Boron supplying alters cadmium retention in root cell walls and glutathione content in Capsicum annuum

Integration of comparative cytology, ionome, transcriptome and metabolome provide a basic framework for the response of foxtail millet to Cd stress

Apart from directly affecting the growth and development of crops, Cd in the soil can easily enter the human body through the food chain and pose a threat to human health. Therefore, understanding the toxicity of Cd to specific crops and the molecular mechanisms of their response to Cd is essential. In this study, hydroponic experiments were utilized to study the response of foxtail millet to Cd stress through phenotypic investigation, enzyme activity determination, ultrastructure, ionome, transcriptome and metabolome. With the increase in cadmium concentration, both the growth and photosynthetic capacity of foxtail millet seedlings are severely inhibited. The ultrastructure of cells is damaged, cells are deformed, chloroplasts swell and disappear, and cell walls thicken. Cd stress affects the absorption, transport, and redistribution of beneficial metal ions in the seedlings. Multi-omics analysis reveals the crucial roles of glycolysis, glutathione metabolism and phenylpropanoid and lignin biosynthesis pathways in Cd detoxification via energy metabolism, the antioxidant system and cell wall changes. Finally, a schematic diagram of foxtail millet in response to Cd stress was we preliminarily drew. This work provides a basic framework for further revealing the molecular mechanism of Cd tolerance in foxtail millet.

Exogenous Hydrogen sulfide attenuates cadmium toxicity to Chrysanthemum (Chrysanthemum indicum) by modulating glutathione synthesis and cadmium adsorption capacity in the cell wall

Soil cadmium (Cd) contamination leads to plant toxicity and poses a risk to human health both directly and indirectly through the food chain. Hydrogen sulfide (H2S), a novel gaseous signaling molecule, has been shown to enhance plant tolerance to various abiotic stresses. In this study, the potential of H2S in mitigating Cd toxicity in chrysanthemum (Chrysanthemum indicum) was investigated through physiological, biochemical and transcriptomic analyses. Results showed that the application of exogenous H2S resulted Cd accumulation in the roots by 21.15 %, while reducing Cd in the aboveground parts by 13.21 %. It was further found that H2S increased the pectin and hemicellulose content by 50.09 % and 49.79 %, respectively, through the regulation of cell wall polysaccharide synthesis-related genes, leading to changes in root functional group content and cell wall adsorption capacity for cadmium ions (Cd2+). Additionally, H2S also promoted the synthesis of GSH and PCs by regulating the expression of genes related to sulfur metabolism, chelating free Cd2+ in the cytoplasm, and reducing their harmful effects on the organelles. It was also found that exogenous H2S may have regulated the expression of transporter proteins by modulating the expression of transcription factors (MYB, AP2/ERF, and WRKY), thereby affecting the uptake, transport, and accumulation of Cd2+. In conclusion, exogenous H2S reduced the free Cd2+ content in the cytoplasm by promoting the adsorption of Cd2+ in the root cell walls and facilitating the synthesis of GSH and PCs in the cells, which in turn alleviated the toxic effects of Cd2+ on chrysanthemum.

Roles and mechanisms of boron in reducing cadmium accumulation in crops

Cadmium (Cd) contamination in farmland soil has become a global environmental concern, resulting in Cd pollution in crops. It is urgent to address this issue, and recent research has suggested that exogenous application of nutrient elements, including boron (B), is an effective strategy to decrease Cd accumulation in crops. Boron, an essential micronutrient for plants, plays a key role in maintaining cell integrity, promoting nitrogen metabolism, and enhancing nutrient absorption. Boron enhances Cd tolerance and reduces Cd accumulation in crops by regulating ROS homeostasis, heavy metal transport proteins, and promoting Cd chelation onto cell walls. However, the precise mechanism by which B regulates Cd accumulation remains unclear. This paper summarizes the physiological, biochemical, and molecular mechanisms by which B regulates Cd accumulation in crops. We believe that the rational application of B in light to moderate Cd-contaminated areas could effectively reduce Cd accumulation in crops, ensuring the safety of agricultural products in these regions.

MIR408-encoded peptide, miPEP408, regulates cadmium stress response through sulfur assimilation pathway

Small peptides encoded by pri-miRNAs (miPEPs), have been identified as significant plant growth and development regulators. However, their roles in plant-environment interactions and heavy metal stress response remain largely unexplored. Here, we demonstrate that Arabidopsis MIR408-encoded peptide (miPEP408) plays a significant role in cadmium (Cd) stress response by modulating the sulfur assimilation pathway. Using a combination of exogenous synthetic peptide assays, CRISPR/Cas9-mediated knockout mutants (miPEP408CR), and overexpression lines (miPEP408OX), we analyzed phenotypic and molecular levels to elucidate the function of miPEP408 under Cd stress. Our results suggest that miPEP408 regulates miR408 expression and its targets in response to Cd exposure. Plants treated with exogenous miPEP408 or overexpressing miPEP408 exhibited reduced glutathione (GSH) levels, suppression of sulfur assimilation pathway genes, and heightened sensitivity to Cd. miPEP408CR plants showed enhanced GSH levels, upregulation of sulfur assimilation genes, and improved Cd detoxification. Furthermore, miPEP408 influenced the expression of Cd transporters and Cd accumulation in plants. In conclusion, this study establishes miPEP408 as a key regulator of Cd stress response in Arabidopsis, functioning through modulation of the sulfur assimilation pathway and metal transporter gene expression. These findings underscore the indispensable role of miPEPs in enhancing plant resilience to heavy metal stress.

