Molecular characterization and bioinformatics analysis of transporter genes associated with Cd-induced phytotoxicity in rice (Oryza sativa L.)

Effect of silicon spraying on rice photosynthesis and antioxidant defense system on cadmium accumulation

Cadmium (Cd) pollution is a serious threat to food safety and human health. Minimizing Cd uptake and enhancing Cd tolerance in plants are vital to improve crop yield and reduce hazardous effects to humans. In this study, we designed three Cd concentration stress treatments (Cd1: 0.20 mg·kg-1, Cd2: 0.60 mg·kg-1, and Cd3: 1.60 mg·kg-1) and two foliar silicon (Si) treatments (CK: no spraying of any material, and Si: foliar Si spraying) to conduct pot experiments on soil Cd stress. The results showed that spraying Si on the leaves reduced the Cd content in brown rice by 4.79-42.14%. Si application increased net photosynthetic rate (Pn) by 1.77-4.08%, stomatal conductance (Gs) by 5.27-23.43%, transpiration rate (Tr) by 2.99-20.50% and intercellular carbon dioxide (CO2) concentration (Ci) by 6.55-8.84%. Foliar spraying of Si significantly increased the activities of superoxide dismutase (SOD) and peroxidase (POD) in rice leaves by 9.84-14.09% and 4.69-53.09%, respectively, and reduced the content of malondialdehyde (MDA) by 7.83-48.72%. In summary, foliar Si spraying protects the photosynthesis and antioxidant system of rice canopy leaves, and is an effective method to reduce the Cd content in brown rice.

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.

Functional characterization of Fagopyrum tataricum ZIP gene family as a metal ion transporter

The zinc/iron-regulated transporter-like proteins (ZIP) family acts as an important transporter for divalent metal cations such as Zn, Fe, Mn, Cu, and even Cd. However, their condition is unclear in Tartary buckwheat (Fagopyrum tataricum). Here, 13 ZIP proteins were identified and were predicted to be mostly plasma membrane-localized. The transient expressions of FtZIP2 and FtZIP6 in tobacco confirmed the prediction. Multiple sequence alignment analysis of FtZIP proteins revealed that most of them had 8 putative transmembrane (TM) domains and a variable region rich in histidine residues between TM3 and TM4, indicating the reliable affinity to metal ions. Gene expression analysis by qRT-PCR showed that FtZIP genes were markedly different in different organs, such as roots, stems, leaves, flowers, fruits and seeds. However, in seedlings, the relative expression of FtZIP10 was notably induced under the CdCl2 treatment, while excessive Zn2+, Fe2+, Mn2+ and Cd2+ increased the transcript of FtZIP5 or FtZIP13, in comparison to normal conditions. Complementation of yeast mutants with the FtZIP family genes demonstrate that FtZIP7/10/12 transport Zn, FtZIP5/6/7/9/10/11 transport Fe, FtZIP12 transports Mn and FtZIP2/3/4/7 transport Cd. Our data suggest that FtZIP proteins have conserved functions of transportation of metal ions but with distinct spatial expression levels.

Integrated morphological, physiological and transcriptomic analyses reveal response mechanisms of rice under different cadmium exposure routes

Rice (Oryza sativa) is one of the major cereal crops and takes up cadmium (Cd) more readily than other crops. Understanding the mechanism of Cd uptake and defense in rice can help us avoid Cd in the food chain. However, studies comparing Cd uptake, toxicity, and detoxification mechanisms of leaf and root Cd exposure at the morphological, physiological, and transcriptional levels are still lacking. Therefore, experiments were conducted in this study and found that root Cd exposure resulted in more severe oxidative and photosynthetic damage, lower plant biomass, higher Cd accumulation, and transcriptional changes in rice than leaf Cd exposure. The activation of phenylpropanoids biosynthesis in both root and leaf tissues under different Cd exposure routes suggests that increased lignin is the response mechanism of rice under Cd stress. Moreover, the roots of rice are more sensitive to Cd stress and their adaptation responses are more pronounced than those of leaves. Quantitative PCR revealed that OsPOX, OsCAD, OsPAL and OsCCR play important roles in the response to Cd stress, which further emphasize the importance of lignin. Therefore, this study provides theoretical evidence for future chemical and genetic regulation of lignin biosynthesis in crop plants to reduce Cd accumulation.

