Long-term cadmium exposure influences the abundance of proteins that impact the cell wall structure in Medicago sativa stems

Responses and resistance capacity of Solanum nigrum L. mediated by three ecological category earthworms in metal-[Cd-As-Cu-Pb]-contaminated soils of North China

Earthworms play vital functions affecting plant growth and metal accumulation from downground to aboveground. Soil metal mobilization may be combined with use of earthworm and hyperaccumulator-Solanum nigrum to improve its remediation efficiency. Understanding the effects of specific-species earthworm belonging to different ecological categories on mechanisms underlying of S. nigrum is critical for metal-polluted remediation. However, seldom studies concerned earthworm-assisted phytoremediation of metal contaminated soil in Northern China. This study investigated the effects of earthworm (Eisenia fetida, Amynthas hupeiensis and Drawida gisti) on S. nigrum with exposure to uncontaminated and [Cd-As-Cu-Pb]-contaminated soil (referred to as S0 and S1) for 60 days, respectively. In S1 soil, A. hupeiensis (anecic) had stronger effects on growth and metal accumulation in the organs (root, stem, and leaf) of S. nigrum than D. gisti (endogeic) and E. fetida (epigeic), attributing to their ecological category. The BAF values of S. nigrum were generally ranking in Cd (0.66-5.13) > As (0.03-1.85) > Cu (0.03-0.06) > Pb (0.01-0.05); the BAFCd values were ranking in leaf (2.34-5.13) > root (1.96-4.14) > stem (0.66-1.33); BAFAs, BAFCu, and BAFPb were root (0.04-1.63) > stem (0.01-0.09) ≈ leaf (0.01-0.06). A. hupeiensis decreased the TF values of S. nigrum from the roots to the shoots. Co-effects of metal stress and earthworm activity on metal uptake by shoots suggested that A. hupeiensis increased the uptake of As, Cu, and Pb (by 56.3 %, 51.5 %, and 16.2 %, p < 0.05), but not Cd, which appeared to remain steady for prolonged durations. Alterations in the integrated biomarker response index version 2 (IBRv2) values demonstrated that A. hupeiensis (12.65) improved the resistance capacity (stimulated GSH, SnGS1, and SnCu-SOD) of S. nigrum under metal-containing conditions, compared with E. fetida and D. gisti (IBRv2 were 9.61 and 9.11). This study may provide insights into the patterns of 'soil-earthworm-plant system' on improving remediation efficiency of S. nigrum, from the perspective of earthworm ecological niche partitioning.

How plants respond to heavy metal contamination: a narrative review of proteomic studies and phytoremediation applications

MAIN CONCLUSION

Heavy metal pollution caused by human activities is a serious threat to the environment and human health. Plants have evolved sophisticated defence systems to deal with heavy metal stress, with proteins and enzymes serving as critical intercepting agents for heavy metal toxicity reduction. Proteomics continues to be effective in identifying markers associated with stress response and metabolic processes. This review explores the complex interactions between heavy metal pollution and plant physiology, with an emphasis on proteomic and biotechnological perspectives. Over the last century, accelerated industrialization, agriculture activities, energy production, and urbanization have established a constant need for natural resources, resulting in environmental degradation. The widespread buildup of heavy metals in ecosystems as a result of human activity is especially concerning. Although some heavy metals are required by organisms in trace amounts, high concentrations pose serious risks to the ecosystem and human health. As immobile organisms, plants are directly exposed to heavy metal contamination, prompting the development of robust defence mechanisms. Proteomics has been used to understand how plants react to heavy metal stress. The development of proteomic techniques offers promising opportunities to improve plant tolerance to toxicity from heavy metals. Additionally, there is substantial scope for phytoremediation, a sustainable method that uses plants to extract, sequester, or eliminate contaminants in the context of changes in protein expression and total protein behaviour. Changes in proteins and enzymatic activities have been highlighted to illuminate the complex effects of heavy metal pollution on plant metabolism, and how proteomic research has revealed the plant's ability to mitigate heavy metal toxicity by intercepting vital nutrients, organic substances, and/or microorganisms.

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

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

Transcriptome Analysis Reveals Novel Insights into the Hyperaccumulator Phytolacca acinosa Roxb. Responses to Cadmium Stress

Cadmium (Cd) is a highly toxic heavy metal that causes serious damage to plant and human health. Phytolacca acinosa Roxb. has a large amount of aboveground biomass and a rapid growth rate, and it has been identified as a novel type of Cd hyperaccumulator that can be harnessed for phytoremediation. However, the molecular mechanisms underlying the response of P. acinosa to Cd2+ stress remain largely unclear. In this study, the phenotype, biochemical, and physiological traits of P. acinosa seeds and seedlings were analyzed under different concentrations of Cd2+ treatments. The results showed higher Cd2+ tolerance of P. acinosa compared to common plants. Meanwhile, the Cd2+ content in shoots reached 449 mg/kg under 10 mg/L Cd2+ treatment, which was obviously higher than the threshold for Cd hyperaccumulators. To investigate the molecular mechanism underlying the adaptability of P. acinosa to Cd stress, RNA-Seq was used to examine transcriptional responses of P. acinosa to Cd stress. Transcriptome analysis found that 61 genes encoding TFs, 48 cell wall-related genes, 35 secondary metabolism-related genes, 133 membrane proteins and ion transporters, and 96 defense system-related genes were differentially expressed under Cd2+ stress, indicating that a series of genes were involved in Cd2+ stress, forming a complex signaling regulatory mechanism. These results provide new scientific evidence for elucidating the regulatory mechanisms of P. acinosa response to Cd2+ stress and new clues for the molecular breeding of heavy metal phytoremediation.

Transcriptome analysis reveals how cadmium promotes root development and accumulates in Apocynum venetum, a promising plant for greening cadmium-contaminated soil

Cadmium (Cd) contamination poses a substantial threat the environment, necessitating effective remediation strategies. Phytoremediation emerges as a cost-efficient and eco-friendly approach for reducing Cd levels in the soil. In this study, the suitability of A. venetum for ameliorating Cd-contaminated soils was evaluated. Mild Cd stress promoted seedling and root growth, with the root being identified as the primary tissue for Cd accumulation. The Cd content of roots ranged from 0.35 to 0.55 mg/g under treatment with 10-50 µM CdCl2·2.5 H2O, and the bioaccumulation factor ranged from 28.78 to 84.43. Transcriptome sequencing revealed 20,292 unigenes, and 7507 nonredundant differentially expressed genes (DEGs) were identified across five comparison groups. DEGs belonging to the "MAPK signaling pathway-plant," "monoterpenoid biosynthesis," and "flavonoid biosynthesis pathway" exhibited higher expression levels in roots compared to stems and leaves. In addition, cytokinin-related DEGs, ROS scavenger genes, such as P450, glutathione-S-transferase (GST), and superoxide dismutase (SOD), and the cell wall biosynthesis-related genes, CSLG and D-GRL, were also upregulated in the root tissue, suggesting that Cd promotes root development. Conversely, certain ABC transporter genes, (e.g, NRAMP5), and some vacuolar iron transporters, predominantly expressed in the roots, displayed a strong correlation with Cd content, revealing the mechanism underlying the compartmentalized storage of Cd in the roots. KEGG enrichment analysis of DEGs showed that the pathways associated with the biosynthesis of flavonoids, lignin, and some terpenoids were significantly enriched in the roots under Cd stress, underscoring the pivotal role of these pathways in Cd detoxification. Our study suggests A. venetum as a potential Cd-contaminated phytoremediation plant and provides insights into the molecular-level mechanisms of root development promotion and accumulation mechanism in response to Cd stress.