Effects of different eco-stoichiometric ratios of calcium and cadmium on the detoxification mechanisms of Capsicum annuum L. under cadmium stress

The eco-stoichiometry of Ca/Cd in soil significantly affects Cd uptake and accumulation by plants in carbonate regions. In this study, the physiological responses and detoxification mechanisms of Capsicum annuum L. (capsicum) were investigated based on the eco-stoichiometric relationship of Ca/Cd in production substrates under varying pH levels (5, 6, and 7). The results revealed that increased Ca/Cd ratio enhanced the Cd accumulation in roots at pH values of 5 and 6. The enrichment of Cd in stems and leaves gradually decreased with varying Ca/Cd ratios at different pH levels. In addition, root vigor, relative chlorophyll content, biomass, and catalase and peroxidase activities increased across various pH levels, while the concentration of protein carbonyl and malondialdehyde decreased. The ability of pectin and cellulose in the cell wall and that of soluble components within the cell to adsorb and partition Cd improved as the Ca/Cd ratio increased at different pH values. Notably, the effects of varying Ca/Cd ratios were most significant at pH 6. Overall, Ca enhanced the tolerance of capsicum to Cd stress, thereby promoting the fixation of Cd in root cells, reducing its transfer to aboveground tissues, and improving both the growth and antioxidant stress response. The effect was attributed to different Ca/Cd stoichiometric ratios, pH levels, and their interactions. These findings enhance the understanding of the mechanism of the interaction between Ca and Cd on crops in the karst agroecosystem.

Multiple insights into differential Cd detoxification mechanisms in new germplasms of mung bean (Vigna radiata L.) and potential mitigation strategy

Long-term cadmium (Cd) exposure inhibits plant growth and development, reduces crop yield and quality, and threatens food security. Exploring the Cd tolerance mechanisms and safe production of crops in Cd-contaminated environment has become a worldwide concern. In this study, mung bean (Vigna radiata L.) cultivar Sulu (SL) and its three mutant lines (20#, 09#, and 06#) were used to compare the difference in Cd absorption, accumulation, and tolerance through pot and field experiments. 20#, 09#, and 06# are Cd-tolerant germplasms of mung bean but exist in different Cd tolerance mechanisms, 20# exhibited the lowest Cd absorption capacity, 09# possessed lower Cd translocation capacity, while 06# accumulated more Cd in protoplasts. Mung bean germplasms with higher Cd tolerance generally showed lower absorption capacity and intracellular accumulation of Cd. Besides, Cd accumulation in mung bean seeds is mainly depended on the absorption and translocation of Cd in roots and the Cd concentration in leaves, exogenous Mn supply inhibited the Cd2+ net influx of roots and Cd accumulation in seeds, this trend was more pronounced in mung bean germplasms with higher Cd accumulation and absorption. Moreover, we characterized a Cd transporter gene VrNramp5, which was differentially expressed in different mung bean lines, overexpression of VrNramp5 increased Cd accumulation and was accompanied by Cd-sensitive phenotype in transgenic mung bean seedlings, and the Cd concentration of mung bean was significantly positively correlated with the expression levels of VrNramp5. Taken together, our findings demonstrated that different Cd tolerance mechanisms exist in mung bean. 20# is the new Cd-tolerant germplasm with low Cd absorption capacity and Cd accumulation in seeds, and has great potential for the safe production of mung bean in Cd-contaminated soils and the breeding of low Cd accumulation crop cultivars.

The induction of polyamines metabolism pathway and membrane stability with silicon alleviate the vanadium toxicity in pepper plants

The vanadium (V) toxicity predominantly is the primary limitation in restraining pepper growth. The silicon (Si) in pepper plants induced the transcript level of the polyamines metabolism pathway genes, including the arginase (CbARG), ornithine decarboxylase (CbODC), arginine decarboxylase (CbADC), N-carbamoylputrescine amidase (CbNCA), Spermidine synthase (CbSPDS), copper binding diamine oxidase (CbCuAO) to overcome the V toxicity. The polyamines, including the Spm, Spd, and Put, induced with Si about 41.37 %, 33.12 %, and 27.90 %, respectively, in V stress. Moreover, the Si application decline in the leaf and root V contents, which was around 49.5 % and 40.74 %, respectively, then the V stress plants. The soluble protein, proline, and Si level in root/leaf with Si treatment significantly induced around 55.55/50.22 %, 42.85/55.35 %, and 49.92/85.29 %, respectively, as compared to the V stress. Si also heightened the nitrate reductase (NR), phosphoenolpyruvate carboxylase (PEPC), and malate dehydrogenase (MDH) levels. Our study revealed that Si maintained the PSII integrity and induced PSII efficiency genes. Si preserves the membrane stability, as evidenced by less accumulation in EL, H2O2, and MDA levels. The Si also induces the AsA-GSH to eliminate the reactive oxygen species (ROS) in the pepper plants. In summary, our research elucidated that Si addition improved pepper plants' tolerance to V toxicity.

Boron's Role in Diminishing Cadmium Concentrations in Rice (Oryza sativa L.): Insights into Absorption Inhibition and Ripening Promotion

Boron, a crucial element for plant growth, has been demonstrated to mitigate cadmium (Cd) absorption in rice seedlings. However, its impact on Cd accumulation in rice grains and the underlying regulatory mechanisms remain poorly understood. The current study explored the roles of boron in reducing Cd accumulation and promoting ripening in rice through pot and hydroponic experiments. The results revealed that the basal boron application (1.5 mg kg-1) decreased grain Cd concentration by 61.1%, primarily due to the synergistic effects of inhibited Cd uptake and transport, along with increased maturation. Boron mitigated the root Cd2+ influx by 32.4% and transport factors by 36.0-47.3% primarily by downregulating the expression of OsNramp5, OsIRT1, and OsHMA2. Moreover, boron enhanced the activities of key sucrose-metabolizing enzymes and increased the relative expression levels of genes associated with sugar metabolism and transport, thereby shortening the rice growth period from 132 to 120 d. Field experiment confirmed that boron application decreased rice grain Cd concentration by 47.7% while promoting earlier maturation. This study elucidates the mechanism behind boron's ability to lower grain Cd levels and highlights its potential as an effective agronomic approach to mitigate food safety risks in rice grown on Cd-contaminated paddy soils.

