Sedum alfredii SaNramp6 Metal Transporter Contributes to Cadmium Accumulation in Transgenic Arabidopsis thaliana

Differentiation and response mechanisms of the endophytic flora of plants ecologically restored in the ilmenite area

Heavy metal contamination in soil is a serious environmental challenge, and abandoned mining areas are of particular concern. In order to rehabilitate the ecology of these areas. In this study, we used ICP-MS and potentiometric method to analyze the soil physicochemical and then endophytic bacteria of remediation plants with the help of 16sRNA sequencing, in order to investigate the ecological remediation of abandoned ilmenite mine and the effect of soil microbiology by seven common plants. The results revealed that the abandonment of ilmenite significantly increased the contents of total phosphorus, total potassium, available potassium, iron, and lead in the surrounding soils. It also affected the richness and diversity of endophytic bacterial communities. Pvi had the highest richness, while Tsi had the lowest richness (P < 0.05). A total of 28 phyla, 69 classes, 171 orders, and 521 genera were identified. A total of nine core OTUs were found: Stenotrophomonas, Chryseobacterium, Lactobacillus, Clostridium_sensu_stricto_12, Prevotella, Lactobacillus, Bradyrhizobium, Nocardioides, and Delftia. Beta diversity analysis revealed that the community structure of the endophytic bacteria differed during the remediation process at the ilmenite site. Functional prediction revealed upregulation of Dco transporter protein function, DNA-binding transcriptional regulators, glyoxalase or related metal-dependent hydrolases, acyl coenzyme A synthetases, ATPase components, amino acid synthesis, and cellular respiration-related functions. Pearson correlation analysis revealed that the SOC, TK, AN, AK, and Zn contents were significantly correlated with α diversity. Redundancy analysis (RDA) revealed that Actinobacteriota was significantly positively correlated with soil SOD, AN, TN, and TK contents. For the first time, this study revealed the interactions among plants, endophytic bacteria and soil pollutants, laying a theoretical basis for screening specific plant endophytic bacteria for ecological restoration.

Zinc Enhances Cadmium Accumulation in Shoots of Hyperaccumulator Solanum nigrum by Improving ATP-Dependent Transport and Alleviating Toxicity

Solanum nigrum is a cadmium (Cd) and zinc (Zn) accumulator with potential for phytoextraction of soil contaminated with heavy metals. However, how Zn affects Cd accumulation in S. nigrum remains unclear. In this study, S. nigrum seedlings were treated with 100 μmol·L-1 Zn (Zn100), 100 μmol·L-1 Cd (Cd100), and the Zn and Cd combination (Zn100+Cd100) for 10 days under hydroponic culture. Compared with Cd100, the Cd content in stems, leaves, and xylem saps was 1.8, 1.6, and 1.3 times more than that in Zn100+Cd100, respectively. In addition, the production of reactive oxygen species in leaves was significantly upregulated in Cd100 compared with the control, and it was downregulated in Zn100. Comparative analyses of transcriptomes and proteomes were conducted with S. nigrum leaves. Differentially expressed genes (DEGs) were involved in Cd uptake, transport, and sequestration, and the upregulation of some transporter genes of Zn transporters (ZIPs), a natural resistance associated macrophage protein (Nramp1), a metal-nicotianamine transporter (YSL2), ATP-binding cassette transporters (ABCs), oligopeptide transporters (OPTs), and metallothionein (MTs) and glutathione S-transferase (GSTs) genes was higher in Zn100+Cd100 than in Cd100. In addition, differentially expressed proteins (DEPs) involved in electron transport chain, ATP, and chlorophyll biosynthesis, such as malate dehydrogenases (MDHs), ATPases, and chlorophyll a/b binding proteins, were mostly upregulated in Zn100. The results indicate that Zn supplement increases Cd accumulation and tolerance in S. nigrum by upregulating ATP-dependent Cd transport and sequestration pathways.

SpSIZ1 from hyperaccumulator Sedum plumbizincicola orchestrates SpABI5 to fine-tune cadmium tolerance

Sedum plumbizincicola is a renowned hyperaccumulator of cadmium (Cd), possesses significant potential for eco-friendly phytoremediation of soil contaminated with Cd. Nevertheless, comprehension of the mechanisms underpinning its Cd stress response remains constrained, primarily due to the absence of a comprehensive genome sequence and an established genetic transformation system. In this study, we successfully identified a novel protein that specifically responds to Cd stress through early comparative iTRAQ proteome and transcriptome analyses under Cd stress conditions. To further investigate its structure, we employed AlphaFold, a powerful tool for protein structure prediction, and found that this newly identified protein shares a similar structure with Arabidopsis AtSIZ1. Therefore, we named it Sedum plumbizincicola SIZ1 (SpSIZ1). Our study revealed that SpSIZ1 plays a crucial role in positively regulating Cd tolerance through its coordination with SpABI5. Overexpression of SpSIZ1 significantly enhanced plant resistance to Cd stress and reduced Cd accumulation. Expression pattern analysis revealed higher levels of SpSIZ1 expression in roots compared to stems and leaves, with up-regulation under Cd stress induction. Importantly, overexpressing SpSIZ1 resulted in lower Cd translocation factors (Tfs) but maintained relatively constant Cd levels in roots under Cd stress, leading to enhanced Cd stress resistance in plants. Protein interaction analysis revealed that SpSIZ1 interacts with SpABI5, and the expression of genes responsive to abscisic acid (ABA) through SpABI5-dependent signaling was significantly up-regulated in SpSIZ1-overexpressing plants with Cd stress treatment. Collectively, our results illustrate that SpSIZ1 interacts with SpABI5, enhancing the expression of ABA downstream stress-related genes through SpABI5, thereby increasing Cd tolerance in plants.

SpCTP3 from the hyperaccumulator Sedum plumbizincicola positively regulates cadmium tolerance by interacting with SpMDH1

Cadmium (Cd) is a heavy metal pollutant mainly originating from the discharge of industrial sewage, irrigation with contaminated water, and the use of fertilizers. The phytoremediation of Cd polluted soil depends on the identification of the associated genes in hyperaccumulators. Here, a novel Cd tolerance gene (SpCTP3) was identified in hyperaccumulator Sedum plumbizincicola. The results of Cd2+ binding and thermodynamic analyses, revealed the CXXC motif in SpCTP3 functions is a Cd2+ binding site. A mutated CXXC motif decreased binding to Cd by 59.93%. The subcellular localization analysis suggested that SpCTP3 is primarily a cytoplasmic protein. Additionally, the SpCTP3-overexpressing (OE) plants were more tolerant to Cd and accumulated more Cd than wild-type Sedum alfredii (NHE-WT). The Cd concentrations in the cytoplasm of root and leaf cells were significantly higher (53.75% and 71.87%, respectively) in SpCTP3-OE plants than in NHE-WT. Furthermore, malic acid levels increased and decreased in SpCTP3-OE and SpCTP3-RNAi plants, respectively. Moreover, SpCTP3 interacted with malate dehydrogenase 1 (MDH1). Thus, SpCTP3 helps regulate the subcellular distribution of Cd and increases Cd accumulation when it is overexpressed in plants, ultimately Cd tolerance through its interaction with SpMDH1. This study provides new insights relevant to improving the Cd uptake by Sedum plumbizincicola.