The impacts of exogenous phosphorus on Cd absorption in perennial ryegrass root cell: Kinetic and mechanism study

Phosphorus (P) is critical to plants in metal-contaminated soils because it participates in various biochemical reactions during plant growth. However, the mechanisms of P in mitigating the toxicity of heavy metals to ryegrass root is still veiled. In this study, the physiological and biochemical dynamics of the ryegrass root under various cadmium (Cd) and P conditions were investigated in a hydroponic system. Cd stress decreased the length of the ryegrass root, but P application enhanced the root elongation to reduce the Cd concentration in the root. Both Cd and P dosages were positively correlated with hemicellulose 1 content, pectin content, and PME activity, while having a negative effect on cellulose content. Moreover, the addition of 80 mg L-1 P increased the contents of pectin and hemicellulose 1 by 2.5 and 5.8% even with 4 mg L-1 Cd. In addition, P supply increased pectin methylesterbase activity under Cd stress, which further changed the extra-cytoplasmic structures and cell wall composition. Thus, exogenous P promoted the immobilization of Cd onto the cell wall and protected protoplast primarily through indirectly regulating the binding capacity of the root cell wall for Cd.

A Study on the Use of the Phyto-Courier Technology in Tobacco Leaves Infected by Agrobacterium tumefaciens

Climate change results in exceptional environmental conditions and drives the migration of pathogens to which local plants are not adapted. Biotic stress disrupts plants' metabolism, fitness, and performance, ultimately impacting their productivity. It is therefore necessary to develop strategies for improving plant resistance by promoting stress responsiveness and resilience in an environmentally friendly and sustainable way. The aim of this study was to investigate whether priming tobacco plants with a formulation containing silicon-stabilised hybrid lipid nanoparticles functionalised with quercetin (referred to as GS3 phyto-courier) can protect against biotic stress triggered by Agrobacterium tumefaciens leaf infiltration. Tobacco leaves were primed via infiltration or spraying with the GS3 phyto-courier, as well as with a buffer (B) and free quercetin (Q) solution serving as controls prior to the biotic stress. Leaves were then sampled four days after bacterial infiltration for gene expression analysis and microscopy. The investigated genes increased in expression after stress, both in leaves treated with the phyto-courier and control solutions. A trend towards lower values was observed in the presence of the GS3 phyto-courier for genes encoding chitinases and pathogenesis-related proteins. Agroinfiltrated leaves sprayed with GS3 confirmed the significant lower expression of the pathogenesis-related gene PR-1a and showed higher expression of peroxidase and serine threonine kinase. Microscopy revealed swelling of the chloroplasts in the parenchyma of stressed leaves treated with B; however, GS3 preserved the chloroplasts' mean area under stress. Furthermore, the UV spectrum of free Q solution and of quercetin freshly extracted from GS3 revealed a different spectral signature with higher values of maximum absorbance (Amax) of the flavonoid in the latter, suggesting that the silicon-stabilised hybrid lipid nanoparticles protect quercetin against oxidative degradation.

Recent Advances in Microbial-Assisted Remediation of Cadmium-Contaminated Soil

Soil contamination with cadmium (Cd) is a severe concern for the developing world due to its non-biodegradability and significant potential to damage the ecosystem and associated services. Industries such as mining, manufacturing, building, etc., rapidly produce a substantial amount of Cd, posing environmental risks. Cd toxicity in crop plants decreases nutrient and water uptake and translocation, increases oxidative damage, interferes with plant metabolism and inhibits plant morphology and physiology. However, various conventional physicochemical approaches are available to remove Cd from the soil, including chemical reduction, immobilization, stabilization and electro-remediation. Nevertheless, these processes are costly and unfriendly to the environment because they require much energy, skilled labor and hazardous chemicals. In contrasting, contaminated soils can be restored by using bioremediation techniques, which use plants alone and in association with different beneficial microbes as cutting-edge approaches. This review covers the bioremediation of soils contaminated with Cd in various new ways. The bioremediation capability of bacteria and fungi alone and in combination with plants are studied and analyzed. Microbes, including bacteria, fungi and algae, are reported to have a high tolerance for metals, having a 98% bioremediation capability. The internal structure of microorganisms, their cell surface characteristics and the surrounding environmental circumstances are all discussed concerning how microbes detoxify metals. Moreover, issues affecting the effectiveness of bioremediation are explored, along with potential difficulties, solutions and prospects.

Tryptophan pretreatment adjusts transcriptome and metabolome profiles to alleviate cadmium toxicity in Arabidopsis

Cadmium (Cd) is highly toxic to all organisms including plants, and recently tryptophan (Trp) pretreatment of plant seedlings is shown to improve Cd tolerance. But the underlying mechanism remains largely unknown. In this study, we used Arabidopsis (Arabidopsis thaliana) to determine the physiological relevance of Trp pretreatment in alleviating Cd toxicity in plants and explore its molecular mechanism with a focus on the metabolic pathways. The results showed that Trp pretreatment maintained the biomass and root lengths, relieved Cd-induced lipid peroxidation, and reduced Cd transport to the shoots, and eventually improved the response against Cd in Arabidopsis seedlings. The integrative analyses of the transcriptome and metabolome further revealed that Trp pretreatment alleviated Cd toxicity not only through a known mechanism of producing a major auxin indole-3-acetic acid and maintaining its levels, but also through two previously unrecognized mechanisms: increasing the area and strength of cell walls by promoting lignification to further reduce Cd entry, and fine-tuning Cd detoxification products derived from sulfur-containing amino acid metabolism. Our findings thereby provide deep mechanical insights into how Trp alleviates Cd toxicity in plants.

Cadmium tolerance and hyperaccumulation in plants - A proteomic perspective of phytoremediation

Cadmium (Cd) is a major environmental pollutant and poses a risk of transfer into the food chain through contaminated plants. Mechanisms underlying Cd tolerance and hyperaccumulation in plants are not fully understood. Proteomics-based approaches facilitate an in-depth understanding of plant responses to Cd stress at the systemic level by identifying Cd-inducible differentially abundant proteins (DAPs). In this review, we summarize studies related to proteomic changes associated with Cd-tolerance mechanisms in Cd-tolerant crops and Cd-hyperaccumulating plants, especially the similarities and differences across plant species. The enhanced DAPs identified through proteomic studies can be potential targets for developing Cd-hyperaccumulators to remediate Cd-contaminated environments and Cd-tolerant crops with low Cd content in the edible organs. This is of great significance for ensuring the food security of an exponentially growing global population. Finally, we discuss the methodological drawbacks in current proteomic studies and propose that better protocols and advanced techniques should be utilized to further strengthen the reliability and applicability of future Cd-stress-related studies in plants. This review provides insights into the improvement of phytoremediation efficiency and an in-depth study of the molecular mechanisms of Cd enrichment in plants.

Morphological and Physiological Indicators and Transcriptome Analyses Reveal the Mechanism of Selenium Multilevel Mitigation of Cadmium Damage in Brassica juncea

Cadmium (Cd) is a common agricultural soil pollutant, which does serious harm to the environment and the human body. In this study, Brassica juncea was treated with different concentrations of CdCl₂ and Na₂SeO₃. Then, physiological indexes and transcriptome were measured to reveal the mechanisms by which Se reduces the inhibition and toxicity of Cd in B. juncea. The results showed that Se alleviated the inhibitive Cd effects on seedling biomass, root length, and chlorophyll, and promoted the adsorption of Cd by pectin and lignin in the root cell wall (CW). Se also alleviated the oxidative stress induced by Cd, and reduced the content of MDA in cells. As a result, SeCys and SeMet alleviated the transport of Cd to the shoots. Transcriptome data showed that the bivalent cation transporter MPP and ABCC subfamily participated in the separation of Cd in vacuoles, CAL1 was related to the chelation of Cd in the cytoplasm of cells, and ZIP transporter 4 reduced the transport of Cd to the shoots. These results indicated that Se alleviated the damage of Cd in plants and decreased its transport to the shoots by improving the antioxidant system, enhancing the ability of the CW to adsorb Cd, reducing the activity of Cd transporters, and chelating Cd.