Boron contributes to enhance antimony tolerance in rice (Oryza sativa L.) by activating antioxidant system, modifying the cell wall component and promoting cell wall deposition of Sb

Boron (B) is essential for plant growth and helps mitigate metal toxicity in various crop plants. However, the potential role and underlying mechanisms of B in alleviating antimony (Sb) toxicity in rice remain unexplored. In this study, we investigated the effects of H₃BO₃ supplementation (30, 50, and 75 μM) on morphological growth, physiological and biochemical traits, Sb content, and the subcellular distribution of Sb in rice plants under 100 μM Sb stress during the seedling stage in a hydroponic system. The results revealed that Sb toxicity severely impaired rice growth, reducing shoot biomass by 38.3%, shoot and root length by 38.9% and 23.2%, and leaf relative water content by 15.5%. Supplementation with 30 μM B mitigated these adverse effects by enhancing photosynthesis and chlorophyll synthesis, restoring root activity, and improving oxidative balance through increased antioxidant enzyme activities in rice tissues. Furthermore, B supplementation significantly reduced Sb concentration in roots by 56.28%, while promoting Sb distribution in the cell wall (CW) fraction. Scanning electron microscopy equipped with energy-dispersive X-ray (SEM-EDS) microanalysis confirmed that B enhanced Sb adsorption on root CWs. Fourier transform infrared spectroscopy (FTIR) analysis indicated increased carboxyl groups in the CWs following B application under Sb treatment. Moreover, B supplementation increased the levels of pectin and hemicellulose and elevated pectin methylesterase (PME) activity by 22.0%, 69.0%, and 29.0% in roots, respectively, thus promoting Sb chelation onto the CWs. Taken together, our results provide a scientific basis and theoretical guidance for applying B to alleviate Sb toxicity in crops.

Roles of boron in preventing cadmium uptake by Capsicum annuum root tips: Novel insights from ultrastructural investigation and single-cell RNA sequencing

Sufficient boron (B) can reduce cadmium (Cd) accumulation in crops; however, the underlying mechanism remains unclear. Ultrastructural analysis and single-cell RNA sequencing were used to investigate the changes of the Casparian strip of hot pepper (Capsicum annuum L.) root tips exposed to Cd under different B supplements, lignin formation-related gene expression and regulation in the endodermis to deeper understand the molecular mechanisms by which B inhibits root Cd uptake. The results showed that the Casparian strip widths significantly increased in hot pepper root tips exposed to Cd under B-sufficient (B1Cd1) conditions compared to those under B-deficient conditions (B0Cd1). Additionally, more cerium precipitates, which indicate H2O2 accumulation, were observed in the Casparian strip region in B0Cd1 than that in B1Cd1. B supplementation markedly enhanced the expression of certain lignin formation-related genes only in the endodermis cells. These genes included transcription factor genes, WRKY (WRKY7/40/41/53) and ERF (ERF2/109), and two types of genes related to lignin formation, namely, PER genes (PER3/9/49/6472 and LAC3) and dirigent protein (DIR) genes (DIR16/21/24/25). It suggests that the main reason for B reducing Cd accumulation in hot peppers is that B-induced endodermal lignification of root tips under Cd exposure is beneficial to prevent Cd influx into the stele via the apoplastic pathway.

SpbZIP60 confers cadmium tolerance by strengthening the root cell wall compartmentalization in Sedum plumbizincicola

Cadmium (Cd) is a prominent heavy metal pollutant that inhibits plant growth and poses risks to human health. Sedum plumbizincicola, as a Cd/Zn/Pb hyperaccumulator species, exhibits robust resistance to heavy metals and effective enrichment capacities. In our previous study, overexpressing SpbZIP60 in Arabidopsis enhanced Cd tolerance; however, the underlying the molecular mechanism remains to be elucidated. Here, we identified SpbZIP60 as a representative Cd stress response factor with nuclear localization and transcriptional activation activity. SpbZIP60 underwent conservative splicing in response to endoplasmic reticulum (ER) stress, while its response to Cd stress is independent of the ER stress-mediated unfolded protein response pathway. Overexpression of SpbZIP60 in S. alfredii increased the Cd tolerance and antioxidant activity. Furthermore, SpbZIP60 increased the content of cell wall components and thickened cell wall under Cd stress. Transcriptome analysis indicated a significant enrichment of differentially expressed genes within the phenylpropanoid metabolism pathway. Besides, the binding of SpbZIP60 to the promoter region of SpBglu resulted in the activation of gene expression, thereby enhancing the process of lignin deposition. Collectively, our results elucidated a molecular regulatory model in which SpbZIP60 increased the thickness of the root cell wall to impede Cd entry into the cell, consequently improving Cd tolerance.

Defense guard: strategies of plants in the fight against Cadmium stress

Soil Cadmium (Cd) contamination is a worldwide problem with negative impacts on human health. Cultivating the Cd-Pollution Safety Cultivar (Cd-PSC) with lower Cd accumulation in edible parts of plants is an environmentally friendly approach to ensure food security with wide application prospects. Specialized mechanisms have been addressed for Cd accumulation in crops. This review provides an extensive generality of molecular regulation mechanisms involved in Cd absorption, transport, detoxification, and tolerance in plants, highlighting key aspects of rhizosphere, apoplast barrier, Cd uptake, transfer, and cellular repair strategies under Cd stress. Additionally, we summarize the possible approaches for lowering the Cd accumulation crops, including molecular-assistant breeding, applying chemical materials, and microbial strategy to decrease Cd content in edible parts and improve Cd tolerance of crops under Cd stress. This review would provide valuable insights for cultivating low Cd accumulated crop cultivars, ultimately contributing to food safety.

Mechanism of foliar application of boron to alleviate cadmium toxicity in winter wheat (Triticum aestivum L.)

