manifested by reduced plant growth and intense cellular damage with a slightly higher Cd translocation from roots to shoots, suggesting that OsZIP16 expression is required for rice growth and development under Cd stress. Conversely, OsZIP16 constitutive overexpression (OE16) lines displayed a growth phenotype compatible to the wide-type with lower Cd translocation ratio from roots to shoots. L16 knock-down lines by RNA interference (L16-R) showed a similar phenotype to the OE16 lines, while the L16 overexpression (L16-OE) lines were phenotypically similar to the C16 lines. The OsZIP16 transcription was upregulated in the L16-R lines but downregulated in the L16-OE lines. Using an antibody against the trimethylation of histone H3 lysine 27 (H3K27me3) followed by chromatin immunoprecipitation (ChIP), we found the reduced H3K27me3 methylation marks surrounding the OsZIP16 gene under Cd stress. Further examination of H3K27me3 in the L16-R lines revealed that the methylation levels were also significantly lower than those in WT. Taken together, these data suggest that the L16 could negatively regulate the OsZIP16 transcriptional expression through an epigenetic mechanism for rice adaption to Cd stress.

Epigenetic regulation influenced by soil microbiota and nutrients: Paving road to epigenome editing in plants

Soil is a complex ecosystem that houses microbes and nutrients that are necessary for plant development. Edaphic properties of the soil and environmental conditions influence microbial growth and nutrient accessibility. Various environmental stimuli largely affect the soil microbes and ionic balance, in turn influencing plants. Soil microflora helps decompose organic matter and is involved in mineral uptake. The combination of soil microbes and mineral nutrients notably affects plant growth. Recent advancements have enabled a deeper understanding of plant genetic/molecular regulators. Deficiencies/sufficiencies of soil minerals and microbes also alter plant gene regulation. Gene regulation mediated by epigenetic mechanisms comprises conformational alterations in chromatin structure, DNA/histone modifications, or involvement of small RNAs. Epigenetic regulation is unique due to its potential to inherit without involving alteration of the DNA sequence. Thus, the compilation study of heritable epigenetic changes driven by nutrient imbalances and soil microbes would facilitate understanding this molecular phenomenon in plants. This information can aid in epigenome editing, which has recently emerged as a promising technology for plant non-transgenic/non-mutagenic modification. Potential epigenetic marks induced by biotic and abiotic stresses in plants could be explored as target sites for epigenome editing. This review discusses novel ways of epigenome editing to create epigenome edited plants with desirable and heritable phenotypes. As plants are sessile and in constant exposure to the soil microbiome and nutrients, epigenetic changes induced by these factors could provide more effective, stable and a sustainable molecular solution for crop improvement.

Mechanisms of low cadmium accumulation in crops: A comprehensive overview from rhizosphere soil to edible parts

Cadmium (Cd) is a toxic heavy metal often found in soil and agricultural products. Due to its high mobility, Cd poses a significant health risk when absorbed by crops, a crucial component of the human diet. This absorption primarily occurs through roots and leaves, leading to Cd accumulation in edible parts of the plant. Our research aimed to understand the mechanisms behind the reduced Cd accumulation in certain crop cultivars through an extensive review of the literature. Crops employ various strategies to limit Cd influx from the soil, including rhizosphere microbial fixation and altering root cell metabolism. Additional mechanisms include membrane efflux, specific transport, chelation, and detoxification, facilitated by metalloproteins such as the natural resistance-associated macrophage protein (Nramp) family, heavy metal P-type ATPases (HMA), zinc-iron permease (ZIP), and ATP-binding cassette (ABC) transporters. This paper synthesizes differences in Cd accumulation among plant varieties, presents methods for identifying cultivars with low Cd accumulation, and explores the unique molecular biology of Cd accumulation. Overall, this review provides a comprehensive resource for managing agricultural lands with lower contamination levels and supports the development of crops engineered to accumulate minimal amounts of Cd.

More effective than direct contact: Nano hydroxyapatite pre-treatment regulates the growth and Cd uptake of rice (Oryza sativa L.) seedlings

Cd contamination in rice urgently needs to be addressed. Nano hydroxyapatite (n-HAP) is an eco-friendly material with excellent Cd fixation ability. However, due to its own high reactivity, innovative application of n-HAP in the treatment of Cd contamination in rice is needed. In this study, we proposed a new application, namely n-HAP pre-treatment, which can effectively reduce Cd accumulation in rice and alleviate Cd stress. The results showed that 80 mg/L n-HAP pre-treatment significantly reduced Cd content in rice shoot by 35.1%. Biochemical and combined transcriptomic-proteomic analysis revealed the possible molecular mechanisms by which n-HAP pre-treatment promoted rice growth and reduced Cd accumulation. (1) n-HAP pre-treatment regulated gibberellin and jasmonic acid synthesis-related pathways, increased gibberellin content and decreased jasmonic acid content in rice root, which promoted rice growth; (2) n-HAP pre-treatment up-regulated gene CATA1 expression and down-regulated gene OsGpx1 expression, which increased rice CAT activity and GSH content; (3) n-HAP pre-treatment up-regulated gene OsZIP1 expression and down-regulated gene OsNramp1 expression, which reduced Cd uptake, increased Cd efflux from rice root cells.

OsPDR20 is an ABCG metal transporter regulating cadmium accumulation in rice

Cadmium (Cd) is a non-essential toxic heavy metal, seriously posing high environmental risks to human health. Digging genetic resources relevant to functional genes is important for understanding the metal absorption and accumulation in crops and bioremediation of Cd-polluted environments. This study investigated a functionally uncharacterized ATP binding cassette transporter G family (ABCG) gene encoding a Pleiotropic Drug Resistance 20 (PDR20) type metal transporter which is localized to the plasma membrane of rice. OsPDR20 was transcriptionally expressed in almost all tissues and organs in lifespan and was strongly induced in roots and shoots of young rice under Cd stress. Ectopic expression of OsPDR20 in a yeast mutant ycf1 sensitive to Cd conferred cellular tolerance with less Cd accumulation. Knockdown of OsPDR20 by RNA interference (RNAi) moderately attenuated root/shoot elongation and biomass, with reduced chlorophylls in rice grown under hydroponic medium with 2 and 10 µmol/L Cd, but led to more Cd accumulation. A field trial of rice grown in a realistic Cd-contaminated soil (0.40 mg/kg) showed that RNAi plants growth and development were also compromised compared to wild-type (WT), with smaller panicles and lower spikelet fertility but little effect on yield of grains. However, OsPDR20 suppression resulted in unexpectedly higher levels of Cd accumulation in rice straw including lower leaves and culm and grain. These results suggest that OsPDR20 is actively involved in Cd accumulation and homeostasis in rice crops. The increased Cd accumulation in the RNAi plants has the potential application in phytoremediation of Cd-polluted wetland soils.