Cadmium-induced oxidative stress responses and acclimation in plants require fine-tuning of redox biology at subcellular level

Cadmium (Cd) is one of the most toxic compounds released into our environment and is harmful to human health, urging the need to remediate Cd-polluted soils. To this end, it is important to increase our insight into the molecular mechanisms underlying Cd stress responses in plants, ultimately leading to acclimation, and to develop novel strategies for economic validation of these soils. Albeit its non-redox-active nature, Cd causes a cellular oxidative challenge, which is a crucial determinant in the onset of diverse signalling cascades required for long-term acclimation and survival of Cd-exposed plants. Although it is well known that Cd affects reactive oxygen species (ROS) production and scavenging, the contribution of individual organelles to Cd-induced oxidative stress responses is less well studied. Here, we provide an overview of the current information on Cd-induced organellar responses with special attention to redox biology. We propose that an integration of organellar ROS signals with other signalling pathways is essential to finetune plant acclimation to Cd stress.

Temporal transcriptome analysis reveals several key pathways involve in cadmium stress response in Nicotiana tabacum L

Tobacco has a strong cadmium (Cd) enrichment capacity, meaning that it can absorb large quantities from the environment, but too much Cd will cause damage to the plant. It is not yet clear how the plant can dynamically respond to Cd stress. Here, we performed a temporal transcriptome analysis of tobacco roots under Cd treatment from 0 to 48 h. The number of differentially expressed genes (DEGs) was found to change significantly at 3 h of Cd treatment, which we used to define the early and middle stages of the Cd stress response. The gene ontology (GO) term analysis indicates that genes related to photosynthesis and fatty acid synthesis were enriched during the early phases of the stress response, and in the middle phase biological process related to metal ion transport, DNA damage repair, and metabolism were enriched. It was also found that plants use precursor mRNA (pre-mRNA) processes to first resist Cd stress, and with the increasing of Cd treatment time, the overlapped genes number of DEGs and DAS increased, suggesting the transcriptional levels and post-transcriptional level might influence each other. This study allowed us to better understand how plants dynamically respond to cadmium stress at the transcriptional and post-transcriptional levels and provided a reference for the screening of Cd-tolerant genes in the future.

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

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

Silicon Actuates Poplar Calli Tolerance after Longer Exposure to Antimony

The presence of antimony (Sb) in high concentrations in the environment is recognized as an emerging problem worldwide. The toxicity of Sb in plant tissues is known; however, new methods of plant tolerance improvement must be addressed. Here, poplar callus (Populus alba L. var. pyramidallis) exposed to Sb(III) in 0.2 mM concentration and/or to silicon (Si) in 5 mM concentration was cultivated in vitro to determine the impact of Sb/Si interaction in the tissue. The Sb and Si uptake, growth, the activity of superoxide dismutase (SOD), catalase (CAT), guaiacol-peroxidase (G-POX), nutrient concentrations, and the concentrations of photosynthetic pigments were investigated. To elucidate the action of Si during the Sb-induced stress, the impact of short and long cultivations was determined. Silicon decreased the accumulation of Sb in the calli, regardless of the length of the cultivation (by approx. 34%). Antimony lowered the callus biomass (by approx. 37%) and decreased the concentrations of photosynthetic pigments (up to 78.5%) and nutrients in the tissue (up to 21.7%). Silicon supported the plant tolerance to Sb via the modification of antioxidant enzyme activity, which resulted in higher biomass production (increased by approx. 35%) and a higher uptake of nutrients from the media (increased by approx. 10%). Silicon aided the development of Sb-tolerance over the longer cultivation period. These results are key in understanding the action of Si-developed tolerance against metalloids.

Proteomic Analysis Reveals the Association between the Pathways of Glutathione and α-Linolenic Acid Metabolism and Lanthanum Accumulation in Tea Plants

Lanthanum can affect the growth and development of the tea plant. Tieguanyin (TGY) and Shuixian (SX) cultivars of Camellia sinensis were selected to explore the mechanism underlying the accumulation of lanthanum (tea plants' most accumulated rare earth element) through proteomics. Roots and fresh leaves of TGY and SX with low- and high-accumulation potential for lanthanum, respectively, were studied; 845 differentially expressed proteins (DEPs) were identified. Gene ontology analysis showed that DEPs were involved in redox processes and related to molecular functions. Kyoto Encyclopedia of Genes and Genomes metabolic pathway analysis showed that DEPs were associated with glutathione (GSH) and α-linolenic acid metabolism, plant pathogen interaction, and oxidative phosphorylation. Thirty-seven proteins in the GSH metabolism pathway showed significant differences, wherein 18 GSH S-transferases showed differential expression patterns in the root system. Compared with the control, expression ratios of GST (TEA004130.1) and GST (TEA032216.1) in TGY leaves were 6.84 and 4.06, respectively, after lanthanum treatment; these were significantly higher than those in SX leaves. The LOX2.1 (TEA011765.1) and LOX2.1 (TEA011776.1) expression ratios in the α-linolenic acid metabolic pathway were 2.44 and 6.43, respectively, in TGY roots, which were significantly higher than those in SX roots. The synthesis of specific substances induces lanthanum-associated defense responses in TGY, which is of great significance for plant yield stability.

Transcriptome alterations of radish shoots exposed to cadmium can be interpreted in the context of leaf senescence

Till now few transcriptome studies have described shoot responses of heavy metal (HM)-sensitive plants to excess Cd and still a unifying model of Cd action is lacking. Using RNA-seq technique, the transcriptome responses of radish (Raphanus sativus L.) leaves to Cd stress were investigated in plants raised hydroponically under control and 5.0 mg L^-1 Cd. The element was mainly accumulated in roots and led to declined biomass and photosynthetic pigments, increased H₂O₂ and lipid peroxidation, and the accumulation of sugars, protein thiols, and phytochelatins. Out of 524 differentially expressed genes (DEGs), 244 and 280 upregulated and downregulated ones were assigned to 82 and 115 GO terms, respectively. The upregulated DEGs were involved in osmotic regulation, protein metabolism, chelators, and carbohydrate metabolisms, whereas downregulated DEGs were related to photosynthesis, response to oxidative stress, glucosinolate, and secondary metabolite biosynthesis. Our transcriptome data suggest that Cd triggers ROS production and photosynthesis decline associated with increased proteolysis through ubiquitin-proteasome system (UPS)- and chloroplast-proteases and in this way brings about re-mobilization of N and C stores into amino acids and sugars. Meanwhile, declined glucosinolate metabolism in favor of chelator synthesis and upregulation of dehydrins as inferred from transcriptome analysis confers shoots some tolerance to the HM-derived ionic/osmotic imbalances. Thus, the induction of leaf senescence might be a major long-term response of HM-sensitive plants to Cd toxicity.

Plant responses to metals stress: microRNAs in focus

Metal toxicity can largely affect the growth and yield of numerous plant species. Plants have developed specific mechanisms to withstand the varying amounts of metals. One approach involves utilization of microRNAs (miRNAs) that are known for cleaving transcripts or inhibiting translation to mediate post-transcriptional control. Use of transcription factors (TFs) or gene regulation in metal detoxification largely depends on metal-responsive miRNAs. Moreover, systemic signals and physiological processes for plants response to metal toxicities are likewise controlled by miRNAs. Therefore, it is necessary to understand miRNAs and their regulatory networks in relation to metal stress. The miRNA-based approach can be important to produce metal-tolerant plant species. Here, we have reviewed the importance of plant miRNAs and their role in mitigating metal toxicities. The current review also discusses the specific advances that have occurred as a result of the identification and validation of several metal stress-responsive miRNAs.