Our previous work shown that boron (B) can inhibit cadmium (Cd) absorption in winter wheat, while the mechanism is not well understood. In this study, the mechanism of foliage spray B on Cd uptake in Cd-contaminated farmland was characterized with three treatments: spraying distilled water (CK), spraying 0.3% B fertilizer (F0.3%), and spraying 0.6% B fertilizer (F0.6%). The spraying treatment was conducted during the jointing stage and booting stage, respectively. The objective was to investigate the impact of different concentrations of B fertilizer application on Cd content, yield, physiological biochemical response, Cd transporter in wheat. Results showed that the yield increased with increasing B spray concentration compared with CK treatment. The wheat yield increased by 1.29%∼12.50% under foliar B application. The Cd concentration in both shoot and root of wheat seedlings were significantly decreased with foliar application of B at jointing stage and booting stage. The Cd concentrations in root and seed were also decreased with B treatment at maturity stage, especially after treatment with F0.6%, the Cd absorption in wheat grains exhibited a significant reduction of 31.20%. In addition, foliar application of B significantly increased antioxidant enzyme activities, include APX, SOD, CAT, and POD. Compared with CK, the MDA content in leaves showed a significant decrease with 30.48%-50.14%, while the GSH content showed an increase with 76.32%-1.05%. The down-regulation of the uptake and transport genes (TaNramp5, TaLCT1 and TaHMA2) and the up-regulation of compartmentalization transport genes (TaTM20 and TaHMA3) may contribute to the reduction of Cd accumulation in shoot. Overall, our results suggest that foliar spraying of B could increase B accumulation and yield, and alleviate Cd toxicity by reducing Cd uptake, enhancing the antioxidant capacity, regulating the expression of Cd genes in wheat.

Synergistic effects of boron and cadmium on the metal enrichment and cell wall immobilization capacity of Cosmosbipinnatus

Cadmium (Cd), as a heavy metal pollutant, can seriously affect plant growth and development. Boron (B), as an indispensable nutrient element, plays an important role in plant growth and cell wall (CW) synthesis. However, the physiological effects of B and Cd on plant growth and the mechanism of Cd chelation by the CW remain unclear. Here, we investigate the effect of exogenous B on Cd accumulation in CW components of Cosmos bipinnatus roots and its mechanism of Cd mitigation. Under B deficiency and single Cd (30 μM) treatments, the growth of C. bipinnatus was significantly inhibited, but the addition of exogenous B significantly increased plant biomass, which increased the Cd content in the underground parts of C. bipinnatus by 20.18% and reduced the Cd translocation factor by 22.22%. Meanwhile, application of exogenous B affected the subcellular Cd content across various Cd forms and alleviated Cd-induced oxidative stress in C. bipinnatus. Additionally, exogenous B and Cd and their mixtures affected the functional groups of the root CW, the proportion of polysaccharide components, the Cd content of polysaccharides, and the polysaccharide uronic acid content of C. bipinnatus. However, B application increased 3-deoxy-oct-2-ulosonic acid content, pectin esterase activity, low esterified pectin content, and its Cd content by 149.52%, 55.69%, 206.38%, and 150.02%, respectively, compared to Cd treatment alone. Thus, our study showed that B mitigates the toxicity of Cd to plants, revealing the effect of B on the physiological aspects of Cd tolerance in plants.

Hydroxamate Siderophores Intensify the Co-Deposition of Cadmium and Silicon as Phytolith-Like Particulates in Rice Stem Nodes: A Natural Strategy to Mitigate Grain Cadmium Accumulation

Sequestration of cadmium (Cd) in rice phytolith can effectively restrict its migration to the grains, but how hydroxamate siderophore (HDS) affects phytolith formation within rice plants especially the fate of Cd and silicon (Si) remains poorly understood. Here, we found that the addition of HDS increased the content of dissolved Si and Cd in soil pore water as well as its absorption by the rice roots during the reproductive growth stage. HDS effectively trapped orthosilicic acid and Cd ions at the third stem nodes of rice plants via hydrogen bonds and chelation interactions, which then rapidly deposited on the xylem cell wall through hydrophobic interactions. Ultimately, Cd was immobilized as phytolith-like particulates in the form of CdSiO3. Field experiments verified that Cd accumulation was significantly reduced by 46.4% in rice grains but increased by 41.2% in rice stems after HDS addition. Overall, this study advances our understanding of microbial metabolites enhancing the instinctive physiological barriers within rice plants.

Integrated proteomics, transcriptomics, and metabolomics offer novel insights into Cd resistance and accumulation in Poa pratensis

Kentucky bluegrass (Poa pratensis L., KB) demonstrates superior performance in both cadmium (Cd) accumulation and tolerance; however, the regulatory mechanisms and detoxification pathways in this species remain unclear. Therefore, phenotype, root ultrastructure, cell wall components, proteomics, transcriptomics, and metabolomics were analyzed under the hydroponic system to investigate the Cd tolerance and accumulation mechanisms in the Cd-tolerant KB variety 'Midnight (M)' and the Cd-sensitive variety 'Rugby II (R)' under Cd stress. The M variety exhibited higher levels of hydroxyl and carboxyl groups as revealed by Fourier transform infrared spectroscopy spectral analysis. Additionally, a reduced abundance of polysaccharide degradation proteins was observed in the M variety. The higher abundance of glutathione S-transferase and content of L-cysteine-glutathione disulfide and oxidized glutathione in the M variety may contribute to better performance of the M variety under Cd stress. Additionally, the R variety had an enhanced content of carboxylic acids and derivatives, increasing the Cd translocation capacity. Collectively, the down-regulation of cell wall polysaccharide degradation genes coupled with the up-regulation of glutathione metabolism genes enhances the tolerance to Cd stress in KB. Additionally, lignification of the endodermis and the increase in carboxylic acids and derivatives play crucial roles in the redistribution of Cd in KB.

