Co-overexpression FIT with AtbHLH38 or AtbHLH39 in Arabidopsis-enhanced cadmium tolerance via increased cadmium sequestration in roots and improved iron homeostasis of shoots

The transcription factor ZmbHLH105 confers cadmium tolerance by promoting abscisic acid biosynthesis in maize

Cadmium (Cd), a highly toxic heavy metal, profoundly impacts crop productivity. The bHLH-type transcription factors regulate plant stress responses, yet their involvement in maize's Cd stress response remains unclear. Here, we studied ZmbHLH105, a maize bHLH gene induced by Cd exposure. Overexpression of ZmbHLH105 in maize seedlings, which were treated with 1.0 mM CdCl2 for 7 days, increased endogenous ABA levels, decreased Cd accumulation, and enhanced Cd stress tolerance. ZmbHLH105 directly bound to promoter regions of two key ABA biosynthesis genes ZmNCED1/2, activating their transcription, thus boosting ABA levels and Cd tolerance. ZmbHLH105-overexpression promoted lignin synthesis, while ZmbHLH105-RNAi attenuated this effect. Exogenous ABA supplementation increased lignin content in Cd-stressed maize roots, suggesting ZmbHLH105-mediated Cd tolerance involves ABA-induced lignin deposition and cell wall thickening. Moreover, Cd transport-related gene expression was suppressed in ZmbHLH105 overexpression lines. Our findings demonstrate that ZmbHLH105 decreases Cd accumulation, improving Cd tolerance by enhancing ABA biosynthesis, increasing lignin deposition, thickening cell walls, and inhibiting Cd absorption in maize roots. This study unveils ZmbHLH105's mechanisms in Cd tolerance, highlighting its potential in breeding low Cd-accumulating crops for food and environment safety.

Comprehensive transcriptome, physiological and biochemical analyses reveal that key role of transcription factor WRKY and plant hormone in responding cadmium stress

Cadmium (Cd) is readily absorbed by tobacco and accumulates in the human body through smoke inhalation, posing threat to human health. While there have been many studies on the negative impact of cadmium in tobacco on human health, the specific adaptive mechanism of tobacco roots to cadmium stress is not well understood. In order to comprehensively investigate the effects of Cd stress on the root system of tobacco, the combination of transcriptomic, biochemical, and physiological methods was utilized. In this study, tobacco growth was significantly inhibited by 50 μM of Cd, which was mainly attributed to the destruction of root cellular structure. By comparing the transcriptome between CK and Cd treatment, there were 3232 up-regulated deferentially expressed genes (DEGs) and 3278 down-regulated DEGs. The obvious differential expression of genes related to the nitrogen metabolism, metal transporters and the transcription factors families. In order to mitigate the harmful effects of Cd, the root system enhances Cd accumulation in the cell wall, thereby reducing the Cd content in the cytoplasm. This result may be mediated by plant hormones and transcription factor (TF). Correlational statistical analysis revealed significant negative correlations between IAA and GA with cadmium accumulation, indicated by correlation coefficients of -0.91 and -0.93, respectively. Conversely, ABA exhibited a positive correlation with a coefficient of 0.96. In addition, it was anticipated that 3 WRKY TFs would lead to a reduction in Cd accumulation. Our research provides a theoretical basis for the systematic study of the specific physiological processes of plant roots under Cd stress.

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

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

Integrative study of transcriptome and microbiome to reveal the response of Rhododendron decorum to cadmium stress

The anomalies of cadmium (Cd) in karst region pose a severe threat to plant growth and development. In this study, the responses of Rhododendron decorum to Cd stress were investigated at physiological, molecular, and endophytic microbial levels, and the potential correlation among these responses was assessed. The Cd stress impeded R. decorum growth and led to an increase in malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels, as well as enhanced superoxide dismutase (SOD) and catalase (CAT) activities. Meanwhile, Cd stress increased the Cd (up to 80 times compared to the control), sodium (Na), aluminum (Al), and zinc (Zn) contents, while decreased the magnesium (Mg) and manganese (Mn) contents in R. decorum leaves. Transcriptome suggested that Cd significantly regulated the pathways including "protein repair", "hormone-mediated signaling pathway", and "ATP-binding cassette (ABC) transporters". Additionally, q-PCR analysis showed that Cd stress significantly up-regulated the expressions of ABCB19-like and pleiotropic drug resistance, while down-regulated the expressions of indole-3-acetic acid-amido synthetase and cytokinin dehydrogenase. The Cd stress influenced the composition of endophytic microbial communities in R. decorum leaves and enhanced the interspecific bacterial associations. Furthermore, the bacterial genera Achromobacter, Aureimonas and fungal genus Vishniacozyma exhibited a high degree of connectivity with other nodes in networks constructed by the metal element contents, differentially expressed genes (DEGs), and microbial communities, respectively. These findings provide a comprehensive insight into the response of R. decorum to Cd-induced stress, which might facilitate the breeding of the Cd-tolerant R. decorum.

GhRCD1 promotes cotton tolerance to cadmium by regulating the GhbHLH12-GhMYB44-GhHMA1 transcriptional cascade

Heavy metal pollution poses a significant risk to human health and wreaks havoc on agricultural productivity. Phytoremediation, a plant-based, environmentally benign, and cost-effective method, is employed to remove heavy metals from contaminated soil, particularly in agricultural or heavy metal-sensitive lands. However, the phytoremediation capacity of various plant species and germplasm resources display significant genetic diversity, and the mechanisms underlying these differences remain hitherto obscure. Given its potential benefits, genetic improvement of plants is essential for enhancing their uptake of heavy metals, tolerance to harmful levels, as well as overall growth and development in contaminated soil. In this study, we uncover a molecular cascade that regulates cadmium (Cd2+) tolerance in cotton, involving GhRCD1, GhbHLH12, GhMYB44, and GhHMA1. We identified a Cd2+-sensitive cotton T-DNA insertion mutant with disrupted GhRCD1 expression. Genetic knockout of GhRCD1 by CRISPR/Cas9 technology resulted in reduced Cd2+ tolerance in cotton seedlings, while GhRCD1 overexpression enhanced Cd2+ tolerance. Through molecular interaction studies, we demonstrated that, in response to Cd2+ presence, GhRCD1 directly interacts with GhbHLH12. This interaction activates GhMYB44, which subsequently activates a heavy metal transporter, GhHMA1, by directly binding to a G-box cis-element in its promoter. These findings provide critical insights into a novel GhRCD1-GhbHLH12-GhMYB44-GhHMA1 regulatory module responsible for Cd2+ tolerance in cotton. Furthermore, our study paves the way for the development of elite Cd2+-tolerant cultivars by elucidating the molecular mechanisms governing the genetic control of Cd2+ tolerance in cotton.

AtERF13 and AtERF6 double knockout fine-tunes growth and the transcriptome to promote cadmium tolerance in Arabidopsis

The toxic heavy metal cadmium (Cd) restricts plant growth. However, how plants fine-tune their growth to modulate Cd resistance has not been determined. Ethylene response factors (ERFs) are key regulators of Cd stress, and Arabidopsis thaliana ERF13 and ERF6 (AtERF13 and AtERF6) negatively regulate growth. We previously demonstrated that AtERF13 is a transcriptional activator that binds a Cd-responsive element. Herein, we report that Arabidopsis plants improve Cd tolerance by repressing AtERF13 and AtERF6. We found that AtERF13 and AtERF6 were strongly downregulated by Cd stress and that AtERF6 weakly bound Cd-responsive elements. Moreover, AtERF13 physically interacted with AtERF6. Importantly, AtERF13 and AtERF6 double knockout mutants, but not single mutants or overexpression lines, grew better, tolerated more Cd and had higher Cd contents than did the wild type. Comparative transcriptome analysis revealed that the double mutants regulate the defense response to cope with Cd toxicity. Accordingly, we propose that, upon Cd stress, Arabidopsis plants repress AtERF13 and AtERF6 to relieve their growth inhibition effects and adjust the transcriptome to adapt to Cd stress, leading to increased Cd tolerance. Our findings thereby provide deep mechanical insights into how dual-function transcription factors fine-tune growth and the transcriptome to promote Cd tolerance in plants.