A transposable element-derived siRNAs involve DNA hypermethylation at the promoter of OsGSTZ4 for cadmium tolerance in rice

Environmental contaminants such as cadmium (Cd) pose high risks to crop production and human health. The genetic basis for regulation of Cd stress-responsive genes for plant adaptation to adverse environments remains poorly understood. In this study, we characterized a rice Zeta family glutathione-S-transferase (OsGSTZ4) gene for Cd detoxification. Heterologous expression of OsGSTZ4 in a yeast (Saccharomyces cerevisiae) conferred cellular Cd tolerance. Transgenic rice overexpressing OsGSTZ4 improved plant growth, attenuated Cd-induced toxicity, and accumulated more Cd in roots. OsGSTZ4 transcription was rapidly induced 3 h after Cd exposure and then declined to the basal level. This was followed by (days after Cd treatment) by CHH hypermethylation (by 41.2 %) at a MITE (Miniature Inverted-repeat Transposable Element) transposable element (TE) inserted in the 5'-untranscribed region (UTR) (-1,722 ∼ -1,392 bp) of OsGSTZ4. Meanwhile, three 24-nt siRNAs derived from the TE (-1,722 ∼ -1,471 bp) were detected and was also rapidly enriched under Cd stress. To validate the possibility that Cd-induced change in OsGSTZ4 expression correlates with the siRNAs-involved CHH methylation through an RdDM (RNA-directed DNA methylation) pathway, genetic analyses were performed. We found that the CHH methylation at the promoter and transcript level of OsGSTZ4 were compromised in the osdrm2 (loss of function for CHH methylation) and osrdr2i (defective in RNA-dependent RNA polymerase 2) but did not change in other types of methyltransferases such as osmet1, ossdg714 or osros1. Promoter deletion analyses confirmed that the siRNA target sequences were essential for the proper expression of OsGSTZ4. Our studies reveal an unusual feedback mechanism by which the Cd-induced rapid OsGSTZ4 expression for Cd tolerance would interplay with the late CHH hypermethylation to silence the TE through the 24-nt siRNAs- and Osdrm2-mediated RdDM pathway, and help understand the diversity of gene regulation via an epigenetic mechanism for rice adaptation to the environmental stress.

Multi-omics analysis reveals differential molecular responses to cadmium toxicity in rice root tip and mature zone

Cadmium (Cd) is a highly toxic heavy metal that can be readily absorbed by plants and enriched in human body. Rice (Oryza sativa L.) yield and grain quality are affected by excessive Cd in the soil. Therefore, understanding the mechanisms of Cd absorption, accumulation and detoxification in the root apex is crucial for developing low-Cd rice cultivars. After Cd treatment, Cd concentration in rice root tips (RT) was 1.4 times higher than that in basal roots (BR). To uncover the distinct molecular responses to Cd toxicity, we conducted transcriptomic, proteomic, and metabolomic analyses on the two root sections. The results revealed that the RT exhibited 1.2-2.0 fold higher transcript or protein abundance of several Cd-related transporters than the BR, including Nramp1, Nramp5, IRT1, and HMA3, thereby contributing to more Cd accumulation in the RT. Furthermore, multi-omics analysis unveiled that the RT had enhanced activity in 'phenylpropanoid metabolism', 'AsA-GSH cycle' and 'tryptophan metabolism', conferring the stronger antioxidant system. While the BR showed higher activation in 'cell wall remodeling' and 'terpenoid biosynthesis'. This comprehensive study provides insights into the regulatory network of genes, proteins and metabolites involved in the differential responses to Cd toxicity between rice root tips and mature zones.

Cadmium toxicity: its' uptake and retaliation by plant defence system and ja signaling

Cadmium (Cd+2) renders multifarious environmental stresses and highly toxic to nearly all living organisms including plants. Cd causes toxicity by unnecessary augmentation of ROS that targets essential molecules and fundamental processes in plants. In response, plants outfitted a repertory of mechanisms to offset Cd toxicity. The main elements of these are Cd chelation, sequestration into vacuoles, and adjustment of Cd uptake by transporters and escalation of antioxidative mechanism. Signal molecules like phytohormones and reactive oxygen species (ROS) activate the MAPK cascade, the activation of the antioxidant system andsynergistic crosstalk between different signal molecules in order to regulate plant responses to Cd toxicity. Transcription factors like WRKY, MYB, bHLH, bZIP, ERF, NAC etc., located downstream of MAPK, and are key factors in regulating Cd toxicity responses in plants. Apart from this, MAPK and Ca2+signaling also have a salient involvement in rectifying Cd stress in plants. This review highlighted the mechanism of Cd uptake, translocation, detoxification and the key role of defense system, MAPKs, Ca2+ signals and jasmonic acid in retaliating Cd toxicity via synchronous management of various other regulators and signaling components involved under stress condition.

Growth and transcriptional response of wheat and rice to the tertiary amine BMVE

Introduction

Seed vigor is largely a product of sound seed development, maturation processes, genetics, and storage conditions. It is a crucial factor impacting plant growth and crop yield and is negatively affected by unfavorable environmental conditions, which can include drought and heat as well as cold wet conditions. The latter leads to slow germination and increased seedling susceptibility to pathogens. Prior research has shown that a class of plant growth regulators called substituted tertiary amines (STAs) can enhance seed germination, seedling growth, and crop productivity. However, inconsistent benefits have limited STA adoption on a commercial scale.

Methods

We developed a novel seed treatment protocol to evaluate the efficacy of 2-(N-methyl benzyl aminoethyl)-3-methyl butanoate (BMVE), which has shown promise as a crop seed treatment in field trials. Transcriptomic analysis of rice seedlings 24 h after BMVE treatment was done to identify the molecular basis for the improved seedling growth. The impact of BMVE on seed development was also evaluated by spraying rice panicles shortly after flower fertilization and subsequently monitoring the impact on seed traits.

Results

BMVE treatment of seeds 24 h after imbibition consistently improved wheat and rice seedling shoot and root growth in lab conditions. Treated wheat seedlings grown to maturity in a greenhouse also resulted in higher biomass than controls, though only under drought conditions. Treated seedlings had increased levels of transcripts involved in reactive oxygen species scavenging and auxin and gibberellic acid signaling. Conversely, several genes associated with increased reactive oxygen species/ROS load, abiotic stress responses, and germination hindering processes were reduced. BMVE spray increased both fresh and mature seed weights relative to the control for plants exposed to 96 h of heat stress. BMVE treatment during seed development also benefited germination and seedling growth in the next generation, under both ambient and heat stress conditions.

Discussion

The optimized experimental conditions we developed provide convincing evidence that BMVE does indeed have efficacy in plant growth enhancement. The results advance our understanding of how STAs work at the molecular level and provide insights for their practical application to improve crop growth.

Cell number regulator 8 from Salix linearistipularis enhances cadmium tolerance in poplar by reducing cadmium uptake and accumulation

Trace metals have relatively high density and high toxicity at low concentrations. Willow (Salix genus) is an excellent phytoremediation species for soil contaminated by trace metal ions. This study identified a cell number regulator (CNR) gene family member in Salix linearistipularis exhibiting strong metal ion resistance: SlCNR8. SlCNR8 expression was affected by various metal ions, including cadmium (Cd), zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn). SlCNR8 overexpression enhanced Cd, Zn, Cu, and Fe resistance in transgenic poplar seedlings (84K) compared with the wild-type (WT). Moreover, transgenic poplar seedlings showed lower root Cd uptake and less Cd accumulation than WT under Cd stress. SlCNR8 was primarily localized to the nucleus and the plasma membrane-like cell periphery. Furthermore, SlCNR8 had transcriptional activation activity in yeast. The transcript levels of multiple metal ion transporters were altered in the roots of transgenic poplar seedlings compared to WT roots under Cd stress. These results suggest that SlCNR8 may enhance Cd resistance in transgenic poplar by reducing Cd uptake and accumulation. This may be related to altered transcription levels of other transporters or to itself. Our study suggests that SlCNR8 can be used as a candidate gene for genetic improvement of phytostabilisation of trace metals by genetic engineering.