The Uptake, Transfer, and Detoxification of Cadmium in Plants and Its Exogenous Effects

Cadmium (Cd) exerts a toxic influence on numerous crucial growth and development processes in plants, notably affecting seed germination rate, transpiration rate, chlorophyll content, and biomass. While considerable advances in Cd uptake and detoxification of plants have been made, the mechanisms by which plants adapt to and tolerate Cd toxicity remain elusive. This review focuses on the relationship between Cd and plants and the prospects for phytoremediation of Cd pollution. We highlight the following issues: (1) the present state of Cd pollution and its associated hazards, encompassing the sources and distribution of Cd and the risks posed to human health; (2) the mechanisms underlying the uptake and transport of Cd, including the physiological processes associated with the uptake, translocation, and detoxification of Cd, as well as the pertinent gene families implicated in these processes; (3) the detrimental effects of Cd on plants and the mechanisms of detoxification, such as the activation of resistance genes, root chelation, vacuolar compartmentalization, the activation of antioxidant systems and the generation of non-enzymatic antioxidants; (4) the practical application of phytoremediation and the impact of incorporating exogenous substances on the Cd tolerance of plants.

CaHMA1 promotes Cd accumulation in pepper fruit

The main planting areas for pepper (Capsicum sp.) are high in cadmium (Cd), which is the most prevalent heavy metal pollutant worldwide. Breeding pepper cultivars with low Cd levels can promote sustainable agricultural production and ensure the safety of pepper products. To identify breeding targets for reducing Cd accumulation in pepper fruits, we performed a genome-wide association study on 186 accessions. Polymorphisms were associated with fruit Cd content in a genomic region containing a homolog of Arabidopsis (Arabidopsis thaliana) Heavy metal-transporting ATPase 1 (HMA1) encoding a P-type ATPase. In two cultivars with contrasting Cd accumulation, transcriptome analysis revealed differentially expressed genes enriched for carbohydrate metabolism and photosynthesis in fruits with high Cd accumulation, and a Cd2+/Zn2+-exporting ATPase gene (HMA). Heterologous expression of CaHMA1 in yeast increases Cd sensitivity. Overexpression of CaHMA1 conferred a severe increase in Cd content in Arabidopsis plants, whereas reduced CaHMA1 expression in pepper fruits decreased Cd content. We propose that CaHMA1 expression may be an important component of the high Cd accumulation in pepper plants.

MsYSL6, A Metal Transporter Gene of Alfalfa, Increases Iron Accumulation and Benefits Cadmium Resistance

Iron (Fe) is necessary for plant growth and development. The mechanism of uptake and translocation in Cadmium (Cd) is similar to iron, which shares iron transporters. Yellow stripe-like transporter (YSL) plays a pivotal role in transporting iron and other metal ions in plants. In this study, MsYSL6 and its promoter were cloned from leguminous forage alfalfa. The transient expression of MsYSL6-GFP indicated that MsYSL6 was localized to the plasma membrane and cytoplasm. The expression of MsYSL6 was induced in alfalfa by iron deficiency and Cd stress, which was further proved by GUS activity driven by the MsYSL6 promoter. To further identify the function of MsYSL6, it was heterologously overexpressed in tobacco. MsYSL6-overexpressed tobacco showed better growth and less oxidative damage than WT under Cd stress. MsYSL6 overexpression elevated Fe and Cd contents and induced a relatively high Fe translocation rate in tobacco under Cd stress. The results suggest that MsYSL6 might have a dual function in the absorption of Fe and Cd, playing a role in the competitive absorption between Fe and Cd. MsYSL6 might be a regulatory factor in plants to counter Cd stress. This study provides a novel gene for application in heavy metal enrichment or phytoremediation and new insights into plant tolerance to toxic metals.

Bacterial Communities Associated with the Roots of Typha spp. and Its Relationship in Phytoremediation Processes

Heavy metal pollution is a severe concern worldwide, owing to its harmful effects on ecosystems. Phytoremediation has been applied to remove heavy metals from water, soils, and sediments by using plants and associated microorganisms to restore contaminated sites. The Typha genus is one of the most important genera used in phytoremediation strategies because of its rapid growth rate, high biomass production, and the accumulation of heavy metals in its roots. Plant growth-promoting rhizobacteria have attracted much attention because they exert biochemical activities that improve plant growth, tolerance, and the accumulation of heavy metals in plant tissues. Because of their beneficial effects on plants, some studies have identified bacterial communities associated with the roots of Typha species growing in the presence of heavy metals. This review describes in detail the phytoremediation process and highlights the application of Typha species. Then, it describes bacterial communities associated with roots of Typha growing in natural ecosystems and wetlands contaminated with heavy metals. Data indicated that bacteria from the phylum Proteobacteria are the primary colonizers of the rhizosphere and root-endosphere of Typha species growing in contaminated and non-contaminated environments. Proteobacteria include bacteria that can grow in different environments due to their ability to use various carbon sources. Some bacterial species exert biochemical activities that contribute to plant growth and tolerance to heavy metals and enhance phytoremediation.

Significance and genetic control of membrane transporters to improve phytoremediation and biofortification processes

Humans frequently consume plant-based foods in their daily life. Contamination of agricultural soils by heavy metals (HMs) is a major food and nutritional security issue. The crop plants grown in HM-contaminated agricultural soil may accumulate more HMs in their edible part, further transferring into the food chain. Consumption of HM-rich crops can cause severe health issues in humans. On the other hand, the low content of the essential HM in the edible part of the crop also causes health problems. Therefore, researchers must try to reduce the non-essential HM in the edible part of the crop plants and improve the essential HMs. Phytoremediation and biofortification are the two strategies for resolving this problem. The genetic component helps to improve the efficiency of phytoremediation and biofortification processes in plants. They help eliminate HMs from soil and improve essential HM content in crop plants. The membrane transporter genes (genetic components) are critical in these two strategies. Therefore, engineering membrane transporter genes may help reduce the non-essential HM content in the edible part of crop plants. Targeted gene editing by genome editing tools like CRISPR could help plants achieve efficient phytoremediation and biofortification. This article covers gene editing's scope, application, and implication to improve the phytoremediation and biofortification processes in non-crop and crop plants.

The Potential of CRISPR/Cas Technology to Enhance Crop Performance on Adverse Soil Conditions

Worldwide food security is under threat in the actual scenery of global climate change because the major staple food crops are not adapted to hostile climatic and soil conditions. Significant efforts have been performed to maintain the actual yield of crops, using traditional breeding and innovative molecular techniques to assist them. However, additional strategies are necessary to achieve the future food demand. Clustered regularly interspaced short palindromic repeat/CRISPR-associated protein (CRISPR/Cas) technology, as well as its variants, have emerged as alternatives to transgenic plant breeding. This novelty has helped to accelerate the necessary modifications in major crops to confront the impact of abiotic stress on agriculture systems. This review summarizes the current advances in CRISPR/Cas applications in crops to deal with the main hostile soil conditions, such as drought, flooding and waterlogging, salinity, heavy metals, and nutrient deficiencies. In addition, the potential of extremophytes as a reservoir of new molecular mechanisms for abiotic stress tolerance, as well as their orthologue identification and edition in crops, is shown. Moreover, the future challenges and prospects related to CRISPR/Cas technology issues, legal regulations, and customer acceptance will be discussed.

Overexpression of Sedum SpHMA2, SpHMA3 and SpNramp6 in Brassica napus increases multiple heavy metals accumulation for phytoextraction

Phytoextraction is an environmentally friendly phytoremediation technology that can reduce the total amount of heavy metals (HMs) in the soil. Hyperaccumulators or hyperaccumulating transgenic plants with biomass are important biomaterials for phytoextraction. In this study, we show that three different HM transporters from the hyperaccumulator Sedum pumbizincicola, SpHMA2, SpHMA3, and SpNramp6, possess Cd transport. These three transporters are located at the plasma membrane, tonoplast, and plasma membrane, respectively. Their transcripts could be strongly stimulated by multiple HMs treatments. To create potential biomaterials for phytoextraction, we overexpressed the three single genes and two combining genes, SpHMA2&SpHMA3 and SpHMA2&SpNramp6, in rapes having high biomass and environmental adaptability, and found that the aerial parts of the SpHMA2-OE3 and SpHMA2&SpNramp6-OE4 lines accumulated more Cd from single Cd-contaminated soil because SpNramp6 transports Cd from root cells to the xylem and SpHMA2 from the stems to the leaves. However, the accumulation of each HM in the aerial parts of all selected transgenic rapes was strengthened in multiple HMs-contaminated soils, probably due to the synergistic transport. The HMs residuals in the soil after the transgenic plant phytoremediation were also greatly reduced. These results provide effective solutions for phytoextraction in both Cd and multiple HMs-contaminated soils.