Transcription factors involved in plant responses to cadmium-induced oxidative stress

Cadmium (Cd) is a heavy metal highly toxic to living organisms. Cd pollution of soils has become a serious problem worldwide, posing a severe threat to crop production and human health. When plants are poisoned by Cd, their growth and development are inhibited, chloroplasts are severely damaged, and respiration and photosynthesis are negatively affected. Therefore, elucidating the molecular mechanisms that underlie Cd tolerance in plants is important. Transcription factors can bind to specific plant cis-acting genes. Transcription factors are frequently reported to be involved in various signaling pathways involved in plant growth and development. Their role in the resistance to environmental stress factors, particularly Cd, should not be underestimated. The roles of several transcription factor families in the regulation of plant resistance to Cd stress have been widely demonstrated. In this review, we summarize the mechanisms of five major transcription factor families-WRKY, ERF, MYB, bHLH, and bZIP-in plant resistance to Cd stress to provide useful information for using molecular techniques to solve Cd pollution problems in the future.

Genome-wide identification of bHLH gene family and its response to cadmium stress in Populus × canescens

The basic helix-loop-helix (bHLH) gene family is integral to various aspects of plant development and the orchestration of stress response. This study focuses on the bHLH genes within Populus × canescens, a poplar species noted for its significant tolerance to cadmium (Cd) stress. Through our comprehensive genomic analysis, we have identified and characterized 170 bHLH genes within the P. canescens genome. These genes have been systematically classified into 22 distant subfamilies based on their evolutionary relationships. A notable conservation in gene structure and motif compositions were conserved across these subfamilies. Further analysis of the promoter regions of these genes revealed an abundance of essential cis-acting element, which are associated with plant hormonal regulation, development processes, and stress response pathway. Utilizing quantitative PCR (qPCR), we have documented the differential regulation of PcbHLHs in response to elevated Cd concentrations, with distinct expression patterns observed across various tissues. This study is poised to unravel the molecular mechanism underpinning Cd tolerance in P. canescens, offering valuable insights for the development of new cultivars with enhanced Cd accumulation capacity and tolerance. Such advancements are crucial for implementing effective phytoremediation strategies to mitigate soil pollution caused by Cd.

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.

Topless-related 2 conferred cadmium accumulation in wheat

Wheat is a vital food crop that faces threats from various abiotic and biotic stresses. Understanding the molecular mechanism of cadmium (Cd) resistance can provide valuable insights into the tolerance of wheat. Plant proteins known as Topless/Topless-Related (TPL/TPR) play a role in growth, development, defense regulation, and stress response. In this study, we identified TaTPR2 as being induced by Cd stress treatment. Upon Cd treatment, wheat plants overexpressing TaTPR2 exhibited better growth compared to wild-type (WT) plants. Moreover, the transgenic lines showed reduced accumulation of reactive oxygen species (ROS), along with significantly higher activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) compared to WT plants. Additionally, the transgenic lines exhibited lower levels of malondialdehyde (MDA) and electrolyte leakage compared to WT plants. Further analysis revealed that TabHLH41 directly binds to the E-box motif of the TaTPR2 promoter and positively regulates its expression. Overall, the overexpression of TaTPR2 in transgenic wheat resulted in reduced accumulation of Cd and ROS. These findings highlight the significance of the TabHLH41-TaTPR2 pathway as a crucial response to Cd stress in wheat.

Alfalfa MsbHLH115 confers tolerance to cadmium stress through activating the iron deficiency response in Arabidopsis thaliana

Cadmium (Cd) pollution severely affects plant growth and development, posing risks to human health throughout the food chain. Improved iron (Fe) nutrients could mitigate Cd toxicity in plants, but the regulatory network involving Cd and Fe interplay remains unresolved. Here, a transcription factor gene of alfalfa, MsbHLH115 was verified to respond to iron deficiency and Cd stress. Overexpression of MsbHLH115 enhanced tolerance to Cd stress, showing better growth and less ROS accumulation in Arabidopsis thaliana. Overexpression of MsbHLH115 significantly enhanced Fe and Zn accumulation and did not affect Cd, Mn, and Cu concentration in Arabidopsis. Further investigations revealed that MsbHLH115 up-regulated iron homeostasis regulation genes, ROS-related genes, and metal chelation and detoxification genes, contributing to attenuating Cd toxicity. Y1H, EMSA, and LUC assays confirmed the physical interaction between MsbHLH115 and E-box, which is present in the promoter regions of most of the above-mentioned iron homeostasis regulatory genes. The transient expression experiment showed that MsbHLH115 interacted with MsbHLH121pro. The results suggest that MsbHLH115 may directly regulate the iron-deficiency response system and indirectly regulate the metal detoxification response mechanism, thereby enhancing plant Cd tolerance. In summary, enhancing iron accumulation through transcription factor regulation holds promise for improving plant tolerance to Cd toxicity, and MsbHLH115 is a potential candidate for addressing Cd toxicity issues.

OsPDR20 is an ABCG metal transporter regulating cadmium accumulation in rice

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

IMA peptides function in iron homeostasis and cadmium resistance

Iron (Fe), an essential micronutrient, participates in photosynthesis, respiration, and many other enzymatic reactions. Cadmium (Cd), by contrast, is a toxic element to virtually all living organisms. Both Fe deficiency and Cd toxicity severally impair crop growth and productivity, finally leading to human health issues. Understanding how plants control the uptake and homeostasis of Fe and combat Cd toxicity thus is mandatory to develop Fe-enriched but Cd-cleaned germplasms for human beings. Recent studies in Arabidopsis and rice have revealed that IRON MAN (IMA) peptides stand out as a key regulator to respond to Fe deficiency by competitively interacting with a ubiquitin E3 ligase, thus inhibiting the degradation of IVc subgroup bHLH transcription factors (TFs), mediated by 26 S proteasome. Elevated expression of IMA confers tolerance to Cd stress in both Arabidopsis and wheat by activating the iron deficiency response. Here, we discuss recent breakthroughs that IMA peptides function in the Fe-deficiency response to attain Fe homeostasis and combat Cd toxicity as a potential candidate for phytoremediation.

Arabidopsis transcription factor WRKY45 confers cadmium tolerance via activating PCS1 and PCS2 expression

Cadmium (Cd) has long been recognized as toxic pollutant to crops worldwide. The biosynthesis of glutathione-dependent phytochelatin (PC) plays crucial roles in the detoxification of Cd in plants. However, its regulatory mechanism remains elusive. Here, we revealed that Arabidopsis transcription factor WRKY45 confers Cd tolerance via promoting the expression of PC synthesis-related genes PCS1 and PCS2, respectively. Firstly, we found that Cd stress induces the transcript levels of WRKY45 and its protein abundance. Accordingly, in contrast to wild type Col-0, the increased sensitivity to Cd is observed in wrky45 mutant, while overexpressing WRKY45 plants are more tolerant to Cd. Secondly, quantitative real-time PCR revealed that the expression of AtPCS1 and AtPCS2 is stimulated in overexpressing WRKY45 plants, but decreased in wrky45 mutant. Thirdly, WRKY45 promotes the expression of PCS1 and PCS2, electrophoresis mobility shift assay analysis uncovered that WRKY45 directly binds to the W-box cis-element of PCS2 promoter. Lastly, the overexpression of WRKY45 in Col-0 leads to more accumulation of PCs in Arabidopsis, and the overexpression of PCS1 or PCS2 in wrky45 mutant plants rescues the phenotypes induced by Cd stress. In conclusion, our results show that AtWRKY45 positively regulates Cd tolerance in Arabidopsis via activating PCS1 and PCS2 expression.