Genome-wide analysis of plant specific YABBY transcription factor gene family in carrot (Dacus carota) and its comparison with Arabidopsis

YABBY gene family is a plant-specific transcription factor with DNA binding domain involved in various functions i.e. regulation of style, length of flowers, and polarity development of lateral organs in flowering plants. Computational methods were utilized to identify members of the YABBY gene family, with Carrot (Daucus carota) 's genome as a foundational reference. The structure of genes, location of the chromosomes, protein motifs and phylogenetic investigation, syntony and transcriptomic analysis, and miRNA targets were analyzed to unmask the hidden structural and functional characteristics YABBY gene family in Carrots. In the following research, it has been concluded that 11 specific YABBY genes irregularly dispersed on all 9 chromosomes and proteins assembled into five subgroups i.e. AtINO, AtCRC, AtYAB5, AtAFO, and AtYAB2, which were created on the well-known classification of Arabidopsis. The wide ranges of YABBY genes in carrots were dispersed due to segmental duplication, which was detected as prevalent when equated to tandem duplication. Transcriptomic analysis showed that one of the DcYABBY genes was highly expressed during anthocyanin pigmentation in carrot taproots. The cis-regulatory elements (CREs) analysis unveiled elements that particularly respond to light, cell cycle regulation, drought induce ability, ABA hormone, seed, and meristem expression. Furthermore, a relative study among Carrot and Arabidopsis genes of the YABBY family indicated 5 sub-families sharing common characteristics. The comprehensive evaluation of YABBY genes in the genome provides a direction for the cloning and understanding of their functional properties in carrots. Our investigations revealed genome-wide distribution and role of YABBY genes in the carrots with best-fit comparison to Arabidopsis thaliana.

Molecular characterization of genes involved in tolerance of cadmium in Triticum aestivum (L.) under Cd stress

The NRAMPs (natural resistance-associated macrophage proteins) are major transporters for the absorption and transport of metals like Pb, Zn, Mn, Fe, and Cd in plants. While NRAMP gene family members have been extensively studied as metal transporters in model and other plants, little information has been reported on their role in Triticum aestivum, particularly in response to Cd stress. Current study reported 13 NRAMP candidates in the genome of T. aestivum. Phylogenetic analysis divided these into three clades. Motif and gene structure study showed that members in the same clades shared the same location and pattern, which further supported the phylogenetic analysis. The analysis of cis-acting elements in promoter sequences of NRAMP genes in wheat identified stress-responsive transcription factor binding sites. Multiple sequence alignment identified the conservation of important residues. Based on RNA-seq and qRT-PCR analysis, Cd stress-responsive variations of TaNRAMP gene expression were reported. This study provides comprehensive data to understand the TaNRAMP gene family, its features, and its expression, which will be a helpful framework for functional research.

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.

Multiple insights into lignin-mediated cadmium detoxification in rice (Oryza sativa)

Cadmium (Cd) is readily absorbed by rice and enters the food chain, posing a health risk to humans. A better understanding of the mechanisms of Cd-induced responses in rice will help in developing solutions to reduce Cd uptake in rice. Therefore, this research attempted to reveal the detoxification mechanisms of rice in response to Cd through physiological, transcriptomic and molecular approaches. The results showed that Cd stress restricted rice growth, led to Cd accumulation and H2O2 production, and resulted cell death. Transcriptomic sequencing revealed glutathione and phenylpropanoid were the major metabolic pathways under Cd stress. Physiological studies showed that antioxidant enzyme activities, glutathione and lignin contents were significantly increased under Cd stress. In response to Cd stress, q-PCR results showed that genes related to lignin and glutathione biosynthesis were upregulated, whereas metal transporter genes were downregulated. Further pot experiment with rice cultivars with increased and decreased lignin content confirmed the causal relationship between increased lignin and reduced Cd in rice. This study provides a comprehensive understanding of lignin-mediated detoxification mechanism in rice under Cd stress and explains the function of lignin in production of low-Cd rice to ensure human health and food safety.