Downregulated expression of TaDeg7 inhibits photosynthetic activity in bread wheat (Triticum aestivum L.)

Deg proteases play critical roles in photoprotection and PSII-repair circle, which remains elusive in cereal crops including wheat. Here, a Deg7-encoding gene TaDeg7 was silenced in wheat via a Barley stripe mosaic virus-induced gene-silencing system (BSMV-VIGS). When the expression level of TaDeg7 was downregulated, the photosynthetic activity including CO2 assimilation rate, actual photochemical efficiency of PSII, and electron transport rate declined while the nonphotochemical quenching increased significantly. When grown in high light, the BSMV:TaDeg7 plants accumulated more soluble sugar, malondialdehyde, and superoxide anion but had lower superoxide dismutase activity and less ascorbic acid. Additionally, the expression levels of TaPsbA and TarbcS were repressed in the BSMV:TaDeg7 plants in high light. The BSMV:TaDeg7 plants also were more sensitive to high-light stress. Collectively, it appeared that TaDeg7 may be a potential target for wheat radiation-use efficiency improvement against high light stress.

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.

A novel gene SpCTP3 from the hyperaccumulator Sedum plumbizincicola redistributes cadmium and increases its accumulation in transgenic Populus × canescens

A cadmium (Cd) tolerance protein (SpCTP3) involved in the Sedum plumbizincicola response to Cd stress was identified. However, the mechanism underlying the Cd detoxification and accumulation mediated by SpCTP3 in plants remains unclear. We compared wild-type (WT) and SpCTP3-overexpressing transgenic poplars in terms of Cd accumulation, physiological indices, and the expression profiles of transporter genes following with 100 μmol/L CdCl₂. Compared with the WT, significantly more Cd accumulated in the above-ground and below-ground parts of the SpCTP3-overexpressing lines after 100 μmol/L CdCl₂ treatment. The Cd flow rate was significantly higher in the transgenic roots than in the WT roots. The overexpression of SpCTP3 resulted in the subcellular redistribution of Cd, with decreased and increased Cd proportions in the cell wall and the soluble fraction, respectively, in the roots and leaves. Additionally, the accumulation of Cd increased the reactive oxygen species (ROS) content. The activities of three antioxidant enzymes (peroxidase, catalase, and superoxide dismutase) increased significantly in response to Cd stress. The observed increase in the titratable acid content in the cytoplasm might lead to the enhanced chelation of Cd. The genes encoding several transporters related to Cd²⁺ transport and detoxification were expressed at higher levels in the transgenic poplars than in the WT plants. Our results suggest that overexpressing SpCTP3 in transgenic poplar plants promotes Cd accumulation, modulates Cd distribution and ROS homeostasis, and decreases Cd toxicity via organic acids. In conclusion, genetically modifying plants to overexpress SpCTP3 may be a viable strategy for improving the phytoremediation of Cd-polluted soil.

Transcriptome analysis of the response of Hypomyces chrysospermus to cadmium stress

Hypomyces chrysospermus is a fungal parasite that grows on Boletus species. One isolated strain of H. chrysospermus from B. griseus was obtained and proved of strong ability to tolerate and absorb cadmium (Cd) by previous research. However, the molecular mechanisms of underlying the resistance of H. chrysospermus to Cd stress have not been investigated. This study aimed to assess the effect of Cd stress on the global transcriptional regulation of H. chrysospermus. A total of 1,839 differentially expressed genes (DEGs) were identified under 120 mg/l Cd stress. Gene ontology (GO) enrichment analysis revealed that large amounts of DEGs were associated with cell membrane components, oxidoreductase activity, and transport activity. KEGG enrichment analysis revealed that these DEGs were mainly involved in the translation, amino acid metabolism, transport and catabolism, carbohydrate metabolism, and folding/sorting and degradation pathways under Cd stress. Moreover, the expression of DEGs encoding transporter proteins, antioxidant enzymes, nonenzymatic antioxidant proteins, detoxification enzymes, and transcription factors was associated with the Cd stress response. These results provide insights into the molecular mechanisms underlying Cd tolerance in H. chrysospermus and serve as a valuable reference for further studies on the detoxification mechanisms of heavy metal-tolerant fungi. Our findings may also facilitate the development of new and improved fungal bioremediation strategies.

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.

PcNRAMP1 Enhances Cadmium Uptake and Accumulation in Populus × canescens

Poplars are proposed for the phytoremediation of heavy metal (HM) polluted soil. Characterization of genes involved in HM uptake and accumulation in poplars is crucial for improving the phytoremediation efficiency. Here, Natural Resistance-Associated Macrophage Protein 1 (NRAMP1) encoding a transporter involved in cadmium (Cd) uptake and transport was functionally characterized in Populus × canescens. Eight putative PcNRAMPs were identified in the poplar genome and most of them were primarily expressed in the roots. The expression of PcNRAMP1 was induced in Cd-exposed roots and it encoded a plasma membrane-localized protein. PcNRAMP1 showed transport activity for Cd²⁺ when expressed in yeast. The PcNRAMP1-overexpressed poplars enhanced net Cd²⁺ influxes by 39–52% in the roots and Cd accumulation by 25–29% in aerial parts compared to the wildtype (WT). However, Cd-induced biomass decreases were similar between the transgenics and WT. Further analysis displayed that the two amino acid residues of PcNRAMP1, i.e., M236 and P405, play pivotal roles in regulating its transport activity for Cd²⁺. These results suggest that PcNRAMP1 is a plasma membrane-localized transporter involved in Cd uptake and transporting Cd from the roots to aerial tissues, and that the conserved residues in PcNRAMP1 are essential for its Cd transport activity in poplars.

Identification and Analysis of bZIP Family Genes in Sedum plumbizincicola and Their Potential Roles in Response to Cadmium Stress

Sedum plumbizincicola (Crassulaceae), a cadmium (Cd)/zinc (Zn)/lead (Pb) hyperaccumulator native to Southeast China, is potentially useful for the phytoremediation of heavy metal-contaminated soil. Basic leucine zipper (bZIP) transcription factors play vital roles in plant growth, development, and abiotic stress responses. However, there has been minimal research on the effects of Cd stress on the bZIP gene family in S. plumbizincicola. In this study, 92 SpbZIP genes were identified in the S. plumbizincicola genome and then classified into 12 subgroups according to their similarity to bZIP genes in Arabidopsis. Gene structure and conserved motif analyses showed that SpbZIP genes within the same subgroup shared similar intron-exon structures and motif compositions. In total, eight pairs of segmentally duplicated SpbZIP genes were identified, but there were no tandemly duplicated SpbZIP genes. Additionally, the duplicated SpbZIP genes were mainly under purifying selection pressure. Hormone-responsive, abiotic and biotic stress-responsive, and plant development-related cis-acting elements were detected in the SpbZIP promoter sequences. Expression profiles derived from RNA-seq and quantitative real-time PCR analyses indicated that the expression levels of most SpbZIP genes were upregulated under Cd stress conditions. Furthermore, a gene co-expression network analysis revealed that most edge genes regulated by hub genes were related to metal transport, responses to stimuli, and transcriptional regulation. Because its expression was significantly upregulated by Cd stress, the hub gene SpbZIP60 was selected for a functional characterization to elucidate its role in the root response to Cd stress. In a transient gene expression analysis involving Nicotiana benthamiana leaves, SpbZIP60 was localized in the nucleus. The overexpression of SpbZIP60 enhanced the Cd tolerance of transgenic Arabidopsis plants by inhibiting ROS accumulation, protecting the photosynthetic apparatus, and decreasing the Cd content. These findings may provide insights into the potential roles of the bZIP family genes during the S. plumbizincicola response to Cd stress.