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.

The transcription factor NAC102 confers cadmium tolerance by regulating WAKL11 expression and cell wall pectin metabolism in Arabidopsis

Cadmium (Cd) toxicity severely limits plant growth and development. Moreover, Cd accumulation in vegetables, fruits, and food crops poses health risks to animals and humans. Although the root cell wall has been implicated in Cd stress in plants, whether Cd binding by cell wall polysaccharides contributes to tolerance remains controversial, and the mechanism underlying transcriptional regulation of cell wall polysaccharide biosynthesis in response to Cd stress is unknown. Here, we functionally characterized an Arabidopsis thaliana NAC-type transcription factor, NAC102, revealing its role in Cd stress responses. Cd stress rapidly induced accumulation of NAC102.1, the major transcript encoding functional NAC102, especially in the root apex. Compared to wild type (WT) plants, a nac102 mutant exhibited enhanced Cd sensitivity, whereas NAC102.1-overexpressing plants displayed the opposite phenotype. Furthermore, NAC102 localizes to the nucleus, binds directly to the promoter of WALL-ASSOCIATED KINASE-LIKE PROTEIN11 (WAKL11), and induces transcription, thereby facilitating pectin degradation and decreasing Cd binding by pectin. Moreover, WAKL11 overexpression restored Cd tolerance in nac102 mutants to the WT levels, which was correlated with a lower pectin content and lower levels of pectin-bound Cd. Taken together, our work shows that the NAC102-WAKL11 module regulates cell wall pectin metabolism and Cd binding, thus conferring Cd tolerance in Arabidopsis.

BRUTUS-LIKE (BTSL) E3 ligase-mediated fine-tuning of Fe regulation negatively affects Zn tolerance of Arabidopsis

The mineral micronutrients zinc (Zn) and iron (Fe) are essential for plant growth and human nutrition, but interactions between the homeostatic networks of these two elements are not fully understood. Here we show that loss of function of BTSL1 and BTSL2, which encode partially redundant E3 ubiquitin ligases that negatively regulate Fe uptake, confers tolerance to Zn excess in Arabidopsis thaliana. Double btsl1 btsl2 mutant seedlings grown on high Zn medium accumulated similar amounts of Zn in roots and shoots to the wild type, but suppressed the accumulation of excess Fe in roots. RNA-sequencing analysis showed that roots of mutant seedlings had relatively higher expression of genes involved in Fe uptake (IRT1, FRO2, and NAS) and in Zn storage (MTP3 and ZIF1). Surprisingly, mutant shoots did not show the transcriptional Fe deficiency response which is normally induced by Zn excess. Split-root experiments suggested that within roots the BTSL proteins act locally and downstream of systemic Fe deficiency signals. Together, our data show that constitutive low-level induction of the Fe deficiency response protects btsl1 btsl2 mutants from Zn toxicity. We propose that BTSL protein function is disadvantageous in situations of external Zn and Fe imbalances, and formulate a general model for Zn-Fe interactions in plants.

Multilayered regulation of iron homeostasis in Arabidopsis

Iron (Fe) is an essential micronutrient for plant growth and development due to its role in crucial processes such as photosynthesis and modulation of the redox state as an electron donor. While Fe is one of the five most abundant metals in the Earth's crust, it is poorly accessible to plants in alkaline soils due to the formation of insoluble complexes. To limit Fe deficiency symptoms, plant have developed a highly sophisticated regulation network including Fe sensing, transcriptional regulation of Fe-deficiency responsive genes, and post-translational modifications of Fe transporters. In this mini-review, we detail how plants perceive intracellular Fe status and how they regulate transporters involved in Fe uptake through a complex cascade of transcription factors. We also describe the current knowledge about intracellular trafficking, including secretion to the plasma membrane, endocytosis, recycling, and degradation of the two main Fe transporters, IRON-REGULATED TRANSPORTER 1 (IRT1) and NATURAL RESISTANCE ASSOCIATED MACROPHAGE PROTEIN 1 (NRAMP1). Regulation of these transporters by their non-Fe substrates is discussed in relation to their functional role to avoid accumulation of these toxic metals during Fe limitation.

Argon-stimulated nitric oxide production and its function in alfalfa cadmium tolerance

Recent results showed that argon may have great potential in both medicines (especially) and agriculture. However, how argon positively influences crop physiology remains elusive. Here, we observed that the stimulation of nitric oxide (NO) production upon cadmium (Cd) stress in hydroponic alfalfa root tissues was strengthened by argon-rich water and/or a NO-releasing compound. The pharmacological results further indicated that above potential source of NO stimulation achieved by argon might be attributed to NO synthase (NOS) and nitrate reductase (NR). Under hydroponic and pot conditions, the improvement of Cd tolerance elicited by argon, confirmed by the alleviation in the plant growth inhibition, oxidative damage, and Cd accumulation, was sensitive to the scavenger of NO. These results suggested a crucial role of argon-induced NO synthesis in response to Cd stress. Subsequent evidence showed that the improved iron homeostasis and increased S-nitrosylation were also dependent on argon-stimulated NO. Above results were matched with the transcriptional profiles of representative target genes involved in heavy metal detoxification, antioxidant defence, and iron homeostasis. Taken together, our results clearly indicated that argon-stimulated NO production contributes to Cd tolerance by favoring important defense strategies against heavy metal exposure.

The TabHLH094-TaMYC8 complex mediates the cadmium response in wheat

In wheat, TaMYC8 is a negative regulator of cadmium (Cd)-responsive ethylene signaling. In this study, we functionally characterized TabHLH094, a basic helix-loop-helix (bHLH) transcription factor (TF) that inhibits the transcriptional activity of TaMYC8. The TabHLH094 protein was found in the nucleus of tobacco epidermal cells and exhibited transcriptional activation activity. Real-time quantitative PCR (RT-qPCR) indicated that TabHLH094 exhibited root-specific, Cd-responsive expression in wheat seedlings. Overexpression of TabHLH094 enhanced the tolerance of wheat seedlings to Cd exposure. The protein-protein interaction between TabHLH094 and TaMYC8 was verified by glutathione S-transferase (GST) pulldown, coimmunoprecipitation (Co-IP), yeast two-hybrid (Y2H), and bimolecular fluorescence complementation (BiFC) analyses. TabHLH094 was found to reduce the ability of TaMYC8 to bind to the TaERF6 promoter. Furthermore, TabHLH094 could also reduce aminocyclopropanecarboxylate oxidase (ACO) and ACC synthase (ACS) activities, both of which are necessary for ethylene biosynthesis. Taken together, these results indicate that TabHLH094 mediates Cd tolerance by regulating the transcriptional activity of TaMYC8 and decreasing ethylene production.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01404-1.

The TabHLH35-TaWAK20-TaSPL5 pathway positively regulates Cd stress in wheat

KEY MESSAGE

Cadmium-induced TaWAK20 regulates the cadmium stress response by phosphorylating TaSPL5 in wheat. Receptor-like kinases (RLKs) are thought to play important roles in responses to abiotic stresses in plants. In this study, we identified a cadmium (Cd)-induced RLK in wheat, TaWAK20, which is a positive regulator of the Cd stress response. TaWAK20 is specifically expressed in root tissue. Overexpression of TaWAK20 significantly improved the tolerance of Cd stress in wheat and decreased Cd accumulation in wheat plants by regulating reactive oxygen species production and scavenging. Yeast one-hybrid assays, electrophoretic mobility shift assays, and firefly luciferase activity analyses demonstrated that the TaWAK20 promoter was bound by the TabHLH35 transcription factor. TaWAK20 interacted with and phosphorylated squamosa promoter binding protein-like 5 (TaSPL5). Furthermore, phosphorylation of TaSPL5 increased its DNA-binding activity. In addition, Arabidopsis-expressing phosphorylated TaSPL5 exhibited greater Cd tolerance than Arabidopsis-expressing unphosphorylated TaSPL5. Taken together, these data identify a TabHLH35-TaWAK20-TaSPL5 module that regulates Cd stress.