A small extent of seawater intrusion significantly enhanced Cd uptake by rice in coastal paddy fields

Paddy fields located around estuaries suffer from seawater intrusion, and how and to what extent salinity levels influence Cd accumulation in rice grains is still unclear. Pot experiments were carried out by cultivating rice under alternating flooding and drainage conditions with different salinity levels (0.2‰, 0.6‰ and 1.8‰). The Cd availability was greatly enhanced at 1.8‰ salinity due to the competition for binding sites by cations and the formation of Cd complexation with anions, which also contributed to Cd uptake by rice roots. The soil Cd fractions were investigated and found that the Cd availability significantly decreased during flooding stage, while it rapidly increased after soil drainage. During drainage stage, Cd availability was greatly enhanced at 1.8‰ salinity mainly attributed to the formation of CdCl_n^2-n. The kinetic model was established to quantitatively evaluate Cd transformation, and it found that the release of Cd from organic matter and Fe-Mn oxides was greatly enhanced at 1.8‰ salinity. The results of pot experiments showed that there was a significant increase in Cd content in rice roots and grains in the treatment of 1.8‰ salinity, because the increasing salinity induced an increase in Cd availability and upregulation of key genes regulating Cd uptake in rice roots. Our findings elucidated the key mechanisms by which high salinity enhanced Cd accumulation in rice grains, and more attention should be given to the food safety of rice cultivated around estuaries.

The Expression of the StNRAMP2 Gene Determined the Accumulation of Cadmium in Different Tissues of Potato

Cadmium (Cd) is a toxic metal that threatens human health when enriched in crops. NRAMPs are a family of natural macrophage proteins reported to play a key role in Cd transport in plants. In order to explore the gene regulation mechanism of potato under Cd stress and the role of NRAMPs family in it, this study analyzed the gene expression differences of two different Cd accumulation levels in potato after 7 days of 50 mg/kg Cd stress and screened out the key genes that may play a major role in the differential accumulation of Cd in different varieties. Additionally, StNRAMP2 was selected for verification. Further verification showed that the StNRAMP2 gene plays an important role in the accumulation of Cd in potato. Interestingly, silencing StNRAMP2 increased Cd accumulation in tubers but significantly decreased Cd accumulation in other sites, suggesting a critical role of StNRAMP2 in Cd uptake and transport in potatoes. To further confirm this conclusion, we performed heterologous expression experiments in which overexpression of StNRAMP2 gene in tomato resulted in a threefold increase in Cd content, which further confirmed the important role of StNRAMP2 in the process of Cd accumulation compared with wild-type plants. In addition, we found that the addition of Cd to the soil increased the activity of the plant antioxidant enzyme system, and silencing StNRAMP2 partially reversed this effect. This suggests that the StNRAMP2 gene plays an important role in plant stress tolerance, and future studies could further explore the role of this gene in other environmental stresses. In conclusion, the results of this study improve the understanding of the mechanism of Cd accumulation in potato and provide experimental basis for remediation of Cd pollution.

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.

Synergistic Effects of Water Management and Silicon Foliar Spraying on the Uptake and Transport Efficiency of Cadmium in Rice (Oryza sativa L.)

To study the synergistic effects of water management and silicon (Si) foliar spraying on the uptake and transport of cadmium (Cd) in rice, we designed four treatments: conventional intermittent flooding + no Si foliar spraying (CK), continuous flooding throughout the growth stage + no Si foliar spraying (W), conventional intermittent flooding + Si foliar spraying (Si) and continuous flooding throughout the growth stage + Si foliar spraying (WSi). The results show that WSi treatment reduced the uptake and translocation of Cd by rice and significantly reduced the brown rice Cd content, with no effect on rice yield. Compared with CK, the Si treatment increased the net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of rice by 6.5-9.4%, 10.0-16.6% and 2.1-16.8%, respectively. The W treatment decreased these parameters by 20.5-27.9%, 8.6-26.8% and 13.3-23.3%, respectively, and the WSi treatment decreased them by 13.1-21.2%, 3.7-22.3% and 2.2-13.7%, respectively. The superoxide dismutase (SOD) and peroxidase (POD) activity decreased by 6.7-20.6% and 6.5-9.5%, respectively, following the W treatment. Following the Si treatment, SOD and POD activity increased by 10.2-41.1% and 9.3-25.1%, respectively, and following the WSi treatment, they increased by 6.5-18.1% and 2.6-22.4%, respectively. Si foliar spraying ameliorated the detrimental effects of continuous flooding throughout the growth stage on photosynthesis and antioxidant enzyme activity. We conclude that synergistic continuous flooding throughout the growth stage, combined with Si foliar spraying, can significantly block Cd uptake and translocation and is therefore an effective means of reducing the accumulation of Cd in brown rice.