CIP1, a CIPK23-interacting transporter, is implicated in Cd tolerance and phytoremediation

Environmental pollution from cadmium (Cd) presents a serious threat to plant growth and development. Therefore, it's crucial to find out how plants resist this toxic metal to develop strategies for remediating Cd-contaminated soils. In this study, we identified CIP1, a transporter protein, by screening interactors of the protein kinase CIPK23. CIP1 is located in vesicles membranes and can transport Cd2+ when expressed in yeast cells. Cd stress specifically induced the accumulation of CIP1 transcripts and functional proteins, particularly in the epidermal cells of the root tip. CIKP23 could interact directly with the central loop region of CIP1, phosphorylating it, which is essential for the efficient transport of Cd2+. A loss-of-function mutation of CIP1 in wild-type plants led to increased sensitivity to Cd stress. Conversely, tobacco plants overexpressing CIP1 exhibited improved Cd tolerance and increased Cd accumulation capacity. Interestingly, this Cd accumulation was restricted to roots but not shoots, suggesting that manipulating CIP1 does not risk Cd contamination of plants' edible parts. Overall, this study characterizes a novel Cd transporter, CIP1, with potential to enhance plant tolerance to Cd toxicity while effectively eliminating environmental contamination without economic losses.

Physiological and molecular mechanisms of ZnO quantum dots mitigating cadmium stress in Salvia miltiorrhiza

This study delved into the physiological and molecular mechanisms underlying the mitigation of cadmium (Cd) stress in the model medicinal plant Salvia miltiorrhiza through the application of ZnO quantum dots (ZnO QDs, 3.84 nm). A pot experiment was conducted, wherein S. miltiorrhiza was subjected to Cd stress for six weeks with foliar application of 100 mg/L ZnO QDs. Physiological analyses demonstrated that compared to Cd stress alone, ZnO QDs improved biomass, reduced Cd accumulation, increased the content of photosynthetic pigments (chlorophyll and carotenoids), and enhanced the levels of essential nutrient elements (Ca, Mn, and Cu) under Cd stress. Furthermore, ZnO QDs significantly lowered Cd-induced reactive oxygen species (ROS) content, including H2O2, O2-, and MDA, while enhancing the activity of antioxidant enzymes (SOD, POD, APX, and GSH-PX). Additionally, ZnO QDs promoted the biosynthesis of primary and secondary metabolites, such as total protein, soluble sugars, terpenoids, and phenols, thereby mitigating Cd stress in S. miltiorrhiza. At the molecular level, ZnO QDs were found to activate the expression of stress signal transduction-related genes, subsequently regulating the expression of downstream target genes associated with metal transport, cell wall synthesis, and secondary metabolite synthesis via transcription factors. This activation mechanism contributed to enhancing Cd tolerance in S. miltiorrhiza. In summary, these findings shed light on the mechanisms underlying the mitigation of Cd stress by ZnO QDs, offering a potential nanomaterial-based strategy for enhancing Cd tolerance in medicinal plants.

Boron-mediated amelioration of copper toxicity in Citrus sinensis seedlings involved reduced concentrations of copper in leaves and roots and their cell walls rather than increased copper fractions in their cell walls

Little information is available on how boron (B) supplementation affects plant cell wall (CW) remodeling under copper (Cu) excess. 'Xuegan' (Citrus sinensis) seedlings were submitted to 0.5 or 350 µM Cu × 2.5 or 25 µM B for 24 weeks. Thereafter, we determined the concentrations of CW materials (CWMs) and CW components (CWCs), the degree of pectin methylation (DPM), and the pectin methylesterase (PME) activities and PME gene expression levels in leaves and roots, as well as the Cu concentrations in leaves and roots and their CWMs (CWCs). Additionally, we analyzed the Fourier transform infrared (FTIR) and X-ray diffraction (XRD) spectra of leaf and root CWMs. Our findings suggested that adding B reduced the impairment of Cu excess to CWs by reducing the Cu concentrations in leaves and roots and their CWMs and maintaining the stability of CWs, thereby improving leaf and root growth. Cu excess increased the Cu fractions in leaf and root pectin by decreasing DPM due to increased PME activities, thereby contributing to citrus Cu tolerance. FTIR and XRD indicated that the functional groups of the CW pectin, hemicellulose, cellulose, and lignin could bind and immobilize Cu, thereby reducing Cu cytotoxicity in leaves and roots.

Phosphorus deficiency-induced cell wall pectin demethylesterification enhances cadmium accumulation in roots of Salix caprea

Regions affected by heavy metal contamination frequently encounter phosphorus (P) deficiency. Numerous studies highlight crucial role of P in facilitating cadmium (Cd) accumulation in woody plants. However, the regulatory mechanism by which P affects Cd accumulation in roots remains ambiguous. This study aims to investigate the effects of phosphorus (P) deficiency on Cd accumulation, Cd subcellular distribution, and cell wall components in the roots of Salix caprea under Cd stress. The results revealed that under P deficiency conditions, there was a 35.4% elevation in Cd content in roots, coupled with a 60.1% reduction in Cd content in shoots, compared to the P sufficiency conditions. Under deficient P conditions, the predominant response of roots to Cd exposure was the increased sequestration of Cd in root cell walls. The sequestration of Cd in root cell walls increased from 37.1% under sufficient P conditions to 66.7% under P deficiency, with pectin identified as the primary Cd binding site under both P conditions. Among cell wall components, P deficiency led to a significant 31.7% increase in Cd content within pectin compared to P sufficiency conditions, but did not change the pectin content. Notably, P deficiency significantly increased pectin methylesterase (PME) activity by regulating the expression of PME and PMEI genes, leading to a 10.4% reduction in the degree of pectin methylesterification. This may elucidate the absence of significant changes in pectin content under P deficiency conditions and the concurrent increase in Cd accumulation in pectin. Fourier transform infrared spectroscopy (FTIR) results indicated an increase in carboxyl groups in the root cell walls under P deficiency compared to sufficient P treatment. The results provide deep insights into the mechanisms of higher Cd accumulation in root mediated by P deficiency.