Populus euphratica plant cadmium tolerance PePCR3 improves cadmium tolerance

Contamination of soils with toxic heavy metals is a major environmental problem. Growing crop plants that can promote the efflux of heavy metals is an effective strategy in contaminated soils. The plant cadmium resistance (PCR) protein is involved in the translocation of heavy metals, specifically zinc and cadmium (Cd). In this study, yeast expressing Populus euphratica PCR3 (PePCR3) showed enhanced Cd tolerance and decreased Cd accumulation under Cd treatment. Real-time quantitative PCR analyses revealed up-regulation of PePCR3 in poplar seedlings under Cd stress. Localization analysis revealed that PePCR3 localizes at the plasma membrane. The plant growth and biomass were greater in PePCR3-overexpressing (OE) transgenic hybrid poplar lines than in wild type (WT). Physiological parameters analyses indicated that, compared with WT, PePCR3-OE transgenic lines were more tolerant to Cd. In addition, more Cd was excreted in the roots of the PePCR3-OE transgenic lines than in those of WT, but the remaining Cd in transgenic lines was more translocated into the stems and leaves. Eight genes encoding transporters showed increased transcript levels in PePCR3-OE transgenic lines under Cd treatment, implying that PePCR3 interacts with other transporters to translocate Cd. Thus, PePCR3 may be an important genetic resource for generating new lines that can enhance Cd translocation to phytoremediation in contaminated soils.

Combined application of biochar and sulfur alleviates cadmium toxicity in rice by affecting root gene expression and iron plaque accumulation

Biochar and sulfur are considered useful amendments for soil cadmium (Cd) contamination remediation. However, there is still a gap in the understanding of how combined biochar and sulfur application affects Cd resistance in rice, and the role of the accumulation of iron plaque and the expression of Cd efflux transporter-related genes are still unclear in this type of treatment. In this study, we screened an effective combination of biochar and sulfur (0.75 % biochar, 60 mg/kg sulfur) that significantly reduced the Cd content of rice roots (32.9 %) and shoots (12.3 %); significantly reduced the accumulation of amino acids and their derivatives, organic acids and their derivatives and flavonoids in rice roots; and altered secondary metabolite production and release. This combined biochar and sulfur application alleviated the toxicity of Cd to rice, in which the enhancement of iron plaque (24.8 %) formation and upregulated expression of heavy metal effector genes (NRAMP3, MTP3, ZIP1) were important factors. These findings show that iron plaque and heavy metal transport genes are involved in the detoxification of rice under the combined application of biochar and sulfur, which provides useful information for the combined treatment of soil Cd pollution.

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

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

Function identification and characterization of Oryza sativa ZRT and IRT-like proteins computationally for nutrition and biofortification in rice

Zinc plays a very critical role and function in all organisms. Its deficiency can cause a serious issue. In Oryza sativa, the ZRT/IRT transporter-like proteins play a role in the zinc metal uptake and transport. Few OsZIPs genes have been validated and characterized for their biological functions and most of OsZIPs are not well physiologically, biochemically and phenotypically characterized. In the current study, they analyzed for their function through subcellular localization, phylogenetic analysis, homology modeling, expression analysis, protein-protein interaction (PPI) network prediction, and prediction of their binding sites. Hierarchical clustering of OsZIP genes based on different anatomical parts and developmental stages also orthologs prediction was identified. The presence of SNPs, SSRs, ESTs, FSTs, MPSS, and SAGE tags were analyzed for useful development of markers. SNPs were identified in all OsZIPs genes and each gene was further classified based on their number and position in the 3'UTR and 5'UTR regions of the gene-specific sequences. Binding clusters and their location on the protein sequences were predicted. We found Changing in residues number and position which were due to partial overlapping and sequence alignment, but they share the same mechanism of binding and transporting Zinc. A wide range of CRISPR Cas9 gRNAs was designed based on single nucleotide polymorphism (SNP) for each OsZIP transporter gene for well-function identification and characterization with genome-wide association studies. Hence this study would provide useful information, understanding, and predicting molecular insights for the future studies that will help for improvement of nutritional quality of rice varieties.Communicated by Ramaswamy H. Sarma.

Cadmium induced a non-coding RNA microRNA535 mediates Cd accumulation in rice

Identifying key regulators related to cadmium (Cd) tolerance and accumulation is the main factor for genetic engineering to improve plants for bioremediation and ensure crop food safety. MicroRNAs (miRNAs), as fine-tuning regulators of genes, participate in various abiotic stress processes. MiR535 is an ancient conserved non-coding small RNA in land plants, positively responding to Cd stress. We investigated the effects of knocking out (mir535) and overexpressing miR535 (mir535 and OE535) under Cd stress in rice plants in this study. The mir535 plants showed better Cd tolerance than wild type (WT), whereas the OE535 showed the opposite effect. Cd accumulated approximately 71.9% and 127% in the roots of mir535 and OE535 plants, respectively, compared to WT, after exposure to 2 µmol/L Cd. In brown rice, the total Cd accumulation of OE535 and mir535 was about 78% greater and 35% lower than WT. When growing in 2 mg/kg Cd of soil, the Cd concentration was significantly lower in mir535 and higher in OE535 than in the WT; afterward, we further revealed the most possible target gene SQUAMOSA promoter binding-like transcription factor 7(SPL7) and it negatively regulates Nramp5 expression, which in turn regulates Cd metabolism. Therefore, the CRISPR/Cas9 technology may be a valuable strategy for creating new rice varieties to ensure food safety.

Inhibition Roles of Calcium in Cadmium Uptake and Translocation in Rice: A Review

Cadmium (Cd) contamination in rice grains is posing a significant threat to global food security. To restrict the transport of Cd in the soil-rice system, an efficient way is to use the ionomics strategy. Since calcium (Ca) and Cd have similar ionic radii, their uptake and translocation may be linked in multiple aspects in rice. However, the underlying antagonistic mechanisms are still not fully understood. Therefore, we first summarized the current knowledge on the physiological and molecular footprints of Cd translocation in plants and then explored the potential antagonistic points between Ca and Cd in rice, including exchange adsorption on roots, plant cell-wall composition, co-transporter gene expression, and transpiration inhibition. This review provides suggestions for Ca/Cd interaction studies on rice and introduces ionomics research as a means of better controlling the accumulation of Cd in plants.

OsHIPP17 is involved in regulating the tolerance of rice to copper stress

Introduction

Heavy metal-associated isoprenylated plant proteins (HIPPs) play vital roles in metal absorption, transport and accumulation in plants. However, so far, only several plant HIPPs have been functionally analyzed. In this study, a novel HIPP member OsHIPP17, which was involved in the tolerance to copper (Cu) was functionally characterized.

Methods

In this study, qRT-PCR, Yeast transgenic technology, Plant transgenic technology, ICP-MS and so on were used for research.