The Role of Membrane Transporters in Plant Growth and Development, and Abiotic Stress Tolerance

The proteins of membrane transporters (MTs) are embedded within membrane-bounded organelles and are the prime targets for improvements in the efficiency of water and nutrient transportation. Their function is to maintain cellular homeostasis by controlling ionic movements across cellular channels from roots to upper plant parts, xylem loading and remobilization of sugar molecules from photosynthesis tissues in the leaf (source) to roots, stem and seeds (sink) via phloem loading. The plant's entire source-to-sink relationship is regulated by multiple transporting proteins in a highly sophisticated manner and driven based on different stages of plant growth and development (PG&D) and environmental changes. The MTs play a pivotal role in PG&D in terms of increased plant height, branches/tiller numbers, enhanced numbers, length and filled panicles per plant, seed yield and grain quality. Dynamic climatic changes disturbed ionic balance (salt, drought and heavy metals) and sugar supply (cold and heat stress) in plants. Due to poor selectivity, some of the MTs also uptake toxic elements in roots negatively impact PG&D and are later on also exported to upper parts where they deteriorate grain quality. As an adaptive strategy, in response to salt and heavy metals, plants activate plasma membranes and vacuolar membrane-localized MTs that export toxic elements into vacuole and also translocate in the root's tips and shoot. However, in case of drought, cold and heat stresses, MTs increased water and sugar supplies to all organs. In this review, we mainly review recent literature from Arabidopsis, halophytes and major field crops such as rice, wheat, maize and oilseed rape in order to argue the global role of MTs in PG&D, and abiotic stress tolerance. We also discussed gene expression level changes and genomic variations within a species as well as within a family in response to developmental and environmental cues.

Genetic system underlying responses of Cryptococcus neoformans to cadmium

Cryptococcus neoformans, basidiomycetous pathogenic yeast, is basically an environmental fungus and, therefore, challenged by ever changing environments. In this study, we focused on how C. neoformans responds to stress caused by cadmium that is one of high-risk pollutants. By tracking phenotypes of the resistance or sensitivity to cadmium, we undertook forward and reverse genetic studies to identify genes involved in cadmium metabolism in C. neoformans. We found that the main route of Cd²⁺ influx is through Mn²⁺ ion transporter, Smf1, which is an ortholog of Nramp (natural resistance-associated macrophage protein 1) of mouse. We found that serotype A strains are generally more resistant to cadmium than serotype D strains and that cadmium resistance of H99, a representative of serotype A strains, was found to be due to a partial defect in SMF1. We found that calcium channel has a subsidiary role for cadmium uptake. We also showed that Pca1 (P-type-ATPase) functions as an extrusion pump for cadmium. We examined the effects of some metals on cadmium toxicity and suggested (i) that Ca²⁺ and Zn²⁺ could exert their protective function against Cd²⁺ via restoring cadmium-inhibited cellular processes and (ii) that Mg²⁺ and Mn²⁺ could have antagonistic roles in an unknown Smf1-independent Cd²⁺ uptake system. We proposed a model for Cd2⁺-response of C. neoformans, which will serve as a platform for understanding how this organism copes with the toxic metal.

Research on the Mechanisms of Plant Enrichment and Detoxification of Cadmium

The heavy metal cadmium (Cd), as one of the major environmentally toxic pollutants, has serious impacts on the growth, development, and physiological functions of plants and animals, leading to deterioration of environmental quality and threats to human health. Research on how plants absorb and transport Cd, as well as its enrichment and detoxification mechanisms, is of great significance to the development of phytoremediation technologies for ecological and environmental management. This article summarises the research progress on the enrichment of heavy metal cadmium in plants in recent years, including the uptake, transport, and accumulation of Cd in plants. The role of plant roots, compartmentalisation, chelation, antioxidation, stress, and osmotic adjustment in the process of plant Cd enrichment are discussed. Finally, problems are proposed to provide a more comprehensive theoretical basis for the further application of phytoremediation technology in the field of heavy metal pollution.

Abandoned Mine Lands Reclamation by Plant Remediation Technologies

Abandoned mine lands (AMLs), which are considered some of the most dangerous anthropogenic activities in the world, are a source of hazards relating to potentially toxic elements (PTEs). Traditional reclamation techniques, which are expensive, time-consuming and not well accepted by the general public, cannot be used on a large scale. However, plant-based techniques have gained acceptance as an environmentally friendly alternative over the last 20 years. Plants can be used in AMLs for PTE phytoextraction, phytostabilization, and phytovolatilization. We reviewed these phytoremediation techniques, paying particular attention to the selection of appropriate plants in each case. In order to assess the suitability of plants for phytoremediation purposes, the accumulation capacity and tolerance mechanisms of PTEs was described. We also compiled a collection of interesting actual examples of AML phytoremediation. On-site studies have shown positive results in terms of soil quality improvement, reduced PTE bioavailability, and increased biodiversity. However, phytoremediation strategies need to better characterize potential plant candidates in order to improve PTE extraction and to reduce the negative impact on AMLs.

Exogenous application of Mn significantly increased Cd accumulation in the Cd/Zn hyperaccumulator Sedum alfredii

Sedum alfredii is a Cd/Zn hyperaccumulator native to China, which was collected from a mined area where Mn content in soil was extremely high, together with Zn and Cd content. We investigated the tolerance and accumulation ability of Mn and its possible association with Cd hyperaccumulation in this plant species by using MP-AES, SR-μ-XRF, and RT-PCR. The results showed that the hyperaccumulating ecotype (HE) S. alfredii exhibited high tolerance to Mn and accumulating around 10,000 and 12,000 mg kg^-1 Mn in roots and shoots, respectively, without exhibiting toxicity under 5000 mg kg^-1 Mn treatment for 4 weeks. Exposure to Cd significantly reduced plant uptake of Mn. In contrast, exogenous Mn application significantly improved root uptake and root-to-shoot translocation of Cd, resulting in the increased Cd accumulation in the shoots of HE S. alfredii. SR-μ-XRF analysis demonstrated that high Mn (20 μM) exposure resulted in higher intensities of Cd localized in both stem vascular bundles and cortex, as well as leaf mesophyll cells, than in those treated with low Mn levels (0.2 μM or 2.0 μM). RT-PCR analysis of several genes possibly involved in Mn/Cd transportation showed that expression of SaNramp3 in roots was significantly reduced under high Mn exposure. These results suggested a significant interaction between Cd and Mn in the HE S. alfredii plants, possibly through their competition for transporters and theoretically provided a strategy to improve the efficiency of Cd extraction from polluted soils by this plant species, after using appropriate nutrient management of Mn.