Transcriptomic Analysis of Differentially Expressed Genes in Arabidopsis thaliana Overexpressing BnMYB2 from Boehmeria nivea under Cadmium Stress

Boehmeria nivea (ramie) is an important fiber crop with strong tolerance to cadmium (Cd). In our previous study, a novel MYB transcription factor gene from ramie, BnMYB2, was found to positively regulate Cd tolerance and accumulation in the transgenic Arabidopsis thaliana lines. Herein, transcriptome sequencing was performed to identify the differentially expressed genes involved in cadmium response between the wild-type (WT) and BnMYB2 overexpressed lines; 1598 differentially expressed genes (DEGs) were detected in the shoot. GO and KEGG analysis indicated that the majority of DEGs belonged to the categories of transcription factors, plant hormone signal transduction and nitrogen metabolism. The expression level of the Ib subgroup bHLH genes (AtbHLH38, AtbHLH39, AtbHLH100 and AtbHLH101) and nitrogen assimilation-related genes (AtNIA1, AtNIA2, AtNIR1 and AtASN2) were significantly higher than that of WT, accompanied with the positive changes in iron (Fe) and total nitrogen content in the shoot of BnMYB2 overexpression lines. Several DEGs belonging to the bZIP transcription factor family or SAUR family were also found up-regulated in the transgenic plants. These results provide important clues for elucidating how the molecular mechanisms of BnMYB2 regulate plant response to Cd stress.

The endo-beta mannase MAN7 contributes to cadmium tolerance by modulating root cell wall binding capacity in Arabidopsis thaliana

The heavy metal cadmium (Cd) is detrimental to crop growth and threatens human health through the food chain. To cope with Cd toxicity, plants employ multiple strategies to decrease Cd uptake and its root-to-shoot translocation. However, genes that participate in the Cd-induced transcriptional regulatory network, including those encoding transcription factors, remain largely unidentified. In this study, we demonstrate that ENDO-BETA-MANNASE 7 (MAN7) is necessary for the response of Arabidopsis thaliana to toxic Cd levels. We show that MAN7 is responsible for mannase activity and modulates mannose content in the cell wall, which plays a role in Cd compartmentalization in the cell wall under Cd toxicity conditions. Additionally, the repression of root growth by Cd was partially reversed via exogenous application of mannose, suggesting that MAN7-mediated cell wall Cd redistribution depends on the mannose pathway. Notably, we identified a basic leucine zipper (bZIP) transcription factor, bZIP44, that acts upstream of MAN7 in response to Cd toxicity. Transient dual-luciferase assays indicated that bZIP44 directly binds to the MAN7 promoter region and activates its transcription. Loss of bZIP44 function was associated with greater sensitivity to Cd treatment and higher accumulation of the heavy metal in roots and shoots. Moreover, MAN7 overexpression relieved the inhibition of root elongation seen in the bzip44 mutant under Cd toxicity conditions. This study thus reveals a pathway showing that MAN7-associated Cd tolerance in Arabidopsis is controlled by bZIP44 upon Cd exposure.

Characterization and Expression Analysis of Four Cadmium-Tolerance-Associated Genes of Avicennia marina (Forsk.)

Mangroves colonize the intertidal area of estuaries (e.g., Pichavaram, Payardia, and Mai Po) with remarkable cadmium (Cd) pollution. A study on the mechanism of mangrove plant response to Cd pollution can help to understand the adaptive characteristics of plants under Cd stress. This study explored the roles of peroxidase (PRX), pectate lyase (PL), and phytosulfokine (PSK) genes in cadmium tolerance of mangrove Avicennia marina. Full-length sequences of four genes (i.e., AmPRX1, AmPRX2, AmPL, and AmPSK) associated with metal tolerance were identified with suppression subtractive hybridization and rapid amplification of cDNA ends. These genes showed the characteristic features of the respective protein family, indicating functions similar to other plant proteins. Real-time quantitative PCR analysis demonstrated that cadmium exposure resulted in differences in expression patterns among the tissues. Our findings emphasize the complex regulatory mechanism of these four genes in response to trace metal pollution and reveal their functions in metabolic signaling during the stress response.

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

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

Birth, life and death of the Arabidopsis IRT1 iron transporter: the role of close friends and foes

IRT1 intracellular dynamics and function are finely controlled through protein-protein interactions. In plants, iron uptake from the soil is tightly regulated to allow optimal growth and development. Iron acquisition in Arabidopsis root epidermal cells requires the IRT1 transporter, which also mediates the entry of non-iron metals. In this mini-review, we describe how protein-protein interactions regulate IRT1 intracellular dynamics and IRT1-mediated metal uptake to maintain iron homeostasis. Recent interactomic data provided interesting clues on IRT1 secretion and the putative involvement of COPI- and COPII-mediated pathways. Once delivered to the plasma membrane, IRT1 can interact with other components of the iron uptake machinery to form an iron acquisition complex that likely optimizes iron entrance in root epidermal cells. Then, IRT1 may be internalized from the plasma membrane. In the past decade, IRT1 endocytosis emerged as an essential mechanism to control IRT1 subcellular localization and thus to tune iron uptake. Interestingly, IRT1 endocytosis and degradation are regulated by its non-iron metal substrates in an ubiquitin-dependent manner, which requires a set of interacting-proteins including kinases, E3 ubiquitin ligases and ESCRT complex subunits. This mechanism is essential to avoid non-iron metal overload in Arabidopsis when the iron is scarce.

Flooding-induced rhizosphere Clostridium assemblage prevents root-to-shoot cadmium translocation in rice by promoting the formation of root apoplastic barriers

Water managements are the most effective agricultural practices for restraining cadmium (Cd) uptake and translocation in rice, which closely correlated with rhizosphere assembly of beneficial microbiome. However, the role of the assemblage of specific microbiota in controlling root-to-shoot Cd translocation in rice remains scarcely clear. The aim of this study was to ascertain how water managements shaped rhizosphere microbiome and mediated root-to-shoot Cd translocation. To disentangle the acting mechanisms of water managements, we performed an experiment monitoring Cd uptake and transport in rice and changes in soil microbial communities in response to continuously flooding and moistening irrigation. Continuously flooding changed rhizosphere microbial communities, leading to the increased abundance of anaerobic bacteria such as Clostridium populations. Weighted gene co-expression network analysis (WGCNA) showed that a dominant OTU163, corresponding to Clostridium sp. CSP1, exhibited a strong negative correlation with root-to-shoot Cd translocation. An integrated analysis of transcriptome and metabolome further indicated that the Clostridium-secreted butyric acid was involved in the regulation of phenylpropanoid pathway in rice roots. The formation of endodermal suberized barriers and lignified xylems was remarkably enhanced in the Clostridium-treated roots, which led to more Cd retained in root cell wall and less Cd in the xylem sap. Collectively, our results indicate that the development of root apoplastic barriers can be orchestrated by beneficial Clostridium strains that are assembled by host plants grown under flooding regime, thereby inhibiting root-to-shoot Cd translocation.

De Novo Transcriptome Assembly, Gene Annotations, and Characterization of Functional Profiling Reveal Key Genes for Lead Alleviation in the Pb Hyperaccumulator Greek Mustard (Hirschfeldia incana L.)