Physiology and proteomics reveal Fulvic acid mitigates Cadmium adverse effects on growth and photosynthetic properties of lettuce

Understanding the molecular mechanisms of plants in response to Cd stress is crucial for improving plants adaptation to Cd stress. Fulvic acid (FA) is an active humic substance that is often used as a soil conditioner. However, there are few reports on the role of FA against Cd stress. The aim of this study was to determine the effects of Fulvic acid on alleviation of Cd toxicity in lettuce (Lactuca sativa L) under hydroponic conditions. Our results showed that 20 μmol/L Cd stress significantly reduced photosynthetic pigment metabolism and the expression of photosynthetic apparatus-related proteins, thereby inhibiting photosynthetic electron transport, net photosynthetic rate and negatively affecting photosynthetic carbon assimilation and growth of lettuce. However, proteomic findings suggest that the application of FA can reduce the adverse effects of Cd contamination. Compared to Cd stress alone, FA significantly increased the expression of Light-harvesting proteins, reaction center and electron transport-related proteins. Further results showed that FA at 0.5 g/L reduced the uptake of Cd by the roots, resulting in a 23.5% reduction in total Cd content in lettuce. Moreover, FA enhanced S metabolism and rebuilt redox homeostasis in cells. Overall, these findings provide new insights into the mechanism of cadmium toxicity mitigation in lettuce by FA. Which is recommended as an eco-friendly tool for improving the photosynthesis performance and biomass of lettuce under Cd stress.

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

Large areas of farmland in southern China are facing environmental problems such as cadmium (Cd) contamination and boron (B) deficiency. The aim of this study was to investigate the biochemical and molecular mechanisms underlying the reduction in Cd accumulation in hot pepper (Capsicum annuum) by B application. A hydroponic experiment was conducted to compare the subcellular distribution of Cd, transcriptome profile, degree of pectin methylation, and glutathione (GSH) synthesis in the roots of hot pepper under different B and Cd conditions. Boron supply promoted root cell wall biosynthesis and pectin demethylation by upregulating related genes and increasing cell wall Cd concentration by 28%. In addition, with the application of B, the proportion of Cd in root cell walls increased from 27% to 37%. Boron supplementation upregulated sulfur metabolism-related genes but decreased cysteine and GSH contents in the roots. As a result, shoot Cd concentration decreased by 27% due to the decrease in GSH, a critical long-distance transport carrier of Cd. Consequently, B supply could reduce the uptake, translocation, and accumulation of Cd in hot pepper by retaining Cd in the root cell walls and decreasing GSH content.

Hydrogen peroxide contributes to cadmium binding on root cell wall pectin of cadmium-safe rice line (Oryza sativa L.)

Cell wall pectin is essential for cadmium (Cd) accumulation in rice roots and hydrogen peroxide (H₂O₂) plays an important role as a signaling molecule in cell wall modification. The role of H₂O₂ in Cd binding in cell wall pectin is unclear. D62B, a Cd-safe rice line, was found to show a greater Cd binding capacity in the root cell wall than a high Cd-accumulating rice line of Wujin4B. In this study, we further investigated the mechanism of the role of H₂O₂ in Cd binding in root cell wall pectin of D62B compared with Wujin4B. Cd treatment significantly increased the H₂O₂ concentration and pectin methyl esterase (PME) activity in the roots of D62B and Wujin4B by 22.45-42.44% and 12.15-15.07%, respectively. The H₂O₂ concentration and PME activity significantly decreased in the roots of both rice lines when H₂O₂ was scavenged by 4-hydroxy-Tempo. The PME activity of D62B was higher than that of Wujin4B. The concentrations of high and low methyl-esterified pectin in the roots of D62B significantly increased when exposed to Cd alone but significantly decreased when exposed to Cd and exogenous 4-hydroxy-Tempo. No significant difference was detected in Wujin4B. Exogenous 4-hydroxy-Tempo significantly decreased the Cd concentration in the cell wall pectin in both rice lines. The modification of H₂O₂ in Cd binding was further explored by adding H₂O₂. The maximum Cd adsorption amounts on the root cell walls of both rice lines were improved by exogenous H₂O₂·H₂O₂ treatment significantly influenced the relative peak area of the main functional groups (hydroxyl, carboxyl), and the groups intensely shifted after Cd adsorption in the root cell wall of D62B, while there was no significant difference in Wujin4B. In conclusion, Cd stress stimulated the production of H₂O₂, thus promoting pectin biosynthesis and demethylation and releasing relative functional groups involved in Cd binding on cell wall pectin, which is beneficial for Cd retention in the roots of Cd-safe rice line.

Unraveling Cadmium Toxicity in Trifolium repens L. Seedling: Insight into Regulatory Mechanisms Using Comparative Transcriptomics Combined with Physiological Analyses

Trifolium repens (T. repens) can accumulate significant amounts of heavy metal ions, and has strong adaptability to wide environmental conditions, and relatively large biomass, which is considered a potential plant for phytoremediation. However, the molecular mechanisms of T. repens involved in Cd tolerance have not yet been studied in detail. This study was conducted to examine the integrative responses of T. repens exposed to a high-level CdCl₂ by investigating the physiological and transcriptomic analyses. The results suggested that T. repens seedlings had a high degree of tolerance to Cd treatment. The roots accumulated higher Cd concentration than leaves and were mainly distributed in the cell wall. The content of MDA, soluble protein, the relative electrolyte leakage, and three antioxidant enzymes (POD, SOD, and APX) was increased with the Cd treatment time increasing, but the CAT enzymes contents were decreased in roots. Furthermore, the transcriptome analysis demonstrated that the differentially expressed genes (DEGs) mainly enriched in the glutathione (GSH) metabolism pathway and the phenylpropanoid biosynthesis in the roots. Overexpressed genes in the lignin biosynthesis in the roots might improve Cd accumulation in cell walls. Moreover, the DEGs were also enriched in photosynthesis in the leaves, transferase activity, oxidoreductase activity, and ABA signal transduction, which might also play roles in reducing Cd toxicity in the plants. All the above, clearly suggest that T. repens employ several different mechanisms to protect itself against Cd stress, while the cell wall biosynthesis and GSH metabolism could be considered the most important specific mechanisms for Cd retention in the roots of T. repens.

Phytoremediation of potentially toxic elements (PTEs) contaminated soils using alfalfa (Medicago sativa L.): A comprehensive review

Soil contamination with potentially toxic elements (PTEs) is an increasing environmental problem, posing serious threats to the living organisms. Phytoremediation is a sustainable and highly accepted technology for remediation of PTE-contaminated soils. Alfalfa has been widely adopted for the phytoremediation of PTE-contaminated soils due to its large biomass productivity, high PTE tolerance, and strong capacity to take up PTEs. However, there are still no literature reviews systematically summarized the potential of alfalfa in the phytoremediation. Therefore, we review the available literatures that present its PTE uptake, phytotoxicity, tolerance mechanisms, and aided techniques improving the phytoremediation efficiency. In this review, alfalfa shows high amounts of PTEs accumulation, especially in their root tissue. Meanwhile, the inner mechanisms of PTE tolerance and accumulation in alfalfa are discussed including: (i) the activation of antioxidant enzyme system, (ii) subcellular localization, (iii) production of glutathione, phytochelatins, and proline, and (iv) regulation of gene expression. Indeed, excessive PTE can overcome the defense system, which causes oxidative damage in alfalfa plants, thereby inhibiting growth and physiological processes and weakening the ability of PTE uptake. Till now, several approaches have been developed to improve the tolerance and/or accumulation of PTE in alfalfa plants as follows: (i) selection of PTE tolerant cultivars, (ii) applying plant growth regulators, (iii) addition of chelating agents, fertilizer, and biochar materials, and (iv) inoculation of soil microbes. Finally, we indicate that the selection of PTE-tolerant cultivars along with inoculation of soil microbes may be an efficient and eco-friendly strategy of the soil PTE phytoremediation.