Results

OsHIPP17 protein was targeted to the nucleus. The Cu concentration reached 0.45 mg/g dry weight due to the overexpression of OsHIPP17 in yeast cells. Meanwhile, the overexpression of OsHIPP17 resulted in the compromised growth of Arabidopsis thaliana (Arabidopsis) under Cu stress. The root length of Oshipp17 mutant lines was also significantly reduced by 16.74- 24.36% under 25 mM Cu stress. The roots of Oshipp17 rice mutant showed increased Cu concentration by 7.25%-23.32%. Meanwhile, knockout of OsHIPP17 decreased the expression levels of OsATX1, OsZIP1, OsCOPT5 or OsHMA5, and increased the expression levels of OsCOPT1 or OsHMA4. Antioxidant enzyme activity was also reduced in rice due to the knockout of OsHIPP17. Moreover, the expression levels of cytokinin-related genes in plants under Cu stress were also affected by overexpression or knockout of OsHIPP17.

Discussion

These results implied that OsHIPP17 might play a role in plant Cu toxic response by affecting the expression of Cu transport genes or cytokinin-related genes. Simultaneously, our work may shed light on the underlying mechanism of how heavy metals affect the plant growth and provide a novel rice genetic source for phytoremediation of heavy metal-contaminated soil.

Deciphering the functional roles of transporter proteins in subcellular metal transportation of plants

MAIN CONCLUSION

The role of transporters in subcellular metal transport is of great significance for plants in coping with heavy metal stress and maintaining their proper growth and development. Heavy metal toxicity is a serious long-term threat to plant growth and agricultural production, becoming a global environmental concern. Excessive heavy metal accumulation not only damages the biochemical and physiological functions of plants but also causes chronic health hazard to human beings through the food chain. To deal with heavy metal stress, plants have evolved a series of elaborate mechanisms, especially a variety of spatially distributed transporters, to strictly regulate heavy metal uptake and distribution. Deciphering the subcellular role of transporter proteins in controlling metal absorption, transport and separation is of great significance for understanding how plants cope with heavy metal stress and improving their adaptability to environmental changes. Hence, we herein introduce the detrimental effects of excessive common essential and non-essential heavy metals on plant growth, and describe the structural and functional characteristics of transporter family members, with a particular emphasis on their roles in maintaining heavy metal homeostasis in various organelles. Besides, we discuss the potential of controlling transporter gene expression by transgenic approaches in response to heavy metal stress. This review will be valuable to researchers and breeders for enhancing plant tolerance to heavy metal contamination.

Regulation of Vicia faba L. Response and Its Effect on Megoura crassicauda Reproduction under Zinc Stress

The heavy metal zinc (Zn) is known to be transmitted in the food chain; however, the effect of Zn stress on beans and herbivorous insects is largely unclear. This study aimed to investigate the resistance of broad bean plants to Zn stress and the consequent changes in their physiological and biochemical metabolism by simulating heavy metal pollution in soil. Simultaneously, the effects of aphid progeny treated with different Zn concentrations on the expression of carbohydrate and related genes were analyzed. The results showed that Zn had no effect on the germination rate of broad beans, but other effects mainly manifested as follows. (1) Chlorophyll content decreased. (2) The total soluble sugar and Zn content in stems and leaves increased with increasing Zn content. (3) The proline content first increased and then decreased with increasing Zn content. (4) The height of the seedlings indicates that low concentrations promote growth and high concentrations inhibit growth. In addition, only the first-generation fecundity decreased significantly when aphids fed on heavy metal broad beans. Continuous high Zn levels increase the trehalose content of aphid F1 and F2, while F3 decreases. These results can not only provide a theoretical basis for exploring the impact of soil heavy metal pollution on ecosystems but also preliminarily evaluate the possibility of broad beans as a means of pollution remediation.

Heavy metals and arsenic stress in food crops: Elucidating antioxidative defense mechanisms in hyperaccumulators for food security, agricultural sustainability, and human health

The spread of heavy metal(loid)s at soil-food crop interfaces has become a threat to sustainable agricultural productivity, food security, and human health. The eco-toxic effects of heavy metals on food crops can be manifested through reactive oxygen species that have the potential to disturb seed germination, normal growth, photosynthesis, cellular metabolism, and homeostasis. This review provides a critical overview of stress tolerance mechanisms in food crops/hyperaccumulator plants against heavy metals and arsenic (HM-As). The HM-As antioxidative stress tolerance in food crops is associated with changes in metabolomics (physico-biochemical/lipidomics) and genomics (molecular level). Furthermore, HM-As stress tolerance can occur through plant-microbe, phytohormone, antioxidant, and signal molecule interactions. Information regarding the avoidance, tolerance, and stress resilience of HM-As should help pave the way to minimize food chain contamination, eco-toxicity, and health risks. Advanced biotechnological approaches (e.g., genome modification with CRISPR-Cas9 gene editing) in concert with traditional sustainable biological methods are useful options to develop 'pollution safe designer cultivars' with increased climate change resilience and public health risks mitigation. Further, the usage of HM-As tolerant hyperaccumulator biomass in biorefineries (e.g., environmental remediation, value added chemicals, and bioenergy) is advocated to realize the synergy between biotechnological research and socio-economic policy frameworks, which are inextricably linked with environmental sustainability. The biotechnological innovations, if directed toward 'cleaner climate smart phytotechnologies' and 'HM-As stress resilient food crops', should help open the new path to achieve sustainable development goals (SDGs) and a circular bioeconomy.

Hydrogen peroxide reduces root cadmium uptake but facilitates root-to-shoot cadmium translocation in rice through modulating cadmium transporters

Cadmium (Cd) contamination in agricultural soils has become a serious worldwide environmental problem threatening crop production and human health. Hydrogen peroxide (H₂O₂) is a critical second messenger in plant response to Cd exposure. However, its role in Cd accumulation in various organs of plants and the mechanistic basis of this regulation remains to be elucidated. In this study, we used electrophysiological and molecular approaches to understand how H₂O₂ regulates Cd uptake and translocation in rice plants. Our results showed that the pretreatment of H₂O₂ significantly reduced Cd uptake by rice roots, which was associated with the downregulation of OsNRAMP1 and OsNRAMP5. On the other hand, H₂O₂ promoted the root-to-shoot translocation of Cd, which might be attributed to the upregulation of OsHMA2 critical for Cd²⁺ phloem loading and the downregulation of OsHMA3 involved in the vacuolar compartmentalization of Cd²⁺, leading to the increased Cd accumulation in rice shoots. Furthermore, such regulatory effects of H₂O₂ on Cd uptake and translocation were notably amplified by the elevated level of exogenous calcium (Ca). Collectively, our results suggest that H₂O₂ can inhibit Cd uptake but increase root to shoot translocation through modulating the transcriptional levels of genes encoding Cd transporters, furthermore, application of Ca can amplify this effect. These findings will broaden our understanding of the regulatory mechanisms of Cd transport in rice plants and provide theoretical foundation for breeding rice for low Cd accumulation.

Physiological and molecular mechanisms of medicinal plants in response to cadmium stress: Current status and future perspective

Medicinal plants have a wide range of uses worldwide. However, the quality of medicinal plants is affected by severe cadmium pollution. Cadmium can reduce photosynthetic capacity, lead to plant growth retardation and oxidative stress, and affect secondary metabolism. Medicinal plants have complex mechanisms to cope with cadmium stress. On the one hand, an antioxidant system can effectively scavenge excess reactive oxygen species produced by cadmium stress. On the other hand, cadmium chelates are formed by chelating peptides and then sequestered through vacuolar compartmentalization. Cadmium has no specific transporter in plants and is generally transferred to plant tissues through competition for the transporters of divalent metal ions, such as zinc, iron, and manganese. In recent years, progress has been achieved in exploring the physiological mechanisms by which medicinal plants responding to cadmium stress. The exogenous regulation of cadmium accumulation in medicinal plants has been studied, and the aim is reducing the toxicity of cadmium. However, research into molecular mechanisms is still lagging. In this paper, we review the physiological and molecular mechanisms and regulatory networks of medicinal plants exposed to cadmium, providing a reference for the study on the responses of medicinal plants to cadmium stress.