Lead (Pb) contamination is a widespread environmental problem due to its toxicity to living organisms. Hirschfeldia incana L., a member of the Brassicaceae family, commonly found in the Mediterranean regions, is characterized by its ability to tolerate and accumulate Pb in soils and hydroponic cultures. This plant has been reported as an excellent model to assess the response of plants to Pb. However, the lack of genomic data for H. incana hinders research at the molecular level. In the present study, we carried out RNA deep transcriptome sequencing (RNA-seq) of H. incana under two conditions, control without Pb(NO₃)₂ and treatment with 100 µM of Pb(NO₃)₂ for 15 days. A total of 797.83 million reads were generated using Illumina sequencing technology. We assembled 77,491 transcript sequences with an average length of 959 bp and N50 of 1330 bp. Sequence similarity analyses and annotation of these transcripts were performed against the Arabidopsis thaliana nr protein database, Gene Ontology (GO), and KEGG databases. As a result, 13,046 GO terms and 138 KEGG maps were created. Under Pb stress, 577 and 270 genes were differentially expressed in roots and aboveground parts, respectively. Detailed elucidation of regulation of metal transporters, transcription factors (TFs), and plant hormone genes described the role of actors that allow the plant to fine-tune Pb stress responses. Our study revealed that several genes related to jasmonic acid biosynthesis and alpha-linoleic acid were upregulated, suggesting these components’ implication in Hirschfeldia incana L responses to Pb stress. This study provides data for further genomic analyses of the biological and molecular mechanisms leading to Pb tolerance and accumulation in Hirschfeldia incana L.

The regulatory role of abscisic acid on cadmium uptake, accumulation and translocation in plants

To date, Cd contamination of cropland and crops is receiving more and more attention around the world. As a plant hormone, abscisic acid (ABA) plays an important role in Cd stress response, but its effect on plant Cd uptake and translocation varies among plant species. In some species, such as Arabidopsis thaliana, Oryza sativa, Brassica chinensis, Populus euphratica, Lactuca sativa, and Solanum lycopersicum, ABA inhibits Cd uptake and translocation, while in other species, such as Solanum photeinocarpum and Boehmeria nivea, ABA severs the opposite effect. Interestingly, differences in the methods and concentrations of ABA addition also triggered the opposite result of Cd uptake and translocation in Sedum alfredii. The regulatory mechanism of ABA involved in Cd uptake and accumulation in plants is still not well-established. Therefore, we summarized the latest studies on the ABA synthesis pathway and comparatively analyzed the physiological and molecular mechanisms related to ABA uptake, translocation, and detoxification of Cd in plants at different ABA concentrations or among different species. We believe that the control of Cd uptake and accumulation in plant tissues can be achieved by the appropriate ABA application methods and concentrations in plants.

Root and shoot biology of Arabidopsis halleri dissected by WGCNA: an insight into the organ pivotal pathways and genes of an hyperaccumulator

Arabidopsis halleri is a hyperaccumulating pseudo-metallophyte and an emerging model to explore molecular basis of metal tolerance and hyperaccumulation. In this regard, understanding of interacting genes can be a crucial aspect as these interactions regulate several biological functions at molecular level in response to multiple signals. In this current study, we applied a weighted gene co-expression network analysis (WGCNA) on root and shoot RNA-seq data of A. halleri to predict the related scale-free organ specific co-expression networks, for the first time. A total of 19,653 genes of root and 18,081 genes of shoot were grouped into 14 modules and subjected to GO and KEGG enrichment analysis. "Photosynthesis" and "photosynthesis-antenna proteins" were identified as the most enriched and common pathway to both root and shoot. Whereas "glucosinolate biosynthesis," "autophagy," and "SNARE interactions in vesicular transport" were specific to root, and "circadian rhythm" was found to be enriched only in shoot. Later, hub and bottleneck genes were identified in each module by using cytoHubba plugin based on Cytoscape and scoring the relevance of each gene to the topology of the network. The modules with the most significant differential expression pattern across control and treatment (Cd-Zn treatment) were selected and their hub and bottleneck genes were screened to validate their possible involvement in heavy metal stress. Moreover, we combined the analysis of co-expression modules together with protein-protein interactions (PPIs), confirming some genes as potential candidates in plant heavy metal stress and as biomarkers. The results from this analysis shed the light on the pivotal functions to the hyperaccumulative trait of A. halleri, giving perspective to new paths for future research on this species.

State-of-the-art OMICS strategies against toxic effects of heavy metals in plants: A review

Environmental pollution of heavy metals (HMs), mainly due to anthropogenic activities, has received growing attention in recent decades. HMs, especially the non-essential carcinogenic ones, including chromium (Cr), cadmium (Cd), mercury (Hg), aluminum (Al), lead (Pb), and arsenic (As), have appeared as the most significant air, water, and soil pollutants, which adversely affect the quantity, quality, and security of plant-based food all over the world. Plants exposed to HMs could experience significant decline in growth and yield. To avoid or tolerate the toxic effects of HMs, plants have developed complicated defense mechanisms, including absorption and accumulation of HMs in cell organelles, immobilization by forming complexes with organic chelates, extraction by using numerous transporters, ion channels, signalling cascades, and transcription elements, among others. OMICS strategies have developed significantly to understand the mechanisms of plant transcriptomics, genomics, proteomics, metabolomics, and ionomics to counter HM-mediated stress stimuli. These strategies have been considered to be reliable and feasible for investigating the roles of genomics (genomes), transcriptomic (coding), mRNA transcripts (non-coding), metabolomics (metabolites), and ionomics (metal ions) to enhance stress resistance or tolerance in plants. The recent developments in the mechanistic understandings of the HMs-plant interaction in terms of their absorption, translocation, and toxicity invasions at the molecular and cellular levels, as well as plants' response and adaptation strategies against these stressors, are summarized in the present review. Transcriptomics, genomics, metabolomics, proteomics, and ionomics for plants against HMs toxicities are reviewed, while challenges and future recommendations are also discussed.

ZAT10 plays dual roles in cadmium uptake and detoxification in Arabidopsis

Cadmium (Cd) is a harmful heavy metal that is risky for plant growth and human health. The zinc-finger transcription factor ZAT10 is highly conserved with ZAT6 and ZAT12, which are involved in Cd tolerance in plants. However, the definite function of ZAT10 in Cd tolerance remains uncertain. Here, we demonstrated that ZAT10 negatively regulated Cd uptake and enhanced Cd detoxification in Arabidopsis. The expression of ZAT10 in plants is induced by Cd treatment. The zat10 mutant plants exhibited a greater sensitivity to Cd stress and accumulated more Cd in both shoot and root. Further investigations revealed that ZAT10 repressed the transcriptional activity of IRT1, which encodes a key metal transporter involved in Cd uptake. Meanwhile, ZAT10 positively regulated four heavy metal detoxification-related genes: NAS1, NAS2, IRT2, and MTP3. We further found that ZAT10 interacts with FIT, but their regulatory relationship is still unclear. In addition, ZAT10 directly bound to its own promoter and repressed its transcription as a negative feedback regulation. Collectively, our findings provided new insights into the dual functions of ZAT10 on Cd uptake and detoxification in plants and pointed to ZAT10 as a potential gene resource for Cd tolerance improvement in plants.

Transcription factor ANAC004 enhances Cd tolerance in Arabidopsis thaliana by regulating cell wall fixation, translocation and vacuolar detoxification of Cd, ABA accumulation and antioxidant capacity

Cadmium (Cd) is toxic to plants, which have evolved multiple strategies to cope with Cd stress. In this study, we identified a nucleus-localized NAC-type transcription factor, ANAC004, which is induced by Cd and involved in regulating Cd resistance in Arabidopsis. First, anac004 mutants exhibited Cd sensitive phenotype and accumulated more Cd (12-23% higher than wild type in roots and shoots); plants overexpressing ANAC004 showed the opposite phenotype and with lower Cd accumulation. Second, ANAC004 enhanced Cd fixation in cell wall hemicellulose, thus reducing Cd²⁺ influx into root cells. Third, ANAC004 was involved in the process of vacuolar Cd compartmentalization by regulating the genes associated with Cd detoxification (PCS1/2, NAS4, ABCC1/2/3, MTP1/3, IREG2 and NRAMP3/4). Fourth, ANAC004 reduced root-to-shoot Cd translocation through down-regulated Cd translocation-related genes (HMA2 and HMA4). Finally, the expression of genes related to ABA synthesis (AAO3, MCSU, and NCED3) and the activities of antioxidant enzymes (SOD, POD and CAT) were all reduced in anac004 mutants, leading to reduced levels of endogenous ABA and increased accumulation of reactive oxygen species (O₂^.- and H₂O₂) and MDA, which ultimately weakened resistance to Cd. Our results suggest that ANAC004 decreases Cd accumulation in Arabidopsis through enhancing cell wall Cd immobilization, increasing vacuolar Cd detoxification, and inhibiting Cd translocation, thus improving Cd resistance, processes that might be mediated by ABA signaling and antioxidant defense systems.