Comparative cytology combined with transcriptomic and metabolomic analyses of Solanum nigrum L. in response to Cd toxicity

Cadmium (Cd) triggers molecular alterations in plants, perturbs metabolites and damages plant growth. Therefore, understanding the molecular mechanism underlying the Cd tolerance in plants is necessary for assessing the persistent environmental impact of Cd. In this study, Solanum nigrum was selected as the test plant to investigate changes in biomass, Cd translocation, cell ultrastructure, metabolites and genes under hydroponic conditions. The results showed that the plant biomass was significantly decreased under Cd stress, and the plant has a stronger Cd transport capability. Transmission electron microscopy revealed that increased Cd concentration gradually damaged the plant organs (roots, stems and leaves) cell ultrastructure, as evidenced by swollen chloroplasts and deformed cell walls. Additionally, metabolomics analyses revealed that Cd stress mainly affected seven metabolism pathways, including 19 differentially expressed metabolites (DEMs). Moreover, 3908 common differentially expressed genes (DEGs, 1049 upregulated and 2859 downregulated) were identified via RNA-seq among five Cd treatments. Meanwhile, conjoint analysis found several DEGs and DEMs, including laccase, peroxidase, D-fructose, and cellobiose etc., are associated with cell wall biosynthesis, implying the cell wall biosynthesis pathway plays a critical role in Cd detoxification. Our comprehensive investigation using multiple approaches provides a molecular-scale perspective on plant response to Cd stress.

The role of silicon in cadmium alleviation by rice root cell wall retention and vacuole compartmentalization under different durations of Cd exposure

Silicon (Si) plays a pivotal role in mitigating phytotoxicity caused by cadmium (Cd). However, few former reports focused on the internal mechanism how Si assisted in alleviating Cd stress in rice under different durations of Cd exposure. Herein, the effects of Si on subcellular distribution of Cd in rice roots under short-term (12 h) and long-term (20 d) Cd exposure were explored. Results showed that Si decreased shoot Cd concentration but had little impact on root Cd levels. Under short-term Cd exposure, subcellular distribution analysis showed that Si increased the ratio of Cd in root cell wall by 23.2~24.0%, and decreased the ratio of Cd in root soluble fraction by 20.6~21.5%. This suggested that Si supply improved root retention of Cd by fixing it on the cell wall and thus restricted intracellular transportation of Cd. Further analysis unraveled that pectin (especially ionic-soluble pectin) of the cell wall was the main binding component, and Si supply induced more Cd accumulation in covalent-soluble pectin and hemicellulose. Moreover, the overexpression of germin-like proteins (GLPs) proved the role of cell wall in moderating Cd toxicity. Under long-term Cd exposure, Si promoted phytochelatin 2 (PC2) and phytochelatin 3 (PC3) synthesis in cytosol, at the same time, Si down-regulated the expression of the Cd efflux-related protein multidrug resistance-associated protein-like ATP-binding cassette transporters (MRP-like ABC transporters) and limited Cd transportation from vacuole to cytosol. Taken together, Si rather predominates in limiting Cd translocation by the cell wall of root under short-term Cd exposure and promoting vacuole compartmentalization to mitigate the Cd toxicity under long-term exposure, instead of reducing the absorption of Cd in rice roots, thereby decreasing Cd delivery into shoots.

Molecular and biochemical mechanisms underlying boron-induced alleviation of cadmium toxicity in rice seedlings

Both cadmium (Cd) contamination and boron (B) deficiency in farmland soils pose a threat to the yield and quality of crops in Southern China. The present study investigated the mechanisms by which B reduces Cd accumulation in rice (Oryza sativa) seedlings. Boron supplementation partially restored the decline in shoot and root biomass caused by Cd treatment (26% and 33%, respectively), with no significant difference between the B+Cd and control groups. We also found that B significantly reduced shoot and root Cd concentrations (by 64% and 25%, respectively) but increased Cd concentration (by 43%) and proportion (from 38% to 55%) in root cell walls. Transcriptome analysis and biochemical tests suggested that B supplementation enhanced lignin and pectin biosynthesis, pectin demethylation, and sulfur and glutathione metabolism. Moreover, B decreased the expression of some Cd-induced transporter-related genes (i.e., HMA2, Nramp1, and several ABC genes). These results indicate that B relieved Cd toxicity and reduced Cd accumulation in rice seedlings by restraining Cd uptake and translocation from root to shoot by improving Cd tolerance and chelation ability. These novel findings would benefit further investigations into how B influences Cd uptake, translocation, detoxification, and accumulation in crops.

Stress response of lettuce (Lactuca sativa) to environmental contamination with selected pharmaceuticals: A proteomic study

Pharmaceutical compounds have been found in rivers and treated wastewaters. They often contaminate irrigation waters and consequently accumulate in edible vegetables, causing changes in plants metabolism. The main objective of this work is to understand how lettuce plants cope with the contamination from three selected pharmaceuticals using a label free proteomic analysis. A lettuce hydroponic culture, grown for 36 days, was exposed to metformin, acetaminophen and carbamazepine (at 1 mg/L), during 8 days, after which roots and leaves were sampled and analysed using a liquid chromatography-mass spectrometry proteomics-based approach. In roots, a total of 612 proteins showed differentially accumulation while in leaves 237 proteins were identified with significant differences over controls. Carbamazepine was the contaminant that most affected protein abundance in roots, while in leaves the highest number of differentially accumulated proteins was observed for acetaminophen. In roots under carbamazepine, stress related protein species such as catalase, superoxide dismutase and peroxidases presented higher abundance. Ascorbate peroxidase increased in roots under metformin. Cell respiration protein species were affected by the presence of the three pharmaceuticals suggesting possible dysregulation of the Krebs cycle. Acetaminophen caused the main differences in respiration pathways, with more emphasis in leaves. Lettuce plants revealed different tolerance levels when contaminants were compared, being more tolerant to metformin presence and less tolerant to carbamazepine. SIGNIFICANCE: The significant increase of emerging contaminants in ecosystems makes essential to understand how these compounds may affect the metabolism of different organisms. Our study contributes with a detailed approach of the main interactions that may occur in plant metabolism when subjected to the stress induced by three different pharmaceuticals (acetaminophen, carbamazepine and metformin).

Cadmium uptake and translocation: selenium and silicon roles in Cd detoxification for the production of low Cd crops: a critical review

Cadmium (Cd) is a primary contaminant in agricultural soils of the world. The ability of Cd uptake, transport, detoxification, and accumulation varies among different plant species and genotypes. Cd is translocated from soil to root by different transporters which are used for essential plant nutrient uptake. A number of strategies have been suggested for decreasing Cd toxicity in Cd contaminated soils. Recently, a lot of research have been carried out on minimizing Cd uptake through selenium (Se) and silicon (Si) applications. Both Se and Si have been reported to mitigate Cd toxicity in different crops. Vacuolar sequestration, formation of phytochelatins, and cell wall adsorption have been reported as effective mechanisms for Cd detoxification. The present review discussed past and current knowledge of literature to better understand Cd toxicity and its mitigation by adopting different feasible and practical approaches.