Phytoremediation: a sustainable environmental technology for heavy metals decontamination

Toxic metal contamination of soil is a major environmental hazard. Chemical methods for heavy metal's (HMs) decontamination such as heat treatment, electroremediation, soil replacement, precipitation and chemical leaching are generally very costly and not be applicable to agricultural lands. However, many strategies are being used to restore polluted environments. Among these, phytoremediation is a promising method based on the use of hyper-accumulator plant species that can tolerate high amounts of toxic HMs present in the environment/soil. Such a strategy uses green plants to remove, degrade, or detoxify toxic metals. Five types of phytoremediation technologies have often been employed for soil decontamination: phytostabilization, phytodegradation, rhizofiltration, phytoextraction and phytovolatilization. Traditional phytoremediation method presents some limitations regarding their applications at large scale, so the application of genetic engineering approaches such as transgenic transformation, nanoparticles addition and phytoremediation assisted with phytohormones, plant growth-promoting bacteria and AMF inoculation has been applied to ameliorate the efficacy of plants as candidates for HMs decontamination. In this review, aspects of HMs toxicity and their depollution procedures with focus on phytoremediation are discussed. Last, some recent innovative technologies for improving phytoremediation are highlighted.

Auxin-Induced SaARF4 Downregulates SaACO4 to Inhibit Lateral Root Formation in Sedum alfredii Hance

Lateral root (LR) formation promotes plant resistance, whereas high-level ethylene induced by abiotic stress will inhibit LR emergence. Considering that local auxin accumulation is a precondition for LR generation, auxin-induced genes inhibiting ethylene synthesis may thus be important for LR development. Here, we found that auxin response factor 4 (SaARF4) in Sedum alfredii Hance could be induced by auxin. The overexpression of SaARF4 decreased the LR number and reduced the vessel diameters. Meanwhile, the auxin distribution mode was altered in the root tips and PIN expression was also decreased in the overexpressed lines compared with the wild-type (WT) plants. The overexpression of SaARF4 could reduce ethylene synthesis, and thus, the repression of ethylene production decreased the LR number of WT and reduced PIN expression in the roots. Furthermore, the quantitative real-time PCR, chromatin immunoprecipitation sequencing, yeast one-hybrid, and dual-luciferase assay results showed that SaARF4 could bind the promoter of 1-aminocyclopropane-1-carboxylate oxidase 4 (SaACO4), associated with ethylene biosynthesis, and could downregulate its expression. Therefore, we concluded that SaARF4 induced by auxin can inhibit ethylene biosynthesis by repressing SaACO4 expression, and this process may affect auxin transport to delay LR development.

Genome-wide analysis of the NRAMP gene family in potato (Solanum tuberosum): Identification, expression analysis and response to five heavy metals stress

NRAMP family genes participate in the absorption and transport of heavy metals such as cadmium (Cd), zinc (Zn), copper (Cu), lead (Pb), iron (Fe) and manganese (Mn) and play an important role in the response to heavy metal stress. There is an abundance of research on these genes in bacteria, plants and fungi, although not in S. tuberosum. A total of 48 members(potato(5), Arabidopsis(7), Tomato(9), pepper(9), rice(12) and tobacco(6)) were identified from 6 species (potato (Solanum tuberosum), Arabidopsis thaliana, Tomato (Solanum lycopersicum), pepper (Capsicum annuum), rice (Oryza sativa) and tobacco (Nicotiana attenuate)) and were classified into four subgroups. Across NRAMP gene family members, there are 15 highly conserved motifs that have similar genetic structures and characteristics. In addition, a total of 16 pairs of colinear genes were found in eight species. Analysis of cis-elements indicated that, in response to abiotic stress, NRAMPs are mainly regulated by phytohormones and transcription factors. In addition, analysis of expression profiles indicated that StNRAMP4 is mainly expressed in the roots. According to a qRT-PCR-based analysis of the StNRAMP family, with the exception of Pb²⁺ stress, StNRAMPs positively responded to stress from Cu²⁺, Cd²⁺, Zn²⁺ and Ni²⁺ and The expression patterns is similar of StNRAMP2, under Pb²⁺, and Cu²⁺ treatment, the relative expression peaked at 24 h. the relative expression peaked at 12 h and was upregulated 428-fold in the roots under Ni²⁺ stress. Under Cd²⁺ stress, StNRAMP3 was upregulated 28-fold in the leaves. StNRAMP1, StNRAMP4 and StNRAMP5 showed significant upregulation under Cu²⁺, Cd²⁺ and Zn²⁺ stress, respectively. Expression of StNRAMPs could be specifically induced by heavy metals, implying their possible role in the transport and absorption of heavy metals. This research explains the colinear characteristics of NRAMPs in several food crop species, which is useful for providing important genetic resources for cultivating food crop that accumulate low amounts of heavy metals and for explaining the biological functions of NRAMPs in plants.

Identification and functional characterization of ABCC transporters for Cd tolerance and accumulation in Sedum alfredii Hance

Cd is one of the potential toxic elements (PTEs) exerting great threats on the environment and living organisms and arising extensive attentions worldwide. Sedum alfredii Hance, a Cd hyperaccumulator, is of great importance in studying the mechanisms of Cd hyperaccumulation and has potentials for phytoremediation. ATP-binding cassette sub-family C (ABCC) belongs to the ABC transporter family, which is deemed to closely associate with multiple physiological processes including cellular homeostasis, metal detoxification, and transport of metabolites. In the present work, ten ABCC proteins were identified in S. alfredii Hance, exhibiting uniform domain structure and divergently clustering with those from Arabidopsis. Tissue-specific expression analysis indicated that some SaABCC genes had significantly higher expression in roots (Sa23221 and Sa88F144), stems (Sa13F200 and Sa14F98) and leaves (Sa13F200). Co-expression network analysis using these five SaABCC genes as hub genes produced two clades harboring different edge genes. Transcriptional expression profiles responsive to Cd illustrated a dramatic elevation of Sa14F190 and Sa18F186 genes. Heterologous expression in a Cd-sensitive yeast cell line, we confirmed the functions of Sa14F190 gene encoding ABCC in Cd accumulation. Our study performed a comprehensive analysis of ABCCs in S. alfredii Hance, firstly mapped their tissue-specific expression patterns responsive to Cd stress, and characterized the roles of Sa14F190 genes in Cd accumulation.

Pseudomonas fluorescens accelerates a reverse and long-distance transport of cadmium and sucrose in the hyperaccumulator plant Sedum alfredii

Plant growth-promoting bacteria (PGPB) can promote root uptake and shoot accumulation of cadmium (Cd) in hyperaccumulator plants, but the mechanisms by which PGPB accelerate root-to-shoot transport of Cd is still unknown. A better understanding of these mechanisms is necessary to develop the strategies that can promote the practical phytoextraction of Cd-polluted soils. In this study, we found that Pseudomonas fluorescens accelerates a reversed and a long-distance transport of Cd and sucrose in Sedum alfredii, by examining the xylem and phloem sap and by quantifying the concentrations of Cd and sucrose in shoot and root. The transcriptome sequencing has revealed the up-regulated expressions of starch metabolism and sucrose biosynthesis related genes in the shoots of Cd hyperaccumulator plant S. alfredii that was inoculated with PGPB P. fluorescens. In addition, the genes of sugar, cation and anion transporters were also up-regulated by bacterial treatment, showing a complicated co-expression network with sucrose biosynthesis related genes. The expression levels of Cd transporter genes, such as ZIP1, ZIP2, HMA2, HMA3 and CAX2, were elevated after PGPB inoculation. As a result, the PGPB successfully colonized the root, and promoted the sucrose shoot-to-root transport and Cd root-to-shoot transport in S. alfredii. Since non-photosynthetic root-associated bacteria usually obtain sugars from photosynthetic plants, our results highlight the importance of PGPB-induced changes in hyperaccumlator plants for both the host and the PGPB.

