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Cadmium (Cd) is one of the most injurious heavy metals, affecting plant growth and development. Melatonin (N-acetyl-5-methoxytryptamine) was discovered in plants in 1995, and it is since known to act as a multifunctional molecule to alleviate abiotic and biotic stresses, especially Cd stress. Endogenously triggered or exogenously applied melatonin re-establishes the redox homeostasis by the improvement of the antioxidant defense system. It can also affect the Cd transportation and sequestration by regulating the transcripts of genes related to the major metal transport system, as well as the increase in glutathione (GSH) and phytochelatins (PCs). Melatonin activates several downstream signals, such as nitric oxide (NO), hydrogen peroxide (H2O2), and salicylic acid (SA), which are required for plant Cd tolerance. Similar to the physiological functions of NO, hydrogen sulfide (H2S) is also involved in the abiotic stress-related processes in plants. Moreover, exogenous melatonin induces H2S generation in plants under salinity or heat stress. However, the involvement of H2S action in melatonin-induced Cd tolerance is still largely unknown. In this review, we summarize the progresses in various physiological and molecular mechanisms regulated by melatonin in plants under Cd stress. The complex interactions between melatonin and H2S in acquisition of Cd stress tolerance are also discussed.

[This corrects the article DOI: 10.3389/fpls.2024.1388924.].

Background Long non-coding RNAs (lncRNAs) have been found to play a vital role in several gene regulatory networks involved in the various biological processes in plants related to stress response. However, systematic analyses of lncRNAs expressed in rice Cadmium (Cd) stress are seldom studied. Thus, we presented the characterization and expression of lncRNAs in rice root development at an early stage in response to Cd stress. Results The lncRNA deep sequencing revealed differentially expressed lncRNAs among Cd stress and normal condition. In the Cd stress group, 69 lncRNAs were up-regulated and 75 lncRNAs were down-regulated. Furthermore, 386 matched lncRNA-mRNA pairs were detected for 120 differentially expressed lncRNAs and 362 differentially expressed genes in cis, and target gene-related pathway analyses exhibited significant variations in cysteine and methionine metabolism pathway-related genes. For the genes in trans, overall, 28,276 interaction relationships for 144 lncRNAs and differentially expressed protein-coding genes were detected. The pathway analyses found that secondary metabolites, such as phenylpropanoids and phenylalanine, and photosynthesis pathway-related genes were significantly altered by Cd stress. All of these results indicate that lncRNAs may regulate genes of cysteine-rich peptide metabolism in cis, as well as secondary metabolites and photosynthesis in trans, to activate various physiological and biochemical reactions to respond to excessive Cd. Conclusion The present study could provide a valuable resource for lncRNA studies in response to Cd treatment in rice. It also expands our knowledge about lncRNA biological function and contributes to the annotation of the rice genome.

Cadmium (Cd) is a non-essential heavy metal, assimilated in plant tissue with other nutrients, disturbing the ions’ homeostasis in plants. The plant develops different mechanisms to tolerate the hazardous environmental effects of Cd. Recently studies found different miRNAs that are involved in Cd stress. In the current study, miR397 mutant lines were constructed to explore the molecular mechanisms of miR397 underlying Cd tolerance. Compared with the genetically modified line of overexpressed miR397 (artificial miR397, amiR397), the lines of downregulated miR397 (Short Tandem Target Mimic miR397, STTM miR397) showed more substantial Cd tolerance with higher chlorophyll a & b, carotenoid and lignin content. ICP-OES revealed higher cell wall Cd and low total Cd levels in STTM miR397 than in the wild-type and amiR397 plants.Further, the STTM plants produced fewer reactive oxygen species (ROS) and lower activity of antioxidants enzymes (e.g., catalase [CAT], malondialdehyde [MDA]) compared with amiR397 and wild-type plants after stress, indicating that silencing the expression of miR397 can reduce oxidative damage. In addition, the different family transporters’ gene expression was much higher in the amiR397 plants than in the wild type and STTM miRNA397. Our results suggest that miR397 plays a role in Cd tolerance in Arabidopsis thaliana. Overexpression of miR397 could decrease Cd tolerance in plants by regulating the expression of LAC 2/4/17, changing the lignin content, which may play an important role in inducing different stress-tolerant mechanisms and protecting the cell from a hazardous condition. This study provides a basis to elucidate the functions of miR397 and the Cd stress tolerance mechanism in Arabidopsis thaliana.Key messageThe miR397 modified lines influence the lignin and Cd content in the plants. The amiR397 plants susceptible to Cd stress have less lignin and high Cd content than STTM miR397 plants, changing the underlying stress regulatory pathways.

