Responses of wheat seedlings to cadmium, mercury and trichlorobenzene stresses

Arabidopsis root growth and development under metal exposure presented in an adverse outcome pathway framework

Due to human activities, soils become more and more polluted with metals, which imposes risks for human health and wildlife welfare. As most of the metals end up in the food chain through accumulation in plants, we need to establish science-based environmental criteria and risk management policies. To meet these necessities, a thorough understanding is required of how these metals accumulate in and affect plants. Many studies have been conducted towards this aim, but strikingly, only a few entries can be found in ecotoxicological databases, especially on Arabidopsis thaliana, which serves as a model species for plant (cell) physiology and genetic studies. As experimental conditions seem to vary considerably throughout literature, extrapolation or comparison of data is rather difficult or should be approached with caution. Furthermore, metal-polluted soils often contain more than one metal, yet limited studies investigated the impact of metal mixtures on plants. This review aims to compile all data concerning root system architecture under Cu, Cd and Zn stress, in single or multi-metal exposure in A. thaliana, and link it to metal-induced responses at different biological levels. Global incorporation into an adverse outcome pathway framework is presented.

Intraspecific variability in membrane proteome, cell growth, and morphometry of the invasive marine neurotoxic dinoflagellate Alexandrium pacificum grown in metal-contaminated conditions

Over the past decades, the occurrence, distribution and intensity of harmful algal blooms involving the dinoflagellate Alexandrium pacificum have increased in marine coastal areas disturbed by anthropogenic inputs. This invasive species produces saxitoxin, which causes the paralytic shellfish poisoning syndrome in humans upon consumption of contaminated seafood. Blooms of A. pacificum have been reported in metal-contaminated coastal ecosystems, suggesting some ability of these microorganisms to adapt to and/or resist in metal stress conditions. This study seeks to characterize the modifications in membrane