Transporters and ascorbate-glutathione metabolism for differential cadmium accumulation and tolerance in two contrasting willow genotypes

Salix matsudana Koidz is a low cadmium (Cd)-accumulating willow, whereas its cultivated variety, Salix matsudana var. matsudana f. umbraculifera Rehd., is a high Cd-accumulating and tolerant willow (HCW). The physiological and molecular mechanisms underlying differential Cd accumulation and tolerance in the two Salix species are poorly understood. Here, we confirmed that the differential Cd translocation capacity from roots to the shoots leads to the differential Cd accumulation in their aboveground parts between these two willow genotypes. Cadmium accumulation happens preferentially in the transport pathway, and Cd is mainly located in the vacuolar, cell wall and intercellular space in HCW bark by cadmium location analysis at tissue and subcellular levels. Comparative transcriptome analysis revealed that higher expressions of several metal transporter genes (ATP-binding cassette transporters, K+ transporters/channels, yellow stripe-like proteins, zinc-regulated transporter/iron-regulated transporter-like proteins, etc.) are involved in root uptake and translocation capacity in HCW; meanwhile, ascorbate-glutathione metabolic pathways play essential roles in Cd detoxification and higher tolerance of the Cd-accumulator HCW. These results lay the foundation for further understanding the molecular mechanisms of Cd accumulation in woody plants and provide new insights into molecular-assisted-screening woody plant varieties for phytoremediation.

Cadmium in plants: uptake, toxicity, and its interactions with selenium fertilizers

Cd is the third major contaminant of greatest hazard to the environment after mercury and lead and is considered as the only metal that poses health risks to both humans and animals at plant tissue concentrations that are generally not phytotoxic. Cd accumulation in plant shoots depends on Cd entry through the roots, sequestration within root vacuoles, translocation in the xylem and phloem, and Cd dilution within the plant shoot throughout its growth. Several metal transporters, processes, and channels are involved from the first step of Cd reaching the root cells and until its final accumulation in the edible parts of the plant. It is hard to demonstrate one step as the pivotal factor to decide the Cd tolerance or accumulation ability of plants since the role of a specific transporter/process varies among plant species and even cultivars. In this review, we discuss the sources of Cd pollutants, Cd toxicity to plants, and mechanisms of Cd uptake and redistribution in plant tissues. The metal transporters involved in Cd transport within plant tissues are also discussed and how their manipulation can control Cd uptake and/or translocation. Finally, we discuss the beneficial effects of Se on plants under Cd stress, and how it can minimize or mitigate Cd toxicity in plants.

A Single Amino Acid Change in Nramp6 from Sedum Alfredii Hance Affects Cadmium Accumulation

SaNramp6 in Sedum alfredii encodes a membrane-localized metal transporter. We isolated the SaNramp6h allele from the hyperaccumulating ecotype (HE) of S. alfredii. When this allele was expressed in transgenic yeast and Arabidopsis thaliana, it enhanced their cadmium (Cd) sensitivity by increased Cd transport and accumulation. We isolated another allele, SaNramp6n, from a nonhyperaccumulating ecotype (NHE) of S. alfredii. Amino acid sequence comparisons revealed three amino acid differences between SaNramp6h and SaNramp6n. We investigated the Cd transport activity of the Nramp6 allele, and determined which residues are essential for the transport activity. We conducted structure-function analyses of SaNramp6 based on site-directed mutagenesis and functional assays of the mutants in yeast and Arabidopsis. The three residues that differed between SaNramp6h and SaNramp6n were mutated. Only the L157P mutation of SaNramp6h impaired Cd transport. The other mutations, S218N and T504A, did not affect the transport activity of SaNramp6h, indicating that these residues are not essential for metal selectivity. Transgenic plants overexpressing SaNramp6hL157P showed altered metal accumulation in shoots and roots. Our results suggest that the conserved site L157 is essential for the high metal transport activity of SaNramp6h. This information may be useful for limiting or increasing Cd transport by other plant natural resistance associated macrophage protein (NRAMP) proteins.

Exogenous abscisic acid (ABA) promotes cadmium (Cd) accumulation in Sedum alfredii Hance by regulating the expression of Cd stress response genes

Sedum alfredii Hance is a zinc (Zn) and cadmium (Cd) hyperaccumulator plant. However, the regulatory role of plant hormones in the Zn or Cd uptake and accumulation of S. alfredii remains unclear. In this work, the growth, Cd accumulation, abscisic acid (ABA) synthesis and catabolism, malonaldehyde (MDA) content, and transcriptional level of some Cd stress response genes under ABA and Cd co-treatment were investigated to reveal the impact of ABA on Cd resistance and Cd accumulation of S. alfredii. The results show that 0.2 mg/L ABA and 100 μmol/L Cd co-treatment enhanced Cd accumulation and growth in S. alfredii, whereas lower or higher ABA concentrations weaken or even reverse this effect, which was positively correlated with endogenous ABA content. The increase in endogenous ABA content might be the results of the increasing ABA synthetase activities and decreasing ABA lytic enzyme, which was induced by the application of 0.2 mg/L ABA under 100 μmol/L Cd treatment. Principal component analysis (PCA) indicated that ABA impacted the expression pattern of Cd stress response genes, which coincided with the Cd accumulation pattern in the shoots of S. alfredii. Cross-over analysis of partial least squares-discriminant analysis (PLS-DA) and correlation analysis indicated that HsfA4c, HMA4 expression in roots, and HMA2, HMA3, CAD, NAS expression in shoots were correlated with endogenous ABA, which suggests that endogenous ABA improves Cd resistance of seedlings, switches the root-to-shoot transporter from HMA2 to HMA4, and transports more Cd into apoplasts to promote Cd accumulation in the shoots of S. alfredii. Taken together, ABA plays an essential role not only in Cd resistance but also in Cd transport from root to shoot in S. alfredii under Cd stress.

Molecular mechanisms in phytoremediation of environmental contaminants and prospects of engineered transgenic plants/microbes

Concerns about emerging environmental contaminants have been growing along with industrialization and urbanization around the globe. Among various options for remediating these contaminants, phytotechnology is suggested as a feasible option to maintain the environmental sustainability. The recent advances in phytoremediation, genetic/molecular/omics/metabolic engineering, and nanotechnology are opening new paths for efficient treatment of emerging organic/inorganic contaminants. In this respect, elucidation of molecular mechanisms and genetic engineering of hyperaccumulator plants is expected to enhance remediation of environmental contaminants. This review was organized to offer valuable insights into the molecular mechanisms of phytoremediation and the prospects of transgenic hyperaccumulators with enhanced stress tolerance to diverse contaminants such as heavy metals and metalloids, xenobiotics, explosives, poly aromatic hydrocarbons (PAHs), petroleum hydrocarbons, pesticides, and nanoparticles. The roles of genoremediation and nanoparticles in augmenting the phytoremediation technology are also described in an interrelated framework with biotechnological prospects (e.g., plant molecular nano-farming). Finally, political debate on the preferential use of crops versus non-crop hyperaccumulators in genoremediation, limitations of transgenics in phytotechnologies, and their public acceptance issues are discussed in the policy framework.

MhNRAMP1 From Malus hupehensis Exacerbates Cell Death by Accelerating Cd Uptake in Tobacco and Apple Calli

Excessive cadmium (Cd) damages plants by causing cell death. The present study discusses the function of natural resistance-associated macrophage protein (NRAMP) on cell death caused by Cd in Malus hupehensis. MhNRAMP1 was isolated from M. hupehensis roots, and its protein was located in the cell membrane as a transmembrane protein characterized by hydrophobicity. MhNRAMP1 expression in the roots was induced by Cd stress and calcium (Ca) deficiency. MhNRAMP1 overexpression increased Cd concentration in yeasts and enhanced their sensitivity to Cd. Phenotypic comparisons of plants under Cd stress revealed that the growth of transgenic tobacco and apple calli overexpressing MhNRAMP1 was worse than that of the wild type (WT). The Cd²⁺ influx of transgenic tobacco roots and apple calli was higher, and the recovery time of the Cd²⁺ influx to a stable state in transgenic apple calli was longer than that of the WT. Cd accumulation and the percentage of apoptotic cells in transgenic lines were higher. Correspondingly, the caspase-1-like and vacuolar processing enzyme (VPE) activities and MdVPEγ expression were higher in transgenic apple calli, but the expression levels of genes that inhibit cell death were lower than those in the WT under Cd stress. Moreover, the Cd translocation from the roots to leaves was increased after MhNRAMP1 overexpression, but the Cd translocation from the leaves to seeds was not affected. These results suggest that MhNRMAP1 exacerbated Cd-induced cell death, which was accomplished by mediating Cd²⁺ uptake and accumulation, as well as stimulating VPE.