Background and aimsCadmium (Cd) contamination is a serious threat to plants and humans. Silicon (Si) was reported to have some alleviative effects on Cd stress in plants. However, whether Si alleviates Cd toxicity in maize genotypes with contrasting root system size are unknown.MethodsEffects of Si application (200 mg kg−1 soil) on shoot and root growth, Cd uptake and transportation under Cd stress (20 mg kg−1 soil) were assessed at the silking and maturity stages of maize genotypes Zhongke11 (deep-rooted) and Shengrui999 (shallow-rooted) in a pot experiment.ResultsApplication of Si significantly increased root dry weight, plant height and root length. Root volume and average root diameter were significantly positively correlated with root Cd concentration, bioaccumulation and translocation factor, respectively, of two maize genotypes at the silking stage. Addition of Si significantly increased Cd concentration, content, bioconcentration and translocation factor in roots of Zhongke11, but reduced the values of these parameters in Shengrui9999 at both growth stages. Grain Cd concentration in the combined Cd and Si treatment was decreased by 14.4% (Zhongke11) and 21.4% (Shengrui999) than that in Cd treatment. Grain yield was significantly negatively correlated with root Cd accumulation. Moreover, addition of Si significantly reduced Cd daily intake and health risk index in maize.ConclusionsThis study demonstrated that addition of Si reduced health risk by eliminating Cd accumulation in maize shoot and grain, and alleviated Cd stress with more profound effects in the shallow-rooted genotype Shengrui999.

Cadmium has been accumulating in the agricultural and ecological environment in recent years due to the release of industrial pollutants. Due to its high solubility, slow degradability and high toxicity, it is highly susceptible to occurring in agricultural fields. The presence of cadmium at low concentrations is harmful to plants. Heavy metal ATPases (HMAs) are proteins that can detoxify high concentrations of heavy metals through vacuole compartmentalization or exocytosis pathways. They have been extensively studied in plants. However, the cadmium transport function of HMAs in wheat has not been explored. In this study, a comprehensive and systematic investigation of HMA gene family members in wheat was conducted. A total of 28 putative TaHMAs were identified. Phylogenetically, these 28 putative TaHMAs were divided into two subgroups: Cu/Ag and Zn/Co/Cd/Pb. The gene structures and conserved motifs were consistent within the same branch and diverse in different branches. The TaHMA gene family is closely related to rice, B. distachyon and A. tauschii. GO analysis results suggest that TaHMAs may be involved in cation transport and membrane components. Protein interaction analysis results suggest that TaHMAs may interact with TaSOD to activate the SOD defense mechanism in wheat. Expression patterns exhibited tissue specificity. Finally, the expression patterns of TaHMAs were validated in the roots and leaves of wheat plants under cadmium stress. Our findings will be valuable for functional studies and applications of HMA gene family members in wheat.

Key messagePopulus euphratica PePCR2 increases Cd resistance by functioning as a Cd extrusion pump and by mediating the expression of genes encoding other transporters.Cadmium (Cd) is a non-essential, toxic metal that negatively affects plant growth. Plant cadmium resistance (PCR) proteins play key roles in the response to heavy metal stress. In this study, we isolated the gene PePCR2 encoding a plant PCR from Populus euphratica. PePCR2 gene transcription was induced by Cd, and its transcript level peaked at 24 h after exposure, at a level approximately 18-fold higher than that at 0 h. The PePCR2 protein was localized to the plasma membrane. Compared with yeast cells harboring the empty vector, yeast cells expressing PePCR2 showed enhanced Cd tolerance and a lower Cd content. Compared with wild-type (WT) plants, poplar overexpressing PePCR2 showed higher Cd resistance. Net Cd2+ efflux measurements showed that Cd2+ efflux from the roots was 1.5 times higher in the PePCR2-overexpressing plants than in WT plants. Furthermore, compared with WT plants, the PePCR2-overexpressing plants showed increased transcript levels of ABCG29, HMA5, PDR2, YSL7, and ZIP1 and decreased transcript levels of NRAMP6, YSL3, and ZIP11 upon exposure to Cd. These data show that PePCR2 increased Cd resistance by acting as a Cd extrusion pump and/or by regulating other Cd2+ transporters to decrease Cd toxicity in the cytosol. The results of this study identify a novel plant gene with potential applications in Cd removal, and provide a theoretical basis for reducing Cd toxicity and protecting food safety.