Manganese and copper additions differently reduced cadmium uptake and accumulation in dwarf Polish wheat (Triticum polonicum L.)

This study investigated the effects of manganese (Mn) and copper (Cu) on dwarf Polish wheat under cadmium (Cd) stress by evaluating plant growth, Cd uptake, translocation, accumulation, subcellular distribution, and chemical forms, and the expression of genes participating in cell wall synthesis, metal chelation, and metal transport. Compared with the control, Mn deficiency and Cu deficiency increased Cd uptake and accumulation in roots, and Cd levels in root cell wall and soluble fractions, but inhibited Cd translocation to shoots. Mn addition reduced Cd uptake and accumulation in roots, and Cd level in root soluble fraction. Cu addition did not affect Cd uptake and accumulation in roots, while it caused a decrease and an increase of Cd levels in root cell wall and soluble fractions, respectively. The main Cd chemical forms (water-soluble Cd, pectates and protein integrated Cd, and undissolved Cd phosphate) in roots were differently changed. Furthermore, all treatments distinctly regulated several core genes that control the main component of root cell walls. Several Cd absorber (COPT, HIPP, NRAMP, and IRT) and exporter genes (ABCB, ABCG, ZIP, CAX, OPT, and YSL) were differently regulated to mediate Cd uptake, translocation, and accumulation. Overall, Mn and Cu differently influenced Cd uptake and accumulation; Mn addition is an effective treatment for reducing Cd accumulation in wheat.

Acetyltransferase OsACE2 acts as a regulator to reduce the environmental risk of oxyfluorfen to rice production

The constant use of the pesticide oxyfluorfen (OFF) in farmland contaminates the soil, posing threats to crop growth and human health. To avoid the contamination of food crops with OFF, it is critically important to understand its absorption and degradation mechanisms. In this study, we characterized a new functional locus encoding an acetyltransferase (OsACE2) that can facilitate OFF degradation in rice. OsACE2 was drastically induced by OFF at 0.04-0.2 mg L^-1 for 6 days and the rice growth was significantly inhibited. To demonstrate the regulatory role of OsACE2 in resistance to OFF toxicity, we generated OsACE2 overexpression (OE) and knockout mutant using genetic transformation and gene-editing technologies (CRISPR/Cas9). The OE plants grown in the hydroponic medium showed improved growth (plant elongation and biomass), increased chlorophyll content, and reduced OFF-induced oxidative stress. The OsACE2-improved growth phenotypes of rice were attributed to the significantly lower OFF accumulation in OE plants. Conversely, knocking out OsACE2 resulted in compromised growth phenotypes compared to the wild-type (WT). Using LC-LTQ-HRMS/MS, five mono-metabolites and eleven conjugates of OFF were characterized through various canonical pathways, such as hydrolysis, oxidation, reduction, glycosylation, acetylation, malonylation, and interaction with amino acids. These metabolites increased in the OE plants, and five acetylated conjugates were reported for the first time. Collectively, OsACE2 plays a primary role in catabolizing OFF residues in rice through multiple degradation pathways and reducing OFF in its growth environment.

Biofortification of Crops to Fight Anemia: Role of Vacuolar Iron Transporters

Plant-based foods provide all the crucial nutrients for human health. Among these, iron (Fe) is one of the essential micronutrients for plants and humans. A lack of Fe is a major limiting factor affecting crop quality, production, and human health. There are people who suffer from various health problems due to the low intake of Fe in their plant-based foods. Anemia has become a serious public health issue due to Fe deficiency. Enhancing Fe content in the edible part of food crops is a major thrust area for scientists worldwide. Recent progress in nutrient transporters has provided an opportunity to resolve Fe deficiency or nutritional problems in plants and humans. Understanding the structure, function, and regulation of Fe transporters is essential to address Fe deficiency in plants and to improve Fe content in staple food crops. In this review, we summarized the role of Fe transporter family members in the uptake, cellular and intercellular movement, and long-distance transport of Fe in plants. We draw insights into the role of vacuolar membrane transporters in the crop for Fe biofortification. We also provide structural and functional insights into cereal crops' vacuolar iron transporters (VITs). This review will help highlight the importance of VITs for improving the Fe biofortification of crops and alleviating Fe deficiency in humans.

Transcriptional Regulatory Network of Plant Cadmium Stress Response

Cadmium (Cd) is a non-essential heavy metal with high toxicity to plants. Plants have acquired specialized mechanisms to sense, transport, and detoxify Cd. Recent studies have identified many transporters involved in Cd uptake, transport, and detoxification. However, the complex transcriptional regulatory networks involved in Cd response remain to be elucidated. Here, we provide an overview of current knowledge regarding transcriptional regulatory networks and post-translational regulation of the transcription factors involved in Cd response. An increasing number of reports indicate that epigenetic regulation and long non-coding and small RNAs are important in Cd-induced transcriptional responses. Several kinases play important roles in Cd signaling that activate transcriptional cascades. We also discuss the perspectives to reduce grain Cd content and improve crop tolerance to Cd stress, which provides a theoretical reference for food safety and the future research of plant varieties with low Cd accumulation.

Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Plants take up a wide range of trace metals/metalloids (hereinafter referred to as trace metals) from the soil, some of which are essential but become toxic at high concentrations (e.g., Cu, Zn, Ni, Co), while others are non-essential and toxic even at relatively low concentrations (e.g., As, Cd, Cr, Pb, and Hg). Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities. Trace metal contamination can cause toxicity and growth inhibition in plants, as well as accumulation in the edible parts to levels that threatens food safety and human health. Understanding the mechanisms of trace metal toxicity and how plants respond to trace metal stress is important for improving plant growth and food safety in contaminated soils. The accumulation of excess trace metals in plants can cause oxidative stress, genotoxicity, programmed cell death, and disturbance in multiple physiological processes. Plants have evolved various strategies to detoxify trace metals through cell-wall binding, complexation, vacuolar sequestration, efflux, and translocation. Multiple signal transduction pathways and regulatory responses are involved in plants challenged with trace metal stresses. In this review, we discuss the recent progress in understanding the molecular mechanisms involved in trace metal toxicity, detoxification, and regulation, as well as strategies to enhance plant resistance to trace metal stresses and reduce toxic metal accumulation in food crops.