The cadmium tolerance enhancement through regulating glutathione conferred by vacuolar compartmentalization in Aspergillus sydowii

Aspergillus was found to be a vital hyperaccumulation species for heavy metal removal with admirable tolerance capacity. But the potential tolerance mechanism has not been completely studied. This study quantified the amounts of total cadmium (Cd), Cd2+, glutathione (GSH), and reactive oxygen species (ROS) in the protoplasts and vacuoles of mycelium. We modulated GSH synthesis using buthionine sulfoximine (BSO) and 2-oxothiazolidine-4-carboxylic acid (OTC) to investigate the subcellular regulatory mechanisms of GSH in the accumulation of Cd. The results confirmed that GSH plays a crucial role in vacuolar compartmentalization under Cd stress. GSH and GSSG as a redox buffer to keep the cellular redox state in balance and GSH as a metal chelating agent to reduce toxicity. When regulating the decreased GSH content with BSO, and increased GSH content with OTC, the system of Cd-GSH-ROS can change accordingly, this also supported that vacuolar compartmentalization is a detoxification strategy that can modulate the transport and storage of substances inside and outside the vacuole reasonably. Interestingly, GSH tended to be distributed in the cytoplasm, the battleground of redox takes place in the cytoplasm but not in the vacuole. These finding potentially has implications for the understanding of tolerance behavior and detoxification mechanisms of cells. In the future bioremediation of Cd in soil, the efficiency of soil remediation can be improved by developing organisms with high GSH production capacity.

Transcriptome and ultrastructural analysis revealed the mechanism of Mercapto-palygorskite on reducing Cd content in wheat

Large areas of crop yields in northern China have faced with cadmium (Cd) contamination problems. Mercapto-modified palygorskite (MP), as a highly efficient immobilization material, could reduce Cd absorption in wheat and alleviate its biotoxicity. However, the molecular mechanism underlying MP-mediated Cd reduction and detoxification processes in wheat is not well understood. This aim of this study was to investigate the biochemical and molecular mechanisms underlying the reduction in Cd accumulation in wheat (Triticum aestivum L.). The results showed that MP application decreased the Cd concentration by 68.91-74.32% (root) and 70.68-77.2% (shoot), and significantly increased the glutathione (GSH) and phytochelatins (PCs) contents in root and shoot. In addition, with the application of MP, the percentage of Cd in the cell walls and organelles of wheat decreased, while that of Cd in soluble components was increased. The content of Cd in all components was significantly reduced. Ultrastructural analysis revealed that MP thickened the cell wall, promoted vesicle formation in the membrane and protected the integrity of intracellular organelles in wheat. Transcriptome analysis further confirmed the above results. MP upregulated the expression of several genes (CCR, CAD COMT and SUS) involved in cell wall component biosynthesis and promoted vesicle formation on cell membranes by upregulating the expression of PLC and IPMK genes. In addition, genes related to antioxidant synthesis (PGD, glnA and GSS) and photosynthesis (Lhca, Lhcb) were altered by MP to alleviate Cd toxicity in wheat. This present work will help to more thoroughly elucidate the molecular mechanism by which wheat defends against Cd contamination under MP application and provide and important research basis for the application of this material in the future.

Unveiling mechanisms of silicon-mediated resistance to chromium stress in rice using a newly-developed hierarchical system

Silicon (Si) has been well-known to enhance plant resistance to heavy-metal stress. However, the mechanisms by which silicon mitigates heavy-metal stress in plants are not clear. In particular, information regarding the role of Si in mediating resistance to heavy-metal stress at a single cell level is still lacking. Here, we developed a hierarchical system comprising the plant, protoplast, and suspension cell subsystems to investigate the mechanisms by which silicon helps to alleviate the toxic effects of trivalent chromium [Cr(III)] in rice. Our results showed that in whole-plant subsystem silicon reduced shoot Cr(III) concentration, effectively alleviating Cr(III) stress in seedlings and causing changes in antioxidant enzyme activities similar to those observed at lower Cr(III) concentrations without silicon added. However, in protoplast subsystem lacking the cell wall, no silicon deposition occurred, leading to insignificant changes in cell survival or antioxidation processes under Cr(III) stress. Conversely, in suspension cell subsystem, silicon supplementation substantially improved cell survival and changes in antioxidant enzyme activities under Cr(III) stress. This is due to the fact that >95% of silicon was on the cell wall, reducing Cr(III) concentration in cells by 7.7%-10.4%. Collectively, the results suggested that the silicon deposited on the cell wall hindered Cr(III) bio-uptake, which consequently delayed Cr(III)-induced changes in antioxidant enzyme activities. This research emphasizes the significance of cell walls in Si-alleviated heavy-metal stress and deepens our understanding of silicon functioning in plants. Furthermore, the hierarchical system has great potential for application in studying the functioning of other elements in plant cell walls.

Amino acid application inhibits root-to-shoot cadmium translocation in Chinese cabbage by modulating pectin methyl-esterification

The exogenous application of amino acids (AAs) generally alleviates cadmium (Cd) toxicity in plants by altering their subcellular distribution. However, the physiological mechanisms underlying AA-mediated cell wall (CW) sequestration of Cd in Chinese cabbage remain unclear. Using two genotypes of Chinses cabbage, Jingcui 60 (Cd-tolerant) and 16-7 (Cd-sensitive), we characterized the root structure, subcellular distribution of Cd, CW component, and related gene expression under the Cd stress. Cysteine (Cys) supplementation led to a reduction in the Cd concentration in the shoots of Jingcui 60 and 16-7 by 65.09 % and 64.03 %, respectively. Addition of Cys alleviated leaf chlorosis in both cultivars by increasing Cd chelation in the root CW and reducing its distribution in the cytoplasm and organelles. We further demonstrated that Cys supplementation mediated the downregulation of PMEI1 expression and improving the activity of pectin methyl-esterase (PME) by 17.98 % and 25.52 % in both cultivars, respectively, compared to the Cd treatment, resulting in an approximate 12.00 %-14.70 % increase in Cd retention in pectin. In contrast, threonine (Thr) application did not significantly alter Cd distribution in the shoots of either cultivar. Taken together, our results suggest that Cys application reduces Cd root-to-shoot translocation by increasing Cd sequestration in the root CW through the downregulation of pectin methyl-esterification.