Declined cadmium accumulation in Na+/H+ antiporter (NHX1) transgenic duckweed under cadmium stress

Cadmium (Cd) is a serious threat to plants health. Though some genes have been reported to get involved in the regulation of tolerance to Cd, the mechanisms underlying this process are not fully understood. Na⁺/H⁺ antiporter (NHX1) plays an important role in Na⁺/H⁺ trafficking. The salt and cadmium stress tolerance were found to be enhanced by NHX1 in duckweed according to our previous study, however, its function in Cd²⁺ flux under Cd stress has not been studied. Here we explored the Cd²⁺ flux in wild type (WT) and NHX1 transgenic duckweed (NHX1) under Cd stress. We found that the Cd²⁺ influx in NHX1 duckweed was significantly declined, followed by an increased Cd²⁺ efflux after 20 min treatment of Cd, which resulted a less accumulation of Cd in NHX1. Reversely, inhibition of NHX1 by amiloride treatment, enhanced Cd²⁺ influx in NHX1 duckweed, subsequently delayed Cd²⁺ efflux in both genotypes of duckweed under Cd²⁺ shock. H⁺ efflux in NHX1 duckweed was lower compare with that in WT with 20 min Cd²⁺ shock. NHX1 also increased the pH value with Cd²⁺ stress in the transgenic rhizoid. These finding suggested a new function of NHX1 in regulation of Cd²⁺ and H⁺ flow during short-term Cd²⁺ shock.

cDNA Library for Mining Functional Genes in Sedum alfredii Hance Related to Cadmium Tolerance and Characterization of the Roles of a Novel SaCTP2 Gene in Enhancing Cadmium Hyperaccumulation

Heavy metal contamination presents serious threats to living organisms. Functional genes related to cadmium (Cd) hypertolerance or hyperaccumulation must be explored to enhance phytoremediation. Sedum alfredii Hance is a Zn/Cd cohyperaccumulator exhibiting abundant genes associated with Cd hypertolerance. Here, we developed a method for screening genes related to Cd tolerance by expressing a cDNA-library for S. alfredii Hance. Yeast functional complementation validated 42 of 48 full-length genes involved in Cd tolerance, and the majority of them were strongly induced in roots and exhibited diverse expression profiles across tissues. Coexpression network analysis suggested that 15 hub genes were connected with genes involved in metabolic processes, response to stimuli, and metal transporter and antioxidant activity. The functions of a novel SaCTP2 gene were validated by heterologous expression in Arabidopsis, responsible for retarding chlorophyll content decrease, maintaining membrane integrity, promoting reactive oxygen species (ROS) scavenger activities, and reducing ROS levels. Our findings suggest a highly complex network of genes related to Cd hypertolerance in S. alfredii Hance, accomplished via the antioxidant system, defense genes induction, and the calcium signaling pathway. The proposed cDNA-library method is an effective approach for mining candidate genes associated with Cd hypertolerance to develop genetically engineered plants for use in phytoremediation.

Construction, characteristics and high throughput molecular screening methodologies in some special breeding populations: a horticultural perspective

Advanced marker technologies are widely used for evaluation of genetic diversity in cultivated crops, wild ancestors, landraces or any special plant genotypes. Developing agricultural cultivars requires the following steps: (i) determining desired characteristics to be improved, (ii) screening genetic resources to help find a superior cultivar, (iii) intercrossing selected individuals, (iv) generating genetically hybrid populations and screening them for agro-morphological or molecular traits, (v) evaluating the superior cultivar candidates, (vi) testing field performance at different locations, and (vii) certifying. In the cultivar development process valuable genes can be identified by creating special biparental or multiparental populations and analysing their association using suitable markers in given populations. These special populations and advanced marker technologies give us a deeper knowledge about the inherited agronomic characteristics. Unaffected by the changing environmental conditions, these provide a higher understanding of genome dynamics in plants. The last decade witnessed new applications for advanced molecular techniques in the area of breeding,with low costs per sample. These, especially, include next-generation sequencing technologies like reduced representation genome sequencing (genotyping by sequencing, restriction site-associated DNA). These enabled researchers to develop new markers, such as simple sequence repeat and single- nucleotide polymorphism, for expanding the qualitative and quantitative information onpopulation dynamics. Thus, the knowledge acquired from novel technologies is a valuable asset for the breeding process and to better understand the population dynamics, their properties, and analysis methods.

Calcium acetate-induced reduction of cadmium accumulation in Oryza sativa: Expression of auto-inhibited calcium-ATPase and cadmium transporters

Calcium (Ca) signalling has an essential role in regulating plant responses to various abiotic stresses. This study applied Ca in various forms (Ca acetate and CaCl₂ ) and concentrations to reduce cadmium (Cd) concentration in rice and propose a possible mechanism through which Ca acts to control the Cd concentration in rice. The results showed that supplementation of Cd-contaminated soil with Ca acetate reduced the Cd concentration in rice after exposure for 7 days in both hydroponic and soil conditions. The possible involvement of the auto-inhibited Ca²⁺ -ATPase gene (ACA) might act to control the primary signal of the Cd stress response. The messages from ACA3 and ACA13 tended to up-regulate the low-affinity cation transporter (OsLCT1) and down-regulate Cd uptake and the Cd translocation transporter, including the genes, natural resistance-associated macrophage protein 5 (Nramp5) and Zn/Cd-transporting ATPase 2 (HMA2), which resulted in a reduction in the Cd concentration in rice. After cultivation for 120 days, the application of Ca acetate into Cd-contaminated soil inhibited Cd uptake of rice. Increasing the Ca acetate concentration in the soil lowered the Cd concentration in rice shoots and grains. Moreover, Ca acetate maintained rice productivity and quality whereas both aspects decreased under Cd stress.

Identification and expression analysis of the GDSL esterase/lipase family genes, and the characterization of SaGLIP8 in Sedum alfredii Hance under cadmium stress

Background

The herb Sedum alfredii (S. alfredii) Hance is a hyperaccumulator of heavy metals (cadmium (Cd), zinc (Zn) and lead (Pb)); therefore, it could be a candidate plant for efficient phytoremediation. The GDSL esterase/lipase protein (GELP) family plays important roles in plant defense and growth. Although the GELP family members in a variety of plants have been cloned and analyzed, there are limited studies on the family's responses to heavy metal-stress conditions.

Methods

Multiple sequence alignments and phylogenetic analyses were performed according to the criteria described. A WGCNA was used to construct co-expression regulatory networks. The roots of S. alfredii seedlings were treated with 100 µM CdCl₂ for qRT-PCR to analyze expression levels in different tissues. SaGLIP8 was transformed into the Cd sensitive mutant strain yeast Δycf1 to investigate its role in resistance and accumulation to Cd.