FeCl3 and Fe2(SO4)3 differentially reduce Cd uptake and accumulation in Polish wheat (Triticum polonicum L.) seedlings by exporting Cd from roots and limiting Cd binding in the root cell walls

Wheat grown in cadmium (Cd)-contaminated soils easily accumulates more Cd in edible parts than the Chinese safety limit (0.1 mg/kg). FeCl₃ and Fe₂(SO₄)₃ have been used to extract Cd from Cd-contaminated soils. Thus, we hypothesized that FeCl₃ and Fe₂(SO₄)₃, used as iron (Fe) fertilizers, can reduce Cd uptake and accumulation in wheat. Here, a hydroponic experiment was performed with three FeCl₃ and Fe₂(SO₄)₃ concentrations under 80 μM CdCl₂ stress on dwarf Polish wheat (Triticum polonicum L., 2n = 4x = 28, AABB) seedlings. Compared with Fe deficiency, FeCl₃ and Fe₂(SO₄)₃ additions competitively reduced Cd concentrations. The reductions were not associated with changes in dry weight and root morphological parameters. FeCl₃ and Fe₂(SO₄)₃ additions reduced Cd concentrations in the following order from smallest to largest reduction: 25 μM Fe₂(SO₄)₃ < 200 μM FeCl₃ < 50 μM FeCl₃ < 100 μM Fe₂(SO₄)₃. Investigation of subcellular distributions showed that the four Fe fertilizers differentially reduced Cd binding in the root cell walls and enhanced root sucrose and trehalose. Cd chemical form analysis revealed that Fe fertilizer addition also differentially reduced root F_E, F_W, and F_NaCl. Transcriptomic analysis revealed that addition of FeCl₃ and Fe₂(SO₄)₃ differentially up-regulated several genes that hydrolyze cell wall polysaccharides and metal transporter genes for Cd uptake (IRT1 and CAX19) and export (ZIP1, ABCG11, ABCG14, ABCG28, ABCG37, ABCG44, and ABCG48) reducing Cd uptake and accumulation. Our results demonstrated that FeCl₃ and Fe₂(SO₄)₃ can reduce Cd accumulation in wheat, and 50 μM FeCl₃ is the most effective treatment.

Genetic Regulation Mechanism of Cadmium Accumulation and Its Utilization in Rice Breeding

Cadmium (Cd) is a heavy metal whose pollution in rice fields leads to varying degrees of Cd accumulation in rice. Furthermore, the long-term consumption of Cd-contaminated rice is harmful to human health. Therefore, it is of great theoretical significance and application value to clarify the genetic regulation mechanism of Cd accumulation in rice and cultivate rice varieties with low Cd accumulation for the safe use of Cd-contaminated soils. This review summarizes the effects of Cd on rice growth, yield, and quality; the physiological and molecular mechanisms of Cd absorption in the roots, loading, and transport of Cd in the xylem, the distribution of Cd in nodes, redistribution of Cd in leaves, and accumulation of Cd in the grains; the regulation mechanism of the Cd stress response; and the breeding of rice with low Cd accumulation. Future directions on the genetic regulation of Cd in rice and application are also discussed. This review provides a theoretical basis for studies exploring the genetic regulation of Cd stress in rice. It also offers a basis for formulating effective strategies to reduce the Cd content in rice.

Elevated CO2 reduces copper accumulation and toxicity in the diatom Thalassiosira pseudonana

The projected ocean acidification (OA) associated with increasing atmospheric CO₂ alters seawater chemistry and hence the bio-toxicity of metal ions. However, it is still unclear how OA might affect the long-term resilience of globally important marine microalgae to anthropogenic metal stress. To explore the effect of increasing pCO₂ on copper metabolism in the diatom Thalassiosira pseudonana (CCMP 1335), we employed an integrated eco-physiological, analytical chemistry, and transcriptomic approach to clarify the effect of increasing pCO₂ on copper metabolism of Thalassiosira pseudonana across different temporal (short-term vs. long-term) and spatial (indoor laboratory experiments vs. outdoor mesocosms experiments) scales. We found that increasing pCO₂ (1,000 and 2,000 μatm) promoted growth and photosynthesis, but decreased copper accumulation and alleviated its bio-toxicity to T. pseudonana. Transcriptomics results indicated that T. pseudonana altered the copper detoxification strategy under OA by decreasing copper uptake and enhancing copper-thiol complexation and copper efflux. Biochemical analysis further showed that the activities of the antioxidant enzymes glutathione peroxidase (GPX), catalase (CAT), and phytochelatin synthetase (PCS) were enhanced to mitigate oxidative damage of copper stress under elevated CO₂. Our results provide a basis for a better understanding of the bioremediation capacity of marine primary producers, which may have profound effect on the security of seafood quality and marine ecosystem sustainability under further climate change.

Genome-wide analysis elucidates the roles of GhHMA genes in different abiotic stresses and fiber development in upland cotton

The heavy metal-binding domain is involved in heavy metal transporting and plays a significant role in plant detoxification. However, the functions of HMAs are less well known in cotton. In this study, a total of 143 GhHMAs (heavy metal-binding domain) were detected by genome-wide identification in G. hirsutum L. All the GhHMAs were classified into four groups via phylogenetic analysis. The exon/intron structure and protein motifs indicated that each branch of the GhHMA genes was highly conserved. 212 paralogous GhHMA gene pairs were identified, and the segmental duplications were the main role to the expansion of GhHMAs. The Ka/Ks values suggested that the GhHMA gene family has undergone purifying selection during the long-term evolutionary process. GhHMA3 and GhHMA75 were located in the plasma membrane, while GhHMA26, GhHMA117 and GhHMA121 were located in the nucleus, respectively. Transcriptomic data and qRT-PCR showed that GhHMA26 exhibited different expression patterns in each tissue and during fiber development or under different abiotic stresses. Overexpressing GhHMA26 significantly promoted the elongation of leaf trichomes and also improved the tolerance to salt stress. Therefore, GhHMA26 may positively regulate fiber elongation and abiotic stress. Yeast two-hybrid assays indicated that GhHMA26 and GhHMA75 participated in multiple biological functions. Our results suggest some genes in the GhHMAs might be associated with fiber development and the abiotic stress response, which could promote further research involving functional analysis of GhHMA genes in cotton.

Cadmium distribution and transformation in leaf cells involved in detoxification and tolerance in barley

Barley is a diagnostic plant that often used in the research of soil pollution by heavy metals, our research explored the detoxification and tolerance mechanism of cadmium(Cd) in barley through pot experiment. We investigated subcellular distribution, chemical forms and oxidative damage of Cd in barley leaves, combing with the transmission electron microscopy and Fourier-transform infrared spectroscopy(FT-IR) to further understand the translocation, transformation characteristics and toxic effect of Cd in cells. The results showed that, the bioaccumulation factors in roots and shoots of barley were ranged of 4.03-7.48 and 0.51-1.30, respectively. Barley reduces the toxic effects by storing Cd in the roots and reducing its transport to the shoots. Compared to the control treatment (0 mg/kg), the percentage of Cd in the cell wall fractions of leaves in 300 mg/kg Cd treatment increased from 34.74 % to 38.41 %; the percentage of the organelle fractions increased from 24.47 % to 56.02 %; and the percentage of soluble fraction decreased from 40.80 % to 5.57 %. We found that 69.13 % of the highly toxic inorganic Cd and water-soluble Cd were converted to less toxic pectates and protein-integrated Cd (50.20 %) and undissolved Cd phosphates (18.93 %). This conversion of Cd was mainly due to its combination with -OH, -NH, -CN, -C-O-C, and -C-O-P groups. Excessive Cd induced a significant (P < 0.05) increase in the levels of peroxidase, malondialdehyde, and cell membrane permeability, which damaged the cell membrane and allowed Cd to enter the organelles. The chloroplasts and mitochondria were destroyed, and eventually the metabolism of intracellular substances was affected, resulting in symptoms of toxicity. Our research provides cellular-scale insight into the mechanisms of Cd tolerance in barley.