Physiological and molecular mechanisms of boron in alleviating cadmium toxicity in Capsicum annuum

Soil cadmium (Cd) contamination threatens food safety and human health, particularly in developing countries. Previously, we have proposed that boron (B) could reduce Cd uptake and accumulation in hot peppers (Capsicum annuum) by regulating the expression of genes related to Cd transport in roots. However, only few studies have examined the role of B in plant leaves under Cd stress. It is unclear how B induces the expression of relevant genes and metabolites in hot pepper leaves and to what extent B is involved in leaf growth and Cd accumulation. The purpose of this study was to investigate the effects of B on growth and Cd accumulation in hot pepper leaves by determining physiological parameters and transcriptome sequencing. The results showed that B application significantly improved the concentration of chlorophyll a and intercellular CO2, stomatal conductance, and photosynthetic and transpiration rates by 18-41 % in Cd-stressed plants. Moreover, B enhanced Cd retention in the cell wall by upregulating the expression levels of pectin-, lignin-, and callose-related genes and improving the activity of pectin methylesterase by 30 %, resulting in an approximate 31 % increase in Cd retention in the cell wall. Furthermore, B application not only enhanced the expression levels of genes related to antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) and their activities by 28-40 %, thereby counteracting Cd-induced oxidative stress, but also improved Cd chelation, sequestration, and exclusion by upregulating the expression levels of genes related to sulfur metabolism, heavy metal-associated isoprenylated plant protein (HIPP), and transporters such as vacuolar cation/proton exchanger (CAX3), metal-nicotianamine transporter (YSL), ATP-binding cassette (ABC), zinc/iron transporters (ZIP) and oxic-compound detoxification (DTX), ultimately reinforcing Cd tolerance. Together, our results suggest that B application reduces the negative effects of Cd on leaf growth, promotes photosynthesis, and decreases Cd transfer to fruits through its sequestration and retention.

NPs-Ca promotes Cd accumulation and enhances Cd tolerance of rapeseed shoots by affecting Cd transfer and Cd fixation in pectin

The use of rapeseed (Brassica napus) as a hyperaccumulator plant has shown great promise for the remediation of cadmium (Cd) contaminated soils. Nanosized materials (NPs) have been shown to mitigate heavy metal toxicity in plants, but it is unknown how l-aspartate nano-calcium (NPs-Ca) affects Cd uptake, transport, and tolerance in rapeseed. A soil pot experiment was conducted with two treatments: a control treatment (CK) with 2.16 g CaCl2 and NPs-Ca treatment with 6.00 g NPs-Ca, to evaluate the effects and mechanisms of NPs-Ca on Cd tolerance in rapeseed. Compared to CaCl2, NPs-Ca promoted Cd transportation from roots to shoots by up-regulating the expression of Cd transport genes (ABCC12, HMA8, NRAM6, ZIP6, CAX4, PCR2, and HIP6). Therefore, NPs-Ca increased Cd accumulation in rapeseed shoots by 39.4%. Interestingly, NPs-Ca also enhanced Cd tolerance in the shoots, resulting in lower hydrogen peroxide (H2O2) accumulation and proline content, as well as higher antioxidant enzyme activities (POD, CAT). Moreover, NPs-Ca reduced the activity of pectin-degrading enzymes (polygalacturonase: PG, β-galactosidase: β-GAL), promoted the activity of pectin methyl esterase (PME), and changed transcription levels of related genes (PME, PMEI, PG, PGIP, and β-GAL). NPs-Ca treatment also significantly increased the Cd content in cell walls by 59.8%, that is, more Cd was immobilized in cell walls, and less Cd entered organelles in shoots of NPs-Ca treatment due to increased pectin content and degree of pectin demethylation. Overall, NPs-Ca increased Cd accumulation in rapeseed shoots by promoting Cd transport from roots to shoots. And meantime, NPs-Ca enhanced Cd tolerance of shoots by inhibiting pectin degradation, promoting pectin demethylation and increasing Cd fixation in pectin. These findings suggest that NPs-Ca can improve the potential of rapeseed as a hyperaccumulator for the remediation of Cd-contaminated soil and the protection of the environment. Furthermore, the study provides a theoretical basis for the application of NPs-Ca in the phytoremediation of Cd-contaminated soils with hyperaccumulating plants.

Transcriptome analysis reveals the molecular mechanism of different forms of selenium in reducing cadmium uptake and accumulation in wheat seedlings

Selenium (Se) can counteract cadmium (Cd) toxicity in wheat, but the molecular mechanism of different Se forms reducing Cd uptake and accumulation in wheat seedlings remain unclear. Here, a hydroponic experiment was conducted to investigate the effects of three Se forms (selenite (Se(IV)), selenate (Se(VI)) and seleno-L-methionine (SeMet)) on Cd2+ influx, Cd subcellular distribution, and Cd accumulation in wheat seedlings, and the underlying molecular mechanisms were investigated through transcriptome analysis. Consequently, Se(IV) and Se(VI) addition significantly reduced root Cd concentration by 74.3% and 80.8%, respectively, and all Se treatments significantly decreased shoot Cd concentration by approximately 34.2%-74.9%, with Se(IV) addition having the most pronounced reducing effect. Transcriptome analysis showed the reduction of Cd accumulation after Se(IV) addition was mainly due to the downregulation of Cd uptake genes. The inhibition of Cd accumulation after Se(VI) addition was not only associated with the downregulation of Cd uptake genes, but also related to the sequestration of Cd in vacuole. For SeMet addition, the reduction of Cd accumulation was mainly related to the sequestration of Cd in vacuole as GSH-Cd. The above findings provide novel insights to understand the effects of different forms of Se on Cd uptake and accumulation and tolerance in wheat.

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.