Effects of Cadmium on Root Morpho-Physiology of Durum Wheat

Durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn.] can accumulate a high level of Cd in grains with a significant variability depending on cultivars. Understanding how this toxic element is distributed in cereal tissues and grains is essential to improve the nutritional quality of cereal-based products. The main objective of this work was to investigate roots of durum wheat plants (cv. Iride) exposed to different Cd concentrations (0.5 and 5.0 μM) to identify the mechanisms involved in Cd management. Results showed that the root morphology was altered by Cd treatment both at macroscopic (increased number of tips and primary root length) and ultrastructural levels (cell membrane system damaged, cell walls thickened and enriched in suberin). On the other side, Cd was localized in vesicles and in cell walls, and the metal colocalized with the phytosiderophore nicotianamine (NA). Overall, data suggest that Cd is chelated by NA and then compartmentalized, through vesicular trafficking, in the root thickened walls reducing Cd translocation to the aerial organs of the plant.

Knowledge Mapping of the Phytoremediation of Cadmium-Contaminated Soil: A Bibliometric Analysis from 1994 to 2021

Cadmium pollution of soil threatens the environmental quality and human health. Phytoremediation of cadmium-contaminated soil has attracted global attention in recent decades. This study aimed to conduct a comprehensive and systematic review of the literature on phytoremediation of cadmium-contaminated soil based on bibliometric analysis. A total of 5494 articles published between 1994 and 2021 were retrieved from the Web of Science Core Collection. Our knowledge mapping presented the authors, journals, countries, institutions, and other basic information to understand the development status of phytoremediation of cadmium-contaminated soil. Based on a keyword cluster analysis, the identified major research domains were "biochar", "Thlaspi caerulescens", "endophytic bacteria", "oxidative stress", "EDTA", and "bioconcentration factor". Overall, this study provided a detailed summary of research trends and hotspots. Based on the keyword co-occurrence and burst analysis, the core concepts and basic theories of this field were completed in 2011. However, the pace of theoretical development has been relatively slow. Finally, future research trends/frontiers were proposed, such as biochar addition, rhizosphere bacterial community manipulation, cadmium subcellular distribution, and health risk assessment.

CF1 reduces grain-cadmium levels in rice (Oryza sativa)

Rice (Oryza sativa) is a leading source of dietary cadmium (Cd), a non-essential heavy metal that poses a serious threat to human health. There are significant variations in grain-Cd levels in natural rice populations, which make the breeding of low-Cd rice a cost-effective way to mitigate grain-Cd accumulation. However, the genetic factors that regulate grain-Cd accumulation have yet to be fully established, thereby hindering the development of low-Cd varieties. Here, we reported a low-Cd quantitative trait locus, CF1, that has the potential to reduce Cd accumulation in rice grains. CF1 is allelic to the metal transporter OsYSL2, which transports Fe from the roots to the shoots. However, it is incapable of binding Cd, and thus, reduces grain-Cd levels indirectly rather than directly in the form of upward delivery. Further analysis showed that high expression levels of CF1 improve Fe nutrition in the shoots, subsequently inhibiting Cd uptake by systemically inhibiting expression of the main Cd uptake gene OsNramp5 in the roots. Compared with the CF1 allele from '02428' (CF1⁰²⁴²⁸ ), higher expression levels of CF1 from 'TQ' (CF1^TQ ) increased the Fe contents and decreased Cd levels in rice grains. In natural rice populations, CF1^TQ was found to be a minor allele, while CF1⁰²⁴²⁸ is present in most japonica rice, suggesting that CF1^TQ could be widely integrated into the japonica rice genome to generate low-Cd varieties. Overall, these results broaden our mechanistic understanding of the natural variation in grain-Cd accumulation, supporting marker-assisted selection of low-Cd rice.

Heterologous Expression of Dehydration-Inducible MfbHLH145 of Myrothamnus flabellifoli Enhanced Drought and Salt Tolerance in Arabidopsis

Myrothamnus flabellifolia is the only woody resurrection plant found in the world. It has a strong tolerance to drought and can survive long-term exposure to desiccated environments. However, few genes related to its drought tolerance have been functionally characterized and the molecular mechanisms underlying the stress tolerance of M. flabellifolia are largely unknown. In this study, we isolated a dehydration-inducible bHLH transcription factor gene MfbHLH145 from M. flabellifolia. Heterologous expression of MfbHLH145 enhanced the drought and salt tolerance of Arabidopsis. It can not only promote root system development under short-term stresses, but also improve growth performance under long-term treatments. Further investigation showed that MfbHLH145 contributes to enhanced leaf water retention capacity through the promotion of stomatal closure, increased osmolyte accumulation, and decreased stress-induced oxidative damage through an increase in antioxidant enzyme activities. These results suggest that MfbHLH145 may be involved in the positive regulation of stress responses in M. flabellifolia. This study provides insight into the molecular mechanism underlying the survival of M. flabellifolia in extreme dehydration conditions.

Long-distance mobile mRNA CAX3 modulates iron uptake and zinc compartmentalization

Iron deficiency in plants can lead to excessive absorption of zinc; however, important details of this mechanism have yet to be elucidated. Here, we report that MdCAX3 mRNA is transported from the leaf to the root, and that MdCAX3 is then activated by MdCXIP1. Suppression of MdCAX3 expression leads to an increase in the root apoplastic pH, which is associated with the iron deficiency response. Notably, overexpression of MdCAX3 does not affect the apoplastic pH in a MdCXIP1 loss-of-function Malus baccata (Mb) mutant that has a deletion in the MdCXIP1 promoter. This deletion in Mb weakens MdCXIP1 expression. Co-expression of MdCAX3 and MdCXIP1 in Mb causes a decrease in the root apoplastic pH. Furthermore, suppressing MdCAX3 in Malus significantly reduces zinc vacuole compartmentalization. We also show that MdCAX3 activated by MdCXIP1 is not only involved in iron uptake, but also in regulating zinc detoxification by compartmentalizing zinc in vacuoles to avoid iron starvation-induced zinc toxicity. Thus, mobile MdCAX3 mRNA is involved in the regulation of iron and zinc homeostasis in response to iron starvation.

MAPK Cascades and Transcriptional Factors: Regulation of Heavy Metal Tolerance in Plants

In nature, heavy metal (HM) stress is one of the most destructive abiotic stresses for plants. Heavy metals produce toxicity by targeting key molecules and important processes in plant cells. The mitogen-activated protein kinase (MAPK) cascade transfers the signals perceived by cell membrane surface receptors to cells through phosphorylation and dephosphorylation and targets various effector proteins or transcriptional factors so as to result in the stress response. Signal molecules such as plant hormones, reactive oxygen species (ROS), and nitric oxide (NO) can activate the MAPK cascade through differentially expressed genes, the activation of the antioxidant system and synergistic crosstalk between different signal molecules in order to regulate plant responses to HMs. Transcriptional factors, located downstream of MAPK, are key factors in regulating plant responses to heavy metals and improving plant heavy metal tolerance and accumulation. Thus, understanding how HMs activate the expression of the genes related to the MAPK cascade pathway and then phosphorylate those transcriptional factors may allow us to develop a regulation network to increase our knowledge of HMs tolerance and accumulation. This review highlighted MAPK pathway activation and responses under HMs and mainly focused on the specificity of MAPK activation mediated by ROS, NO and plant hormones. Here, we also described the signaling pathways and their interactions under heavy metal stresses. Moreover, the process of MAPK phosphorylation and the response of downstream transcriptional factors exhibited the importance of regulating targets. It was conducive to analyzing the molecular mechanisms underlying heavy metal accumulation and tolerance.