Changes in phenotype and gene expression under lead stress revealed key genetic responses to lead tolerance in Medicago sativa L

Lead (Pb) is a serious heavy metal soil pollutant. It can be absorbed and accumulated by plant roots and impact plant growth. Medicago sativa L. (alfalfa) is a low-input forage and potential bioenergy crop, and improving its yield and quality has always been a focus of the alfalfa breeding industry. Little is known about the mechanism by which alfalfa responds to Pb stress at the molecular level. In this study, three alfalfa genotypes (a lead-resistant type (LR), a lead-sensitive type (LS) and an intermediate type (IN)) with contrasting abilities to resist lead were exposed to different durations of Pb treatment. Next-generation sequencing (NGS)-based RNA-seq technology was employed to characterize the root transcriptomes of three genotypes of alfalfa and identify differentially expressed genes (DEGs) during Pb stress. Genotypes LR and LS displayed different mechanisms of tolerance. In LR, the accumulation of more resistant substances was induced by the upregulation of sucrose synthase, glucan endo-1,3-beta-glucosidase, beta-amylase 3, probable trehalose-phosphate phosphatase J, 6-phosphofructo-2-kinase delta-1-pyrroline-5-carboxylate synthase (P5CS) and δ-ornithine aminotransferase (δ-OAT). In addition, flavin monooxygenase (YUCCA), 4-coumarate:CoA ligase-like protein (4CL), cinnamoyl-CoA reductase-like protein (CCR), ferulate 5-hydroxylase (F5H) and caffeic acid O-methyltransferase (COMT) were upregulated, leading to root development in a short time under Pb stress. Further study of the expression levels of metal transport-related genes, such as NRAMP (metal transporter), MATE (multidrug and toxin extrusion), HIPPs (heavy metal-associated isoprenylated plant proteins), MTP (metal tolerance protein), and ABC transporter, suggested that these genes were differentially expressed after lead treatment in the three alfalfa genotypes. Our research provides useful information for further studies on the molecular mechanism of Pb resistance in Medicago sativa L.

Boron supply alleviates cadmium toxicity in rice (Oryza sativa L.) by enhancing cadmium adsorption on cell wall and triggering antioxidant defense system in roots

Cadmium (Cd) pollution is a key concern globally that affects plant growth and productivity. Boron (B) is a micronutrient that helps in the formation of the primary cell wall (CW) and alleviates negative effects of toxic elements on plant growth. Nonetheless, knowledge about how B can reduce Cd toxicity in rice seedlings is not enough, particularly regarding CW-Cd adsorption. Therefore, the current experiment investigated the alleviative role of B on Cd toxicity in rice seedling. The experiment was carried out with 0 μM and 30 μM H₃BO₃ under 50 μM Cd toxicity in hydroponics. The results showed that Cd exposure alone inhibited plant growth parameters and caused lipid peroxidation. Moreover, Cd toxicity led to obvious visible toxicity symptoms on the leaves. However, increasing the availability of B alleviated Cd toxicity by reducing Cd concentration in plant tissues and improving antioxidative system. Moreover, cell wall pectin and hemicellulose adsorbed a significant amount of Cd. Fourier-Transform Infrared spectroscopy (FTIR) spectra exhibited that cell wall functional groups were increased by B application. Scanning electron microscopy (SEM) equipped with energy-dispersive X-ray (EDX) microanalysis confirmed the higher Cd binding onto CW. The findings of this investigation showed that B could mitigate Cd stress by decreasing Cd uptake and encouraging Cd adsorption on CW, and activation of the protective mechanisms. The present results might help to increase rice productivity on Cd polluted soils.

Effect of cadmium in the microalga Chlorella sorokiniana: A proteomic study

Cadmium is one of the most common heavy metals in contaminated aquatic environments and one of the most toxic contaminants for phytoplankton. Nevertheless, there are not enough studies focused on the effect of this metal in algae. Through a proteomic approach, this work shows how Cd can alter the growth, cell morphology and metabolism of the microalga Chlorella sorokiniana. Using the sequential window acquisition of all theoretical fragment ion spectra mass spectrometry (SWATH-MS), we concluded that exposure of Chlorella sorokiniana to 250 μM Cd²⁺ for 40 h caused downregulation of different metabolic pathways, such as photosynthesis, oxidative phosphorylation, glycolysis, TCA cycle and ribosomal proteins biosynthesis. However, photorespiration, antioxidant enzymes, gluconeogenesis, starch catabolism, and biosynthesis of glutamate, cysteine, glycine and serine were upregulated, under the same conditions. Finally, exposure to Cd also led to changes in the metabolism of carotenoids and lipids. In addition, the high tolerance of Chlorella sorokiniana to Cd points to this microalga as a potential microorganism to be used in bioremediation processes.

Long-Term Cd Exposure Alters the Metabolite Profile in Stem Tissue of Medicago sativa

As a common pollutant, cadmium (Cd) is one of the most toxic heavy metals accumulating in agricultural soils through anthropogenic activities. The uptake of Cd by plants is the main entry route into the human food chain, whilst in plants it elicits oxidative stress by unbalancing the cellular redox status. Medicago sativa was subjected to chronic Cd stress for five months. Targeted and untargeted metabolic analyses were performed. Long-term Cd exposure altered the amino acid composition with levels of asparagine, histidine and proline decreasing in stems but increasing in leaves. This suggests tissue-specific metabolic stress responses, which are often not considered in environmental studies focused on leaves. In stem tissue, profiles of secondary metabolites were clearly separated between control and Cd-exposed plants. Fifty-one secondary metabolites were identified that changed significantly upon Cd exposure, of which the majority are (iso)flavonoid conjugates. Cadmium exposure stimulated the phenylpropanoid pathway that led to the accumulation of secondary metabolites in stems rather than cell wall lignification. Those metabolites are antioxidants mitigating oxidative stress and preventing cellular damage. By an adequate adjustment of its metabolic composition, M. sativa reaches a new steady state, which enables the plant to acclimate under chronic Cd stress.

Wheat (Triticum aestivum L.) grains uptake of lead (Pb), transfer factors and prediction models for various types of soils from China

Lead (Pb) contaminated in farmlands has become a deep threat to global food security and human health. In this study, the bioavailability of Pb in 18 types of soil to wheat (Triticum aestivum L.) grains were investigated, and reliable empirical models of Pb in wheat grains were established based on soil properties. The results showed that the average bioconcentration factor (BCF_{grain/total-Pb}) in acidic soils was approximately 3.30 times than that in alkaline soils (ANOVA P < 0.05). Significant positive relationships between wheat grain Pb concentration and soil total Pb or EDTA extractable Pb were presented through the results of simple linear regressions (P < 0.001). The stepwise multiple linear regression models indicated that soil pH and soil total Pb were determined to be the two most reliable and reasonable factors in predicting wheat grain Pb concentration, with 83.8% explanation of variation. Soil total Pb compared with EDTA extractable Pb was applied to better improve prediction models in describing Pb transfer from soils to wheat grains. Furthermore, grouped models divided into two parts with pH of 7.5 also generated well prediction in wheat grain Pb concentration. Our prediction models were successfully verified within 95% prediction intervals for published literature data (including other wheat varieties). Moreover, the results indicated that ungrouped models performed better in predicting accuracy within 400 mg kg^-1 of soil total Pb, and grouped models showed better extrapolation stability when Pb in soil were overly high. Our results in the study were conduce to evaluate food security of Pb in contaminated agricultural soils.