Boron decreases cadmium accumulation in water spinach by enhancing cadmium retention in the root cell walls

Cadmium (Cd) contamination and boron (B) deficiency are two major challenges associated with the farmland soils in Southern China. Therefore, this study was conducted to examine the impacts of B supply on Cd accumulation in water spinach (Ipomoea aquatica) using a cultivar (T308) with high Cd accumulation. The study further investigated the physiological mechanism behind the changes in Cd accumulation due to B supply. The findings revealed that B supply substantially reduced the Cd concentration in the leaves of water spinach by 41.20% and 37.16% under the Cd stress of 10 μM and 25 μM, respectively. Subcellular distribution of Cd showed that the Cd content as well as its proportion in root cell wall (RCW) increased significantly after B supply. Fourier transform infrared spectroscopy showed significant enrichment of negatively charged groups (such as -OH, -COOH, and -NH2) in the RCW after B supply. Overall, B supply also enhanced covalently bound pectin (CSP) content as well as the Cd content linked with CSP under Cd stress. These observations revealed that B regulated the Cd chelation in RCW, thereby reducing the amassment of Cd in water spinach.

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

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

Effect of boron supply on the uptake and translocation of cadmium in Capsicum annuum

Large areas of soil in southern China are contaminated with cadmium (Cd) and are deficient in boron (B). Previously, we suggested that B supplementation could reduce Cd accumulation in hot peppers (Capsicum annuum L.); however, the physiological mechanisms underlying this reduction remain unclear. In this study, the uptake and translocation of Cd in hot pepper plants were investigated using hydroponic experiments with different B and Cd treatments. A pot experiment was performed to verify whether B decreased the Cd concentration in hot peppers by minimizing the Cd translocation rate. The results of the dose- and time-dependent experiments showed that B supplementation reduced root Cd uptake and root-to-shoot Cd translocation. Additionally, B supplementation increased the root length, diameter, volume, surface area, and number of root forks and tips, as well as improving the relative absorbance of carboxyl groups under Cd exposure, leading to enhanced Cd fixation in the cell walls of the roots. As a result, the fruit Cd concentration decreased because B inhibited Cd translocation from the roots. Overall, the results demonstrate that B supplementation can reduce Cd accumulation in hot peppers by promoting normal root growth and development and by limiting the uptake and translocation of Cd.

Root cell wall remodeling: A way for exopolysaccharides to mitigate cadmium toxicity in rice seedling

Exopolysaccharides (EPS) are macromolecules with environment beneficial properties. Currently, numerous studies focus on the absorption of heavy metals by EPS, but less attention has been paid to the effects of EPS on the plants. This study explored the effects of EPS from Lactobacillus plantarum LPC-1 on the structure and function of cell walls in rice seedling roots under cadmium (Cd) stress. The results showed that EPS could regulate the remodeling process of the cell walls of rice roots. EPS affects the synthesis efficiency and the content of the substances that made up the cell wall, and thus plays an essential role in limiting the uptake and transport of Cd in rice root. Furthermore, EPS could induce plant resistance to heavy metals by regulating the lignin biosynthesis pathway in rice roots. Finally, the cell wall remodeling induced by EPS likely contributes to plant stress responses by activating the reactive oxygen species (ROS) signaling.

Characterization of cadmium accumulation mechanism between eggplant (Solanum melongena L.) cultivars

Excessive cadmium (Cd) accumulation in vegetables due to farmland pollution constitutes a serious threat to human health. Eggplant has a tendency to accumulate Cd. To investigate the mechanism of the differences in Cd accumulation levels between high-Cd (BXGZ) and low-Cd (MYQZ) eggplant cultivar, physiological and biochemical indicators and mRNA expression of eggplant were examined using photosynthetic apparatus, biochemical test kits, Fourier transform infrared (FTIR) spectroscopy and transcriptome sequencing, etc. The results of biochemical test kits and FTIR revealed that MYQZ enhanced pectin methylesterase (PME) activity, and lignin and pectin content in the root cell wall, which was associated with the upregulation of PME, cinnamyl-alcohol dehydrogenase and peroxidase (PODs). Higher levels of cysteine and glutathione (GSH) contents and upregulation of genes associated with sulfur metabolism, as well as higher expression of ATP-binding cassette transporters (ABCs), cation exchangers (CAX) and metal tolerance proteins (MTPs) were observed in MYQZ. In BXGZ, the higher stomatal density and stomatal aperture as well as higher levels of Ca²⁺ binding protein-1 (PCaP1) and aquaporins and lower levels of A2-type cyclins (CYCA2-1) are consistent with an enhanced transpiration rate in BXGZ. Furthermore, a more developed root system was shown to be associated with higher levels of auxin response factor (ARF19), GATA transcription factors (GATA4, 5 and 11) and auxin efflux carrier component (PIN5) in BXGZ. In conclusion, highly active PME, and higher levels of lignin and pectin in MYQZ are expected to reduce Cd toxicity, while Cd translocation can be inhibited with the help of ABC and other Cd transporters. As for BXGZ, the uptake and translocation of Cd were enhanced by the developed root system and stronger transpiration.

Interaction between Boron and Other Elements in Plants

Boron (B) is an essential mineral nutrient for growth of plants, and B deficiency is now a worldwide problem that limits production of B deficiency-sensitive crops, such as rape and cotton. Agronomic practice has told that balanced B and other mineral nutrient fertilizer applications is helpful to promote crop yield. In recent years, much research has reported that applying B can also reduce the accumulation of toxic elements such as cadmium and aluminum in plants and alleviate their toxicity symptoms. Therefore, the relation between B and other elements has become an interesting issue for plant nutritionists. Here we summarize the research progress of the interaction between B and macronutrients such as nitrogen, phosphorus, calcium, potassium, magnesium, and sulfur, essential micronutrients such as iron, manganese, zinc, copper, and molybdenum, and beneficial elements such as sodium, selenium, and silicon. Moreover, the interaction between B and toxic elements such as cadmium and aluminum, which pose a serious threat to agriculture, is also discussed in this paper. Finally, the possible physiological mechanisms of the interaction between B and other elements in plants is reviewed. We propose that the cell wall is an important intermediary between interaction of B and other elements, and competitive inhibition of elements and related signal transduction pathways also play a role. Currently, research on the physiological role of B in plants mainly focuses on its involvement in the structure and function of cell walls, and our understanding of the details for interactions between B and other elements also tend to relate to the cell wall. However, we know little about the metabolic process of B inside cells, including its interactions with other elements. More research is needed to address the aforementioned research questions in future.