Melatonin (MT) and nitric oxide (NO) act as signaling molecules that can enhance cadmium (Cd) stress resistance in plants. However, little information is available about the relationship between MT and NO during seedling growth under Cd stress. We hypothesize that NO may be involved in how MT responds to Cd stress during seedling growth. The aim of this study is to evaluate the relationship and mechanism of response. The results indicate that different concentrations of Cd inhibit the growth of tomato seedlings. Exogenous MT or NO promotes seedling growth under Cd stress, with a maximal biological response at 100 μM MT or NO. The promotive effects of MT-induced seedling growth under Cd stress are suppressed by NO scavenger 2-4-carboxyphenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (cPTIO), suggesting that NO may be involved in MT-induced seedling growth under Cd stress. MT or NO decreases the content of hydrogen peroxide (H2O2), malonaldehyde (MDA), dehydroascorbic acid (DHA), and oxidized glutathione (GSSG); improves the content of ascorbic acid (AsA) and glutathione (GSH) and the ratios of AsA/DHA and GSH/GSSG; and enhances the activities of glutathione reductase (GR), monodehydroascorbic acid reductase (MDHAR), dehydroascorbic acid reductase (DHAR), ascorbic acid oxidase (AAO), and ascorbate peroxidase (APX) to alleviate oxidative damage. Moreover, the expression of genes associated with the ascorbate–glutathione (AsA-GSH) cycle and reactive oxygen species (ROS) are up-regulated by MT or NO under Cd conditions, including AAO, AAOH, APX1, APX6, DHAR1, DHAR2, MDHAR, and GR. However, NO scavenger cPTIO reverses the positive effects regulated by MT. The results indicate that MT-mediated NO enhances Cd tolerance by regulating AsA-GSH cycle and ROS metabolism.

Selenium (Se) can promote the growth and resistance of agricultural crops as fertilizers, while the role of nano-selenium (nano-Se) against Cd remains unclear in pepper plants ( Capsicum annuum L.). Biofortification with nano-Se observably restored Cd stress by decreasing the level of Cd in plant tissues and boosting the accumulation in biomass. The Se compounds transformed by nano-Se were primarily in the form of SeMet and MeSeCys in pepper tissues. Differential metabolites and the genes of plant signal transduction and lignin biosynthesis were measured by employing transcriptomics and determining target metabolites. The number of lignin-related genes ( PAL , CAD , 4CL , and COMT ) and contents of metabolites (sinapyl alcohol, phenylalanine, p -coumaryl alcohol, caffeyl alcohol, and coniferaldehyde) were remarkably enhanced by treatment with Cd1Se0.2, thus, maintaining the integrity of cell walls in the roots. It also enhanced signal transduction by plant hormones and responsive resistance by inducing the biosynthesis of genes ( BZR1 , LOX3 , and NCDE1 ) and metabolites (brassinolide, abscisic acid, and jasmonic acid) in the roots and leaves. In general, this study can enable a better understanding of the protective mechanism of nano-Se in improving the capacity of plants to resist environmental stress.

Cadmium (Cd) contamination has become a major environmental issue and has toxic effects on agricultural crops. Selenium (Se) is an essential trace element that plays an important role due to its impact on several physiological and biochemical processes in plants. This study addresses the mechanistic insights into the role of SeNPs in enhancing Cd stress tolerance, thereby contributing to sustainable nano-agronomic strategies for the soils contaminated with heavy metals (HMs). The current experimental approach involved a pot trial conducted in a greenhouse with two levels of cadmium stress (C 1  = control; C 2  = 20 mg kg −1 of soil w/w using CdCl 2 ), and four levels of SeNPs (0, 25, 50, and 75 mg L −1 ) in mungbean plants. The findings indicated that cd stressed conditions significantly affected the productivity of mungbean plants. The optimal level of SeNPs significantly enhanced the growth, biomass, and photosynthetic traits of mungbean. This study also indicated that Cd-stressed conditions increased lipid peroxidation and membrane damage. Moreover, SeNPs application increased soluble protein (110.10%), and decreased the proline accumulation (64.45%) and malondialdehyde (MDA) contents (64.25%) in comparison with control (0 mg L −1 of SeNPs). Furthermore, the SeNPs also decreased the leaf Cd contents by 64.95% and grain Cd contents by 64.88% by reduced Cd uptake. An optimum level of SeNPs offers great potential as an eco-friendly and feasible method for mitigating the effects of Cd stress on mungbean plants. However, long-term environmental impact of Se NPs, including their bioavailability, accumulation, and potential toxicity, is crucial for ensuring their safe and sustainable use in agriculture.