Integrative transcriptomics reveals genotypic impact on sugar beet storability

An integrative comparative transcriptomic approach on six sugar beet varieties showing different amount of sucrose loss during storage revealed genotype-specific main driver genes and pathways characterizing storability. Sugar beet is next to sugar cane one of the most important sugar crops accounting for about 15% of the sucrose produced worldwide. Since its processing is increasingly centralized, storage of beet roots over an extended time has become necessary. Sucrose loss during storage is a major concern for the sugar industry because the accumulation of invert sugar and byproducts severely affect sucrose manufacturing. This loss is mainly due to ongoing respiration, but changes in cell wall composition and pathogen infestation also contribute. While some varieties can cope better during storage, the underlying molecular mechanisms are currently undiscovered. We applied integrative transcriptomics on six varieties exhibiting different levels of sucrose loss during storage. Already prior to storage, well storable varieties were characterized by a higher number of parenchyma cells, a smaller cell area, and a thinner periderm. Supporting these findings, transcriptomics identified changes in genes involved in cell wall modifications. After 13 weeks of storage, over 900 differentially expressed genes were detected between well and badly storable varieties, mainly in the category of defense response but also in carbohydrate metabolism and the phenylpropanoid pathway. These findings were confirmed by gene co-expression network analysis where hub genes were identified as main drivers of invert sugar accumulation and sucrose loss. Our data provide insight into transcriptional changes in sugar beet roots during storage resulting in the characterization of key pathways and hub genes that might be further used as markers to improve pathogen resistance and storage properties.

Transcriptional regulation and expression network responding to cadmium stress in a Cd-tolerant perennial grass Poa Pratensis

Kentucky bluegrass has good capability to absorb and accumulate cadmium (Cd) through developed root system, thus having potential phytoremediation function in Cd contaminated soils. Understanding the molecular mechanisms of Cd tolerance and accumulation in this species will be crucial to generating novel Cd-tolerance cultivars through genetic improvement, while it has not well documented yet. In the present study, comparative transcriptome analysis was performed for the seedlings of high Cd-tolerant genotype (M) and low Cd-tolerant genotype (R) under Cd stress. A total of 7022 up-regulated and 1033 down-regulated transcripts were identified in M genotype, whereas, only 850 up-regulated and 846 down-regulated transcripts were detected in R. Further transcriptional regulation analysis in M genotype showed that Dof, MADS25, BBR-BPC, B3, bZIP23 and MYB30 might be the hub transcription factors in response to Cd stress due to the orchestrated multiple functional genes associated with carbohydrate, lipid and secondary metabolism, as well as signal transduction. Differential expressed genes involved in auxin, ethylene, brassinosteroid and ABA signalling formed signal transduction cascades, which interacted with hub transcription factors, thereby finally orchestrated the expression of multiple genes associated with cell wall and membrane stability, cell elongation and Cd tolerance, including IAAs, ARFs, SnRK2, PP2C, PIFs, BES1/BZR1, CCR, CAD, FATB, fabF and HACD. Additionally, post-transcriptional modification of CIPKs, MAPKs, WAXs, UBCs, and E3 ubiquitin ligases were identified and also involved in plant signalling pathways and abiotic resistance. The study could contribute to our understanding the transcriptional regulation and complex internal network associated with Cd tolerance in Kentucky bluegrass.

Biotechnological Perspectives of Omics and Genetic Engineering Methods in Alfalfa

For several decades, researchers are working to develop improved major crops with better adaptability and tolerance to environmental stresses. Forage legumes have been widely spread in the world due to their great ecological and economic values. Abiotic and biotic stresses are main factors limiting legume production, however, alfalfa (Medicago sativa L.) shows relatively high level of tolerance to drought and salt stress. Efforts focused on alfalfa improvements have led to the release of cultivars with new traits of agronomic importance such as high yield, better stress tolerance or forage quality. Alfalfa has very high nutritional value due to its efficient symbiotic association with nitrogen-fixing bacteria, while deep root system can help to prevent soil water loss in dry lands. The use of modern biotechnology tools is challenging in alfalfa since full genome, unlike to its close relative barrel medic (Medicago truncatula Gaertn.), was not released yet. Identification, isolation, and improvement of genes involved in abiotic or biotic stress response significantly contributed to the progress of our understanding how crop plants cope with these environmental challenges. In this review, we provide an overview of the progress that has been made in high-throughput sequencing, characterization of genes for abiotic or biotic stress tolerance, gene editing, as well as proteomic and metabolomics techniques bearing biotechnological potential for alfalfa improvement.

Transcriptome analysis reveals insight into molecular hydrogen-induced cadmium tolerance in alfalfa: the prominent role of sulfur and (homo)glutathione metabolism

BACKGROUND

Hydrogen gas (H2) is hypothesised to play a role in plants that are coping with stresses by regulating signal transduction and gene expression. Although the beneficial role of H2 in plant tolerance to cadmium (Cd) has been investigated previously, the corresponding mechanism has not been elucidated. In this report, the transcriptomes of alfalfa seedling roots under Cd and/or hydrogen-rich water (HRW) treatment were first analysed. Then, the sulfur metabolism pathways were focused on and further investigated by pharmacological and genetic approaches.

RESULTS

A total of 1968 differentially expressed genes (DEGs) in alfalfa seedling roots under Cd and/or HRW treatment were identified by RNA-Seq. The DEGs were classified into many clusters, including glutathione (GSH) metabolism, oxidative stress, and ATP-binding cassette (ABC) transporters. The results validated by RT-qPCR showed that the levels of relevant genes involved in sulfur metabolism were enhanced by HRW under Cd treatment, especially the genes involved in (homo)glutathione metabolism. Additional experiments carried out with a glutathione synthesis inhibitor and Arabidopsis thaliana cad2-1 mutant plants suggested the prominent role of glutathione in HRW-induced Cd tolerance. These results were in accordance with the effects of HRW on the contents of (homo)glutathione and (homo)phytochelatins and in alleviating oxidative stress under Cd stress. In addition, the HRW-induced alleviation of Cd toxicity might also be caused by a decrease in available Cd in seedling roots, achieved through ABC transporter-mediated secretion.

CONCLUSIONS

Taken together, the results of our study indicate that H2 regulated the expression of genes relevant to sulfur and glutathione metabolism and enhanced glutathione metabolism which resulted in Cd tolerance by activating antioxidation and Cd chelation. These results may help to elucidate the mechanism governing H2-induced Cd tolerance in alfalfa.

Contributions of root cell wall polysaccharides to Cu sequestration in castor (Ricinus communis L.) exposed to different Cu stresses

Cell wall polysaccharides play a vital role in binding with toxic metals such as copper (Cu) ions. However, it is still unclear whether the major binding site of Cu in the cell wall varies with different degrees of Cu stresses. Moreover, the contribution of each cell wall polysaccharide fraction to Cu sequestration with different degrees of Cu stresses also remains to be verified. The distribution of Cu in cell wall polysaccharide fractions of castor (Ricinus communis L.) root was investigated with various Cu concentrations in the hydroponic experiment. The results showed that the hemicellulose1 (HC1) fraction fixed 44.9%-67.8% of the total cell wall Cu under Cu stress. In addition, the pectin fraction and hemicelluloses2 (HC2) fraction also contributed to the Cu binding in root cell wall, accounting for 11.0%-25.9% and 14.1%-26.6% of the total cell wall Cu under Cu treatments, respectively. When the Cu levels were ≤25 μmol/L, pectin and HC2 contributed equally to Cu storage in root cell wall. However, when the Cu level was higher than 25 μmol/L, the ability of the pectin to bind Cu was easy to reach saturation. Much more Cu ions were bound on HC1 and HC2 fractions, and the HC2 played a much more important role in Cu binding than pectin. Combining fourier transform infrared (FT-IR) and two-dimensional correlation analysis (2D-COS) techniques, the hemicellulose components were showed not only to accumulate most of Cu in cell wall, but also respond fastest to Cu stress.

Characteristics of cadmium immobilization in the cell wall of root in a cadmium-safe rice line (Oryza sativa L.)