Prediction of Cd Accumulation in Wheat (Triticum aestivum L.) and Simulation Calculation of Lime or Zn Fertilizer Remediated Soil

Soil Cd contamination to wheat raise wide concerns over food safety. It is essential to find the key factors affecting Cd accumulation in wheat and to establish a predictive model. The effects of pH, Zn, Ca, and DOM on the accumulation of Cd in wheat were investigated using hydroponic experiments. The results showed that Zn was the most important factor inhibiting Cd uptake in wheat. Models were developed to predict the Cd contents in wheat tissues based on the ion concentration. Meanwhile, the available Cd contents in soil were predicted using a geochemical multi-surface model (MSM) which is suitable for various soils and conditions. The combination of the hydroponic accumulation model and MSM exhibits good predictions of wheat-Cd (R² = 0.822-0.862, RMSE = 0.317-0.533). The results of this study can quantitatively predict the accumulation of Cd in wheat and provide a reference for soil remediation and safe wheat production.

Cloning and Functional Characterization of SpZIP2

Zinc (Zn)-regulated and iron (Fe)-regulated transporter-like proteins (ZIP) are key players involved in the accumulation of cadmium (Cd) and Zn in plants. Sedum plumbizincicola X.H. Guo et S.B. Zhou ex L.H. Wu (S. plumbizincicola) is a Crassulaceae Cd/Zn hyperaccumulator found in China, but the role of ZIPs in S. plumbizincicola remains largely unexplored. Here, we identified 12 members of ZIP family genes by transcriptome analysis in S. plumbizincicola and cloned the SpZIP2 gene with functional analysis. The expression of SpZIP2 in roots was higher than that in the shoots, and Cd stress significantly decreased its expression in the roots but increased its expression in leaves. Protein sequence characteristics and structural analysis showed that the content of alanine and leucine residues in the SpZIP2 sequence was higher than other residues, and several serine, threonine and tyrosine sites can be phosphorylated. Transmembrane domain analysis showed that SpZIP2 has the classic eight transmembrane regions. The evolutionary analysis found that SpZIP2 is closely related to OsZIP2, followed by AtZIP11, OsZIP1 and AtZIP2. Sequence alignment showed that most of the conserved sequences among these members were located in the transmembrane regions. A further metal sensitivity assay using yeast mutant Δyap1 showed that the expression of SpZIP2 increased the sensitivity of the transformants to Cd but failed to change the resistance to Zn. The subsequent ion content determination showed that the expression of SpZIP2 increased the accumulation of Cd in yeast. Subcellular localization showed that SpZIP2 was localized to membrane systems, including the plasma membrane and endoplasmic reticulum. The above results indicate that ZIP member SpZIP2 participates in the uptake and accumulation of Cd into cells and might contribute to Cd hyperaccumulation in S. plumbizincicola.

A Golgi-Located Transmembrane Nine Protein Gene TMN11 Functions in Manganese/Cadmium Homeostasis and Regulates Growth and Seed Development in Rice

Metal transporters play crucial roles in plant nutrition, development, and metal homeostasis. To date, several multi-proteins have been identified for metal transport across the plasma membrane and tonoplast. Nevertheless, Golgi endomembrane metal carriers and their mechanisms are less documented. In this study, we identified a new transmembrane nine (TMN) family gene, TMN11, which encodes a Mn transport protein that was localized to the cis-Golgi endomembrane in rice. OsTMN11 contains a typically conserved long luminal N-terminal domain and nine transmembrane domains. OsTMN11 was ubiquitously expressed over the lifespan of rice and strongly upregulated in young rice under excess Mn(II)/Cd(II) stress. Ectopic expression of OsTMN11 in an Mn-sensitive pmr1 mutant (PMR1 is a Golgi-resident Mn exporter) yeast (Saccharomyces cerevisiae) restored the defective phenotype and transported excess Mn out of the cells. As ScPMR1 mediates cellular Mn efflux via a vesicle-secretory pathway, the results suggest that OsTMN11 functions in a similar manner. OsTMN11 knockdown (by RNAi) compromised the growth of young rice, manifested as shorter plant height, reduced biomass, and chlorosis under excessive Mn and Cd conditions. Two lifelong field trials with rice cropped in either normal Mn supply conditions or in Cd-contaminated farmland demonstrated that knockdown of OsTMN11 impaired the capacity of seed development (including panicle, spikelet fertility, seed length, grain weight, etc.). The mature RNAi plants contained less Mn but accumulated Cd in grains and rice straw, confirming that OsTMN11 plays a fundamental role in metal homeostasis associated with rice growth and development even under normal Mn supply conditions.

Physiological and Molecular Mechanisms of Plant Responses to Copper Stress

Copper (Cu) is an essential micronutrient for humans, animals, and plants, and it participates in various morphological, physiological, and biochemical processes. Cu is a cofactor for a variety of enzymes, and it plays an important role in photosynthesis, respiration, the antioxidant system, and signal transduction. Many studies have demonstrated the adverse effects of excess Cu on crop germination, growth, photosynthesis, and antioxidant activity. This review summarizes the biological functions of Cu, the toxicity of excess Cu to plant growth and development, the roles of Cu transport proteins and chaperone proteins, and the transport process of Cu in plants, as well as the mechanisms of detoxification and tolerance of Cu in plants. Future research directions are proposed, which provide guidelines for related research.

A metal chaperone OsHIPP16 detoxifies cadmium by repressing its accumulation in rice crops

Cadmium (Cd) is an environmentally polluted toxic heavy metal and seriously risks food safety and human health through food chain. Mining genetic potentials of plants is a crucial step for limiting Cd accumulation in rice crops and improving environmental quality. This study characterized a novel locus in rice genome encoding a Cd-binding protein named OsHIPP16, which resides in the nucleus and near plasma membrane. OsHIPP16 was strongly induced by Cd stress. Histochemical analysis with pHIPP16::GUS reveals that OsHIPP16 is primarily expressed in root and leaf vascular tissues. Expression of OsHIPP16 in the yeast mutant strain ycf1 sensitive to Cd conferred cellular tolerance. Transgenic rice overexpressing OsHIPP16 (OE) improved rice growth with increased plant height, biomass, and chlorophyll content but with a lower degree of oxidative injury and Cd accumulation, whereas knocking out OsHIPP16 by CRISPR-Cas9 compromised the growth and physiological response. A lifelong trial with Cd-polluted soil shows that the OE plants accumulated much less Cd, particularly in brown rice where the Cd concentrations declined by 11.76-34.64%. Conversely, the knockout oshipp16 mutants had higher levels of Cd with the concentration in leaves being increased by 26.36-35.23% over the wild-type. These results suggest that adequate expression of OsHIPP16 would profoundly contribute to Cd detoxification by regulating Cd accumulation in rice, suggesting that both OE and oshipp16 mutant plants have great potentials for restricting Cd acquisition in the rice crop and phytoremediation of Cd-contaminated wetland soils.