TmNAS3 from Triticum monococum directly regulated by TmbHLH47 increases Fe content of wheat grain

Biofortification is an effective way to enhance wheat grain Fe content. However, Fe overload inhibits the growth and development of wheat. In this work, the impact of Triticum monococcum nicotianamine synthase 3 (TmNAS3) on Fe accumulation in wheat grain was analyzed. Transgenic wheat expressing TmNAS3 was obtained via Agrobacterium-mediated transformation. The concentrations of Fe in the grains of two transgenic wheat lines were 62.42 μg/g and 68.75 μg/g, while that in the non-transgenic line (NT) was only 29.51 μg/g. Exogenous Fe application induced the expression of natural resistance-associated macrophage protein 3 (NRAMP3), NRAMP6, yellow stripe-like protein 3 (YSL3), YSL6, and vacuolar iron transporter 2 in transgenic wheat. The transcription factor that bound to the TmNAS3 promoter was identified, and the findings suggested that TmbHLH47 directly interacted and promoted the transcription of TmNAS3. Moreover, TmbHLH47 was observed to bind directly to the G-box in TmNAS3 promoter and regulated the transcriptional level of TmNAS3. Our findings contribute a TmbHLH47/TmNAS3 transcriptional pathway and thereby provide a potential strategy for improving the Fe concentration of wheat through genetic engineering.

Stress signaling convergence and nutrient crosstalk determine zinc-mediated amelioration against cadmium toxicity in rice

Consumption of rice (Oryza sativa L.) is one of the major pathways for heavy metal bioaccumulation in humans over time. Understanding the molecular responses of rice to heavy metal contamination in agriculture is useful for eco-toxicological assessment of cadmium (Cd) and its interaction with zinc (Zn). In certain crops, the impacts of Cd stress or Zn nutrition on the biophysical chemistry and gene expression have been widely investigated, but their molecular interactions at transcriptomic level, particularly in rice roots, are still elusive. Here, hydroponic investigations were carried out with two rice genotypes (Yinni-801 and Heizhan-43), varying in Cd contents in plant tissues to determine their transcriptomic responses upon Cd15 (15 µM) and Cd15+Zn50 (50 µM) treatments. High throughput RNA-sequencing analysis confirmed that 496 and 2407 DEGs were significantly affected by Cd15 and Cd15+Zn50, respectively, among which 1016 DEGs were commonly induced in both genotypes. Multitude of DEGs fell under the category of protein kinases, such as calmodulin (CaM) and calcineurin B-like protein-interacting protein kinases (CBL), indicating a dynamic shift in hormonal signal transduction and Ca2+ involvement with the onset of treatments. Both genotypes expressed a mutual regulation of transcription factors (TFs) such as WRKY, MYB, NAM, AP2, bHLH and ZFP families under both treatments, whereas genes econding ABC transporters (ABCs), high affinity K+ transporters (HAKs) and Glutathione-S-transferases (GSTs), were highly up-regulated under Cd15+Zn50 in both genotypes. Zinc addition triggered more signaling cascades and detoxification related genes in regulation of immunity along with the suppression of Cd-induced DEGs and restriction of Cd uptake. Conclusively, the effective integration of breeding techniques with candidate genes identified in this study as well as economically and technologically viable methods, such as Zn nutrient management, could pave the way for selecting cultivars with promising agronomic qualities and reduced Cd for sustainable rice production.

Ectopic expression of IMA small peptide genes confers tolerance to cadmium stress in Arabidopsis through activating the iron deficiency response

Increasing cadmium (Cd) pollution severely affects plant growth and development, posing risks to human health via food chains. The Cd toxicity could be mitigated by improving Fe nutrient in plants. IMA1 and IMA3, two novel small peptides functionally epistatic to the key transcription factor bHLH39 but independent of bHLH104, were recently identified as the newest additions to the Fe regulatory cascade, but their roles in Cd uptake and toxicity remain not addressed. Here, the functions of two IMAs and two transcription factors related to Cd tolerance were verified. Overexpression of either bHLH39 or bHLH104 in Arabidopsis showed weak roles in Cd tolerance, but overexpression of IMAs, which activates the Fe-deficient response, significantly enhanced Cd tolerance, showing greater root elongation, biomass and chlorophyll contents. The Cd contents did not show significant difference among the overexpression lines. Further investigations revealed that the tolerance of transgenic plants to Cd mainly depended on higher Fe accumulation, which decreased the MDA contents and enhanced root elongation under Cd exposure, finally contributing to attenuating Cd toxicity. Taken together, the results suggest that increasing Fe accumulation is promising for improving plant tolerance to Cd toxicity and that IMAs are potential candidates for solving Cd toxicity problem.

Cadmium interference with iron sensing reveals transcriptional programs sensitive and insensitive to reactive oxygen species

Iron (Fe) is an essential micronutrient whose uptake is tightly regulated to prevent either deficiency or toxicity. Cadmium (Cd) is a non-essential element that induces both Fe deficiency and toxicity; however, the mechanisms behind these Fe/Cd-induced responses are still elusive. Here we explored Cd- and Fe-associated responses in wild-type Arabidopsis and in a mutant that overaccumulates Fe (opt3-2). Gene expression profiling revealed a large overlap between transcripts induced by Fe deficiency and Cd exposure. Interestingly, the use of opt3-2 allowed us to identify additional gene clusters originally induced by Cd in the wild type but repressed in the opt3-2 background. Based on the high levels of H2O2 found in opt3-2, we propose a model where reactive oxygen species prevent the induction of genes that are induced in the wild type by either Fe deficiency or Cd. Interestingly, a defined cluster of Fe-responsive genes was found to be insensitive to this negative feedback, suggesting that their induction by Cd is more likely to be the result of an impaired Fe sensing. Overall, our data suggest that Fe deficiency responses are governed by multiple inputs and that a hierarchical regulation of Fe homeostasis prevents the induction of specific networks when Fe and H2O2 levels are elevated.

The Mechanism of Metal Homeostasis in Plants: A New View on the Synergistic Regulation Pathway of Membrane Proteins, Lipids and Metal Ions

Heavy metal stress (HMS) is one of the most destructive abiotic stresses which seriously affects the growth and development of plants. Recent studies have shown significant progress in understanding the molecular mechanisms underlying plant tolerance to HMS. In general, three core signals are involved in plants' responses to HMS; these are mitogen-activated protein kinase (MAPK), calcium, and hormonal (abscisic acid) signals. In addition to these signal components, other regulatory factors, such as microRNAs and membrane proteins, also play an important role in regulating HMS responses in plants. Membrane proteins interact with the highly complex and heterogeneous lipids in the plant cell environment. The function of membrane proteins is affected by the interactions between lipids and lipid-membrane proteins. Our review findings also indicate the possibility of membrane protein-lipid-metal ion interactions in regulating metal homeostasis in plant cells. In this review, we investigated the role of membrane proteins with specific substrate recognition in regulating cell metal homeostasis. The understanding of the possible interaction networks and upstream and downstream pathways is developed. In addition, possible interactions between membrane proteins, metal ions, and lipids are discussed to provide new ideas for studying metal homeostasis in plant cells.