A newly-constructed bifunctional bacterial consortium for removing butyl xanthate and cadmium simultaneously from mineral processing wastewater: Experimental evaluation, degradation and biomineralization

Due to the technological limitations associated with beneficiation technology, large amounts of flotation reagents and heavy metals remain in mineral processing wastewater. Unfortunately, however, no treatment methods are available to mitigate the resulting pollution by them. In this study, a bacterial consortium SDMC (simultaneously degrade butyl xanthate and biomineralize cadmium) was constructed in an effort to simultaneously degrade butyl xanthate (BX) and biomineralize cadmium (Cd) by screening and domesticating two different bacterial species including Hypomicrobium and Sporosarcina. SDMC is efficient in removing the combined pollution due to BX and Cd with a 100% degradation rate for BX and 99% biomineralization rate for Cd within 4 h. Besides, SDMC can tolerate high concentrations of Fe(III) (0-40 mg/L). It has an excellent ability to utilize Fe(III) for enhanced removal of the combined pollutants. SDMC can effectively remove pollutants with a pH range of 6-9. Further, we discussed pathways for potential degradation and biomineralization: Cd(BX)₂-Cd²⁺, BX^-; BX^--CS₂, butyl perxanthate (BPX); Cd²⁺-(Ca0.67,Cd0.33)CO₃. The removal of the combined pollutants primarily entails decomposition, degradation, and biomineralization, C-O bond cleavage, and microbially induced carbonate precipitation (MICP). SDMC is a simple, efficient, and eco-friendly bifunctional bacterial consortium for effective treatment of BX-Cd combined pollution in mineral processing wastewater.

Arbuscular mycorrhizal fungus regulates cadmium accumulation, migration, transport, and tolerance in Medicago sativa

Cadmium (Cd) pollution in croplands is a global environmental problem. Measures to improve the tolerance of sensitive crops and reduce pollutant absorption and accumulation are needed in contaminated agricultural areas, and inoculation with rhizosphere microorganisms to regulate plant resistance and heavy metal transport can provide an effective solution. A pot experiment was conducted to analyse the impact of arbuscular mycorrhizal fungi (AMF) on alfalfa oxidase activity, heavy metal resistance genes and transport proteins, metabolism, and other biochemical regulation mechanisms that lead to complexation, compartmentalisation, efflux, enrichment, and antioxidant detoxification pathways. The AMF reduced shoot and protoplasm Cd inflow, and promoted organic compound production (e.g., by upregulating HM-Res4 for 1.2 times), to complex with Cd, reducing its biological toxicity. The AMF increased the ROS scavenging efficiency and osmotic regulatory substance content of the alfalfa plants, reduced oxidative stress (ROS dereased), and maintained homeostasis. It also alleviated Cd inhibition of photosynthetic electron transport, tricarboxylic acid circulation, and nitrogen assimilation. These AMF effects improved leaf and root biomass by 43.87% and 59.71% and facilitated recovery of a conservative root economic strategy. It is speculated that AMF induces the resistance signal switch by regulating the negative feedback regulation mode of indole acetic acid upward transport and methyl jasmonate downward transmission in plants.

Biochar-supported nanoscale zero-valent iron can simultaneously decrease cadmium and arsenic uptake by rice grains in co-contaminated soil

Cadmium (Cd) and Arsenic (As) in rice grains are a primary exposure source for human beings. However, the simultaneous stabilization of Cd and As in soil becomes difficult due to the opposite properties of those. In this study, we investigated the simultaneous effects of biochar-supported nanoscale zero-valent iron (nZVI-BC) and water management on the decrease of Cd and As bioaccumulation in rice grain. Compared to the control, 0.25-1.00% nZVI-BC coupled with alternate wetting and drying (AWD) management simultaneously decreased the bioaccumulation of Cd and As in rice grains by 15.85-69.16% and 23.06-59.45%, respectively. The cancer risk associated with rice consumption effectively reduced by 15.60-52.41% after the application of nZVI-BC, and the lowest cancer risk was detected in 1.00% nZVI-BC under AWD management. Furthermore, rice cultivated under AWD management had a lower total cancer risk than that cultivated under continuous flooded (CF) management with the same amendment of type and dose. The reduction of soil Cd and As availability and the formation of iron plaque dominated the decrease of Cd and As uptake by rice grains. The elevated soil pH was responsible for Cd adsorption, and the dominant mechanism for As immobilization was the formation of complexes. The iron plaque was double-edged, promoting and inhibiting Cd uptake by rice, wherein the inhibition was predominant under aerobic conditions. In addition, iron plaque was a barrier to preventing the As accumulation by rice, a larger amount of As was immobilized on the iron plaque with nZVI-BC treatment. This study sheds new insights on the simultaneous remediation of Cd and As co-contaminated paddy fields.