OsNAC15 Regulates Tolerance to Zinc Deficiency and Cadmium by Binding to OsZIP7 and OsZIP10 in Rice

Zinc (Zn) deficiency and cadmium (Cd) stress are severe threats to the growth and development of plants. Increasing Zn content and/or decreasing Cd content in grain are also important objectives of rice breeding. However, the molecular mechanisms of Zn deficiency tolerance (ZDT) and Cd stress tolerance (CDT) are largely unknown in rice. Here, we report that a NAM/CUC2-like transcription factor, OsNAC15, contributes to ZDT and CDT in rice. Knockout of OsNAC15 reduced ZDT and CDT at the vegetative stage. OsNAC15 expresses in all tissues of different developmental stages, and is repressed by Zn deficiency and induced by Cd stress. OsNAC15 is a functional transcription factor with transactivation and DNA binding activities. Expression analysis of rice ZIP family genes suggested that the knockout of OsNAC15 activates or inhibits their transcriptions under Zn deficiency or Cd stress conditions. The yeast one-hybrid assay, transient transcriptional activity assay using the dual-luciferase reporter system and electrophoretic mobility shift assay demonstrated that OsNAC15 directly binds to the zinc deficiency-responsive element motifs in the promoters of OsZIP7 and OsZIP10 to repress their transcriptions. The OsNAC15–OsZIP7/10 module is an essential foundation for further study on the regulatory mechanisms of ZDT and CDT in rice.

Genetic Approaches for Iron and Zinc Biofortification and Arsenic Decrease in Oryza sativa L. Grains

Rice is the staple diet to half of the world's population, being a major source of carbohydrates, vitamins, and some essential elements. However, rice naturally contains low amounts of essential minerals such as iron (Fe) and zinc (Zn), which are drastically decreased after milling. Thus, populations that consume mostly rice may have micronutrient deficiency, which is associated with different diseases. On the other hand, rice irrigated by flooding has a high ability to accumulate arsenic (As) in the grain. Therefore, when rice is grown in areas with contaminated soil or irrigation water, it represents a risk factor for consumers, since As is associated with cancer and other diseases. Different strategies have been used to mitigate micronutrient deficiencies such as Fe and Zn and to prevent As from entering the food chain. Each strategy has its positive and its negative sides. The development of genetically biofortified rice plants with Fe and Zn and with low As accumulation is one of the most promising strategies, since it does not represent an additional cost for farmers, and gives benefits to consumers as well. Considering the importance of genetic improvement (traditional or molecular) to decrease the impact of micronutrient deficiencies such as Fe and Zn and contamination with As, this review aimed to summarize the major efforts, advances, and challenges for genetic biofortification of Fe and Zn and decrease in As content in rice grains.

Advances in molecular mechanisms underlying cadmium uptake and translocation in rice

The increasing cadmium (Cd) pollution in paddy fields has severely threatened China's ecological and food safety. Cultivation of low Cd accumulation varieties to reduce Cd content in rice or cultivation of Cd-tolerant varieties for phytoremediation are considered effective methods to control Cd pollution in paddy fields. However, the underlying molecular mechanism of Cd absorption and transport by rice plants needs to be deciphered to cultivate these varieties. Here, we summarized the molecular mechanisms underlying Cd absorption and transport in rice, as well as the variation of Cd accumulation among rice varieties, the QTLs related to Cd accumulation in rice, and discusses the direction of future research.

Functional Characterization of MaZIP4, a Gene Regulating Copper Stress Tolerance in Mulberry (Morus atropurpurea R.)

ZIP4 (zinc transporter 4) plays important roles in transporting Cu²⁺ ions in plants, which may contribute to the maintenance of plant metal homeostasis in growth, plant development and normal physiological metabolism. However, ZIP4 transporters have not been described in mulberry and the exact function of ZIP4 transporters in regulating the homeostasis of Cu in mulberry remains unclear. In this study, a new ZIP4 gene (MaZIP4) was isolated and cloned from Morus atropurpurea R. Phylogenetic analysis of amino sequences suggested that the amino-acid sequence of the MaZIP4 protein shows high homology with other ZIP4 proteins of Morus notabilis, Trema orientale, Ziziphus jujube and Cannabis sativa. In addition, a MaZIP4 silenced line was successfully constructed using virus-induced gene silencing (VIGS). The analysis of MaZIP4 expression by quantitative real-time PCR in mulberry showed that the level of MaZIP4 expression increased with increasing Cu concentration until the Cu concentration reached 800 ppm. Relative to the blank (WT) and the negative controls, malondialdehyde (MDA) levels increased significantly and rose with increasing Cu concentration in the MaZIP4 silenced line, whereas the soluble protein and proline content, superoxide dismutase (SOD) and peroxidase (POD) activities of these transgenic plants were lower. These results indicated that MaZIP4 may play an important role in the resistance of mulberry to Cu stress.

Advances in Genes-Encoding Transporters for Cadmium Uptake, Translocation, and Accumulation in Plants

Cadmium (Cd) is a heavy metal that is highly toxic for plants, animals, and human beings. A better understanding of the mechanisms involved in Cd accumulation in plants is beneficial for developing strategies for either the remediation of Cd-polluted soils using hyperaccumulator plants or preventing excess Cd accumulation in the edible parts of crops and vegetables. As a ubiquitous heavy metal, the transport of Cd in plant cells is suggested to be mediated by transporters for essential elements such as Ca, Zn, K, and Mn. Identification of the genes encoding Cd transporters is important for understanding the mechanisms underlying Cd uptake, translocation, and accumulation in either crop or hyperaccumulator plants. Recent studies have shown that the transporters that mediate the uptake, transport, and accumulation of Cd in plants mainly include members of the natural resistance-associated macrophage protein (Nramp), heavy metal-transporting ATPase (HMA), zinc and iron regulated transporter protein (ZIP), ATP-binding cassette (ABC), and yellow stripe-like (YSL) families. Here, we review the latest advances in the research of these Cd transporters and lay the foundation for a systematic understanding underlying the molecular mechanisms of Cd uptake, transport, and accumulation in plants.

OsZIP11 is a trans-Golgi-residing transporter required for rice iron accumulation and development

Iron (Fe) is a mineral nutrient necessary for plant growth and development. Whether the rice ZRT/IRT-like protein family metal transporter OsZIP11 is involved in Fe transport has not been functionally defined. The objective of the study is to figure out the essential role of the uncharacterized OsZIP11 played in rice growth, development, and iron accumulation, particularly in seeds. Transient subcellular location assays show that OsZIP11 was targeted to the trans-Golgi network. OsZIP11 was preferentially expressed in the rice tissues (or organs) at later flowering and seed development stages. Transcripts of OsZIP11 were significantly induced under Fe but not under zinc (Zn), copper (Cu) or manganese (Mn) deficiency. Yeast (Saccharomyces cerevisiae) transformed with OsZIP11 sequences displayed an active iron input which turned out that excessive iron accumulated in the cells. Knocking out OsZIP11 by CRISPR-Cas9 approach led to the attenuated rice growth and physiological phenotypes, depicting shorter plant height, reduced biomass, chlorosis (a symptom of lower chlorophyll concentration), and over-accumulation of malondialdehyde (complex representing the peroxidation of membrane lipids) in rice plantlets. The field trials demonstrated that OsZIP11 mutation impaired the capacity of seed development, with shortened panicle and seed length, compromised spikelet fertility, and reduced grain per plant or 1000-grain weight. Knocking out OsZIP11 also lowered the accumulation of iron in the brown rice by 48-51% compared to the wild-type. Our work pointed out that OsZIP11 is required for iron acquisition for rice growth and development.