Recent advances in physiological and molecular mechanisms of heavy metal accumulation in plants

Among abiotic stress, the toxicity of metals impacts negatively on plants' growth and productivity. This toxicity promotes various perturbations in plants at different levels. To withstand stress, plants involve efficient mechanisms through the implication of various signaling pathways. These pathways enhance the expression of many target genes among them gene coding for metal transporters. Various metal transporters which are localized at the plasma membrane and/or at the tonoplast are crucial in metal stress response. Furthermore, metal detoxification is provided by metal-binding proteins like phytochelatins and metallothioneins. The understanding of the molecular basis of metal toxicities signaling pathways and tolerance mechanisms is crucial for genetic engineering to produce transgenic plants that enhance phytoremediation. This review presents an overview of the recent advances in our understanding of metal stress response. Firstly, we described the effect of metal stress on plants. Then, we highlight the mechanisms involved in metal detoxification and the importance of the regulation in the response to heavy metal stress. Finally, we mentioned the importance of genetic engineering for enhancing the phytoremediation technique. In the end, the response to heavy metal stress is complex and implicates various components. Thus, further studies are needed to better understand the mechanisms involved in response to this abiotic stress.

Genome-wide investigation of bHLH genes and expression analysis under different biotic and abiotic stresses in Helianthus annuus L

The basic helix-loop-helix (bHLH) transcription factors play important roles in many processes such as plant growth, metabolism and response to biotic/abiotic stresses. Sunflower (Helianthus annuus) is a major oil crop, cultivated throughout the world. However, no systematic characterization of bHLH gene members in sunflower (HabHLH) and their functions involved in drought, cadmium tolerance and Orobanche cumana resistance has been reported yet. In this study, 183 HabHLH genes were identified and named according to their chromosomal locations. We classified these proteins into 21 subfamilies by phylogenetic tree analysis. Subsequently, DNA-binding patterns, sequence analysis, duplication analysis and gene structures were analyzed. All of the HabHLH genes were randomly distributed on 17 chromosomes, and 10 pairs of tandem duplicated genes and one pair of segmental duplicated genes were detected in the HabHLH family. Among the duplicated gene pairs, eight pairs of HabHLH genes suffer from positive selection. Moreover, qRT-PCR results revealed significant up-regulated expression of HabHLH024 gene in response to both abiotic (cadmium, drought) and biotic (Orobanche cumana) stresses, suggesting its important functions in response to different stresses. Therefore, HabHLH024 would be the potential candidate gene for the sunflower tolerance breeding.

Challenges in microbially and chelate-assisted phytoextraction of cadmium and lead - A review

Cadmium (Cd) and lead (Pb) are ubiquitously present in surface soils, due to anthropogenic activities, causing threat to ecological and human health because of their carcinogenic nature. They accumulate in large quantities in the environment and affect negatively soil microbiota, plants, animals, and humans. For the cleanup of Cd/Pb polluted soils, different plant species have been studied. Many plants have shown the potential to hyperaccumulate Cd/Pb in their above-ground tissues. These plants decrease soil pH by root exudation or by releasing H⁺ ions, and this, in turn, increases the bioavailability of Cd/Pb for plant uptake. Different environmental processes related to soil organic matter, microorganisms, pH, genetic modifications, and various soil-borne chelating agents affect the potential of phytoremediation technology. Review papers trying to identify a single factor influencing the phytoremediation of heavy metals are available in the literature. However, an integrated approach dealing with different factors involved in the remediation of both metals is scarcely discussed. The main focus of this review is to discuss the phytoextraction technique for Cd/Pb removal from contaminated sites along with detoxification mechanisms. Further, the challenges in the Cd/Pb phytoextraction and different options available to cope with these challenges are also discussed. The update on the relevant findings on the use of microorganisms and amendments in enhancing the Cd/Pb phytoextraction is also provided. Finally, the areas to be explored in future research for the removal of Cd/Pb by integrated strategies have been discussed.

Heavy metal detoxification mechanisms by microalgae: Insights from transcriptomics analysis

Heavy metal pollution in ecosystem is a global threat. The associated toxicity and carcinogenic nature of heavy metals/metalloids such as mercury, cadmium, lead, and arsenic are imposing a severe risk to both ecological diversity and human lives. Harnessing the adaptive feature of microalgae for remediating toxic heavy metal has reached a milestone in past few decades. Transcriptomics analyses have provided mechanistic insights to map the dynamics of cellular events under heavy metal stress, thus deciphering the strategic responses of microalgae. Here, the present review comprehensively addresses the elicited molecular responses of microalgae to detoxify the heavy metal stress. The review highlights the intricate role of biochemical components and signaling networks mediating stress responsive transitions of microalgae at physiological level. Furthermore, the differential gene expression signifying the transporters involved in uptake, distribution/sequestration, and efflux of heavy metal has also been reviewed. In a nutshell, this study provided a comprehensive understanding of the molecular mechanisms adopted by microalgae at transcriptome level to nullify the oxidative stress while detoxifying the heavy metals.

Comparative transcriptomics analysis reveals differential Cd response processes in roots of two turnip landraces with different Cd accumulation capacities

Understanding the molecular mechanisms of cadmium (Cd) tolerance and accumulation in plants is important to address Cd pollution. In the present study, we performed comparative transcriptome analysis to identify the Cd response processes in the roots of two turnip landraces, KTRG-B14 (high-Cd accumulation) and KTRG-B36 (low-Cd accumulation). Two common enhanced processes, glutathione metabolism and antioxidant system, were identified in both landraces. However, some differential antioxidant processes are likely employed by two landraces, namely, several genes encoding peptide methionine sulfoxide reductases and thioredoxins were up-regulated in B14, whereas flavonoid synthesis was potentially induced to fight against oxidative stress in B36. In addition to the commonly upregulated ZINC INDUCED FACILITATOR 1-like gene in two landraces, different metal transporter-encoding genes identified in B14 (DETOXIFICATION 1) and B36 (PLANT CADMIUM RESISTANCE 2-like, probable zinc transporter 10, and ABC transporter C family member 3) were responsible for Cd accumulation and distribution in cells. Several genes that encode extensins were specifically upregulated in B14, which may improve Cd accumulation in cell walls or regulate root development to absorb more Cd. Meanwhile, the induced high-affinity nitrate transporter 2.1-like gene was also likely to contribute to the higher Cd accumulation in B14. However, Cd also caused some toxic symptoms in both landraces. Cd stress might inhibit iron uptake in both landraces whereas many apoenzyme-encoding genes were influenced in B36, which may be attributed to the interaction between Cd and other metal ions. This study provides novel insights into the molecular mechanism of plant root response to Cd at an early stage. The transporters and key enzymes identified in this study are helpful for the molecular-assisted breeding of low- or high-Cd-accumulating plant resources.

Iron and copper micronutrients influences cadmium accumulation in rice grains by altering its transport and allocation

Cadmium (Cd) contamination in rice paddy fields constitutes a serious health issue in some parts of China. Here we study the potential for remediation of Cd contaminated alkaline paddy soil with low iron (Fe) and high copper (Cu) background by altering the concentrations of Fe and Cu in the growing media, which has been only seldom considered. We assessed how these two micronutrients (Cu and Fe) affect the absorption and transport of Cd in rice. Adding Cu significantly increased rice biomass and grain yield by reducing root Cd influx and Cd upward transport which, consequently, lowered Cd concentrations in roots, culms and leaves. However, excessive Cu also promoted a relatively higher Cd allocation in grains, especially under Fe deficiency, likely because Cu significantly increased the proportion of bioavailable Cd in leaves. Contrastingly, Fe did not alleviate the toxic effects of Cd on rice growth and yield, but it significantly reduced Cd transfer towards grains, which might be attributed to a sharp decrease in the proportion of bioavailable Cd in leaves. Our results demonstrated that Cd remediation may be achieved through altering Fe and Cu inputs, such that Cd accumulation in rice grains is reduced.