Results

We analyzed GELP family members from genomic data of S. alfredii. A phylogenetic tree divided the 80 identified family members into three clades. The promoters of the 80 genes contained certain elements related to abiotic stress, such as TC-rich repeats (defense and stress responsiveness), heat shock elements (heat stress) and MYB-binding sites (drought-inducibility). In addition, 66 members had tissue-specific expression patterns and significant responses to Cd stress. In total, 13 hub genes were obtained, based on an existing S. alfredii transcriptome database, that control 459 edge genes, which were classified into five classes of functions in a co-expression subnetwork: cell wall and defense function, lipid and esterase, stress and tolerance, transport and transcription factor activity. Among the hub genes, Sa13F.102 (SaGLIP8), with a high expression level in all tissues, could increase Cd tolerance and accumulation in yeast when overexpressed.

Conclusion

Based on genomic data of S. alfredii, we conducted phylogenetic analyses, as well as conserved domain, motif and expression profiling of the GELP family under Cd-stress conditions. SaGLIP8 could increase Cd tolerance and accumulation in yeast. These results indicated the roles of GELPs in plant responses to heavy metal exposure and provides a theoretical basis for further studies of the SaGELP family's functions.

Identification and comprehensive analysis of the characteristics and roles of leucine-rich repeat receptor-like protein kinase (LRR-RLK) genes in Sedum alfredii Hance responding to cadmium stress

Sedum alfredii Hance is a Zn/Cd co-hyperaccumulator and its underlying molecular mechanism of Cd tolerance is worthy to be elucidated. Although numerous studies have reported the uptake, sequestration and detoxification of Cd in S. alfredii Hance, how it senses Cd-stress stimuli and transfers signals within tissues remains unclear. Leucine-rich repeat receptor-like protein kinases (LRR-RLKs) are vital for plant growth, development, immunity and signal transduction. Till now, there is lack of comprehensive studies addressing their functions in S. alfredii Hance responding to Cd stress. In the present study, we identified 60 LRR-RLK genes in S. alfredii Hance based on transcriptome analysis under Cd stress. They were categorized into 11 subfamilies and most of them had highly conserved protein structures and motif compositions. The inter-family diversity provided evidence for their functional divergence, supported by their expression level and profile in tissues under Cd stress. Co-expression network analysis revealed that the most highly connected hubs, Sa0F.522, Sa0F.1036, Sa28F.115 and Sa1F.472, were closely related with other genes involved in metal transport, stimulus response and transcription regulations. Of the ten hub genes exhibiting differential expression dynamics under the short-term Cd stress (Sa0F.522, Sa0F.1036 and Sa28F.115) were dramatically induced in the whole plant. Among them, Sa0F.522 gene was heterologously expressed in a Cd-sensitive yeast cell line and its function in Cd signal perception was confirmed. For the first time, our findings performed a comprehensive analysis of LRR-RLKs in S. alfredii Hance, mapped their expression patterns under Cd stress, and identified the key roles of Sa0F.522, Sa0F.1036 and Sa28F.115 in Cd signal transduction.

Expression of the Tobacco Non-symbiotic Class 1 Hemoglobin Gene Hb1 Reduces Cadmium Levels by Modulating Cd Transporter Expression Through Decreasing Nitric Oxide and ROS Level in Arabidopsis

Hemoglobin (Hb) proteins are ubiquitous in plants, and non-symbiotic class 1 hemoglobin (Hb1) is involved in various biotic and abiotic stress responses. Here, the expression of the tobacco (Nicotiana tabacum) hemoglobin gene NtHb1 in Arabidopsis (Arabidopsis thaliana) showed higher cadmium (Cd) tolerance and lower accumulations of Cd, nitric oxide (NO), and reactive oxygen species (ROS) like hydrogen peroxide (H₂O₂). NtHb1-expressing Arabidopsis exhibited a reduced induction of NO levels in response to Cd, suggesting scavenging of NO by Hb1. In addition, transgenic plants had reduced accumulation of ROS and increased activities of antioxidative enzymes (catalase, superoxide dismutase, and glutathione reductase) in response to Cd. While the expression of the Cd exporters ABC transporter (PDR8) and Ca²⁺/H⁺ exchangers (CAXs) was increased, that of the Cd importers iron responsive transporter 1 (IRT1) and P-type 2B Ca²⁺ ATPase (ACA10) was reduced in response to Cd. When Col-0 plants were treated with the NO donor sodium nitroprusside (SNP) and H₂O₂, the expression pattern of Cd transporters (PDR8, CAX3, IRT1, and ACA10) was reversed, suggesting that NtHb1 expression decreased the Cd level by regulating the expression of Cd transporters via decreased NO and ROS. Correspondingly, NtHb1-expressing Arabidopsis showed increased Cd export. In summary, the expression of NtHb1 reduces Cd levels by regulating Cd transporter expression via decreased NO and ROS levels in Arabidopsis.

Comparative transcriptome analysis revealed key factors for differential cadmium transport and retention in roots of two contrasting peanut cultivars

BACKGROUND

Peanut is the world's fourth largest oilseed crop that exhibits wide cultivar variations in cadmium (Cd) accumulation. To establish the mechanisms of Cd distribution and accumulation in peanut plants, eight cDNA libraries from the roots of two contrasting cultivars, Fenghua 1 (low-Cd cultivar) and Silihong (high-Cd cultivar), were constructed and sequenced by RNA-sequencing. The expression patterns of 16 candidate DEGs were validated by RT-qPCR analysis.

RESULTS

A total of 75,634 genes including 71,349 known genes and 4484 novel genes were identified in eight cDNA libraries, among which 6798 genes were found to be Cd-responsive DEGs and/or DEGs between these two cultivars. Interestingly, 183 DEGs encoding ion transport related proteins and 260 DEGs encoding cell wall related proteins were identified. Among these DEGs, nine metal transporter genes (PDR1, ABCC4 and ABCC15, IRT1, ZIP1, ZIP11, YSL7, DTX43 and MTP4) and nine cell wall related genes (PEs, PGIPs, GTs, XYT12 CYP450s, LACs, 4CL2, C4H and CASP5) showed higher expression in Fenghua 1 than in Silihong.

CONCLUSIONS

Both the metal transporters and cell wall modification might be responsible for the difference in Cd accumulation and translocation between Fenghua 1 and Silihong. These findings would be useful for further functional analysis, and reveal the molecular mechanism responsible for genotype difference in Cd accumulation.

Identification, Expression Analysis of the Hsf Family, and Characterization of Class A4 in Sedum Alfredii Hance under Cadmium Stress

Sedum alfredii Hance, a cadmium (Cd)/zinc (Zn)/lead (Pb) co-hyperaccumulating species, is a promising phytoremediation candidate because it accumulates substantial amounts of heavy metal ions without showing any obvious signs of poisoning. The heat shock transcription factor (Hsf) family plays crucial roles in plant growth, development, and stress responses. Although the roles of some Hsfs in abiotic stress have been well studied in model plants, the Hsf family has not been systematically investigated in heavy metal hyperaccumulators. Here, we comprehensively analyzed the Hsf gene family in S. alfredii based on a transcriptome under Cd stress. There were 22 Hsf members that were identified and phylogenetically clustered into three classes, namely, SaHsfA, SaHsfB, and SaHsfC. All of the three classes shared similar motifs. The expression profiles of the 22 Hsf members showed significant differences: 18 SaHsfs were responsive to Cd stress, as were multiple SaHsp genes, including SaHsp18.1, SaHsp22, SaHsp26.5, SaHsp70, SaHsp90, and SaHsp101. Two class A4 members, SaHsfA4a and SaHsfA4c, exhibited transcriptional activation activities. Overexpression of SaHsfA4a and SaHsfA4c in transgenic yeast indicated an improved tolerance to Cd stress and Cd accumulation. Our results suggest SaHsfs play important regulatory roles in heavy metal stress responses, and provide a reference for further studies on the mechanism of heavy metal stress regulation by SaHsfs.