Cultivating cadmium (Cd)-safe rice lines, which show low Cd accumulation in brown rice, is generally beneficial to ensure food safety. The Cd retention in root of Cd-safe rice line D62B plays an important role in its low Cd translocation from root to shoot. To understand the mechanism of Cd retention in root, a hydroponic experiment was conducted to investigate the subcellular distribution of Cd and the contribution of polysaccharides to Cd binding to the root cell wall of a Cd-safe rice line D62B with a common rice line Luhui17 as a control material. D62B retained more Cd in the root by sequestrated a higher proportion of Cd in the cell wall, further it transferred less Cd to shoot. Close to half of the Cd in the root cell wall of D62B was accumulated in the hemicellulose 1 (HC1), and the proportions of HC1 in it were 1.2-1.7 times higher than these of Luhui17. The proportion of Cd in the pectin showed a dose-dependent increase in two rice lines. D62B contained significantly higher uronic acid concentrations of the pectin and greater pectin methyl esterase (PME) activities than Luhui17 in the root cell wall. These results indicated that a superior Cd binding capacity of the cell wall polysaccharides in D62B played an important role in its Cd retention in root.

Protein Carbonylation As a Biomarker of Heavy Metal, Cd and Pb, Damage in Paspalum fasciculatum Willd. ex Flüggé

Heavy metal tolerant plants have phytoremediation potential for the recovery of contaminated soils, and the characterization of their metabolic adaptation processes is an important starting point to elucidate their tolerance mechanisms at molecular, biochemical and physiological levels. In this research, the effects of Cd and Pb on growth and protein carbonylation in tissues of Paspalum fasciculatum exposed to 30 and 50 mg·Kg⁻¹ Cd and Pb respectively were determined. P. fasciculatum seedlings exposed to metals grew more than controls until 60 days of cultivation and limited their oxidative effects to a reduced protein group. Carbonyl indexes in leaf and root proteins reached a significant increase concerning their controls in plants exposed 30 days to Cd and 60 days to Pb. From the combined approach of Western Blot with Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE) and protein analysis by Matrix Asisted Laser Desorption/Ionisation-Time Of Flight (MALDI-TOF/TOF) mass spectrometry, chloroplastic proteins were identified into the main oxidative stress-inducible proteins to Cd and Pb, such as subunits α, γ of ATP synthetase, Chlorophyll CP26 binding protein, fructose-bisphosphate aldolase and long-chain ribulose bisphosphate carboxylase (RuBisCO LSU). Cd generated damage in the photosynthetic machinery of the leaves of P. fasciculatum into the first 30 days of treatment; five of the oxidized proteins are involved in photosynthesis processes. Moreover, there was a proteolytic fragmentation of the RuBisCO LSU. Results showed that intrinsic tolerance of P. fasciculatum to these metals reached 60 days in our conditions, along with the bioaccumulating appreciable quantities of metals in their roots.

The Dynamics of the Cell Wall Proteome of Developing Alfalfa Stems

In this study, the cell-wall-enriched subproteomes at three different heights of alfalfa stems were compared. Since these three heights correspond to different states in stem development, a view on the dynamics of the cell wall proteome during cell maturation is obtained. This study of cell wall protein-enriched fractions forms the basis for a description of the development process of the cell wall and the linking cell wall localized proteins with the evolution of cell wall composition and structure. The sequential extraction of cell wall proteins with CaCl2, EGTA, and LiCl-complemented buffers was combined with a gel-based proteome approach and multivariate analysis. Although the highest similarities were observed between the apical and intermediate stem regions, the proteome patterns are characteristic for each region. Proteins that bind carbohydrates and have proteolytic activity, as well as enzymes involved in glycan remobilization, accumulate in the basal stem region. Beta-amylase and ferritin likewise accumulate more in the basal stem segment. Therefore, remobilization of nutrients appears to be an important process in the oldest stem segment. The intermediate and apical regions are sites of cell wall polymer remodeling, as suggested by the high abundance of proteins involved in the remodeling of the cell wall, such as xyloglucan endoglucosylase, beta-galactosidase, or the BURP-domain containing polygalacturonase non-catalytic subunit. However, the most striking change between the different stem parts is the strong accumulation of a DUF642-conserved domain containing protein in the apical region of the stem, which suggests a particular role of this protein during the early development of stem tissues.

Cadmium and Plant Development: An Agony from Seed to Seed

Anthropogenic pollution of agricultural soils with cadmium (Cd) should receive adequate attention as Cd accumulation in crops endangers human health. When Cd is present in the soil, plants are exposed to it throughout their entire life cycle. As it is a non-essential element, no specific Cd uptake mechanisms are present. Therefore, Cd enters the plant through transporters for essential elements and consequently disturbs plant growth and development. In this review, we will focus on the effects of Cd on the most important events of a plant's life cycle covering seed germination, the vegetative phase and the reproduction phase. Within the vegetative phase, the disturbance of the cell cycle by Cd is highlighted with special emphasis on endoreduplication, DNA damage and its relation to cell death. Furthermore, we will discuss the cell wall as an important structure in retaining Cd and the ability of plants to actively modify the cell wall to increase Cd tolerance. As Cd is known to affect concentrations of reactive oxygen species (ROS) and phytohormones, special emphasis is put on the involvement of these compounds in plant developmental processes. Lastly, possible future research areas are put forward and a general conclusion is drawn, revealing that Cd is agonizing for all stages of plant development.

Does long-term cadmium exposure influence the composition of pectic polysaccharides in the cell wall of Medicago sativa stems?

BACKGROUND

The heavy metal cadmium (Cd) accumulates in the environment due to anthropogenic influences. It is unessential and harmful to all life forms. The plant cell wall forms a physical barrier against environmental stress and changes in the cell wall structure have been observed upon Cd exposure. In the current study, changes in the cell wall composition and structure of Medicago sativa stems were investigated after long-term exposure to Cd. Liquid chromatography coupled to mass spectrometry (LC-MS) for quantitative protein analysis was complemented with targeted gene expression analysis and combined with analyses of the cell wall composition.

RESULTS

Several proteins determining for the cell wall structure changed in abundance. Structural changes mainly appeared in the composition of pectic polysaccharides and data indicate an increased presence of xylogalacturonan in response to Cd. Although a higher abundance and enzymatic activity of pectin methylesterase was detected, the total pectin methylation was not affected.

CONCLUSIONS

An increased abundance of xylogalacturonan might hinder Cd binding in the cell wall due to the methylation of its galacturonic acid backbone. Probably, the exclusion of Cd from the cell wall and apoplast limits the entry of the heavy metal into the symplast and is an important factor during tolerance acquisition.

Changes in the Proteome of Medicago sativa Leaves in Response to Long-Term Cadmium Exposure Using a Cell-Wall Targeted Approach

Accumulation of cadmium (Cd) shows a serious problem for the environment and poses a threat to plants. Plants employing various cellular and molecular mechanisms to limit Cd toxicity and alterations of the cell wall structure were observed upon Cd exposure. This study focuses on changes in the cell wall protein-enriched subproteome of alfalfa (Medicago sativa) leaves during long-term Cd exposure. Plants grew on Cd-contaminated soil (10 mg/kg dry weight (DW)) for an entire season. A targeted approach was used to sequentially extract cell wall protein-enriched fractions from the leaves and quantitative analyses were conducted with two-dimensional difference gel electrophoresis (2D DIGE) followed by protein identification with matrix-assisted laser desorption/ionization (MALDI) time-of-flight/time of flight (TOF/TOF) mass spectrometry. In 212 spots that showed a significant change in intensity upon Cd exposure a single protein was identified. Of these, 163 proteins are predicted to be secreted and involved in various physiological processes. Proteins of other subcellular localization were mainly chloroplastic and decreased in response to Cd, which confirms the Cd-induced disturbance of the photosynthesis. The observed changes indicate an active defence response against a Cd-induced oxidative burst and a restructuring of the cell wall, which is, however, different to what is observed in M. sativa stems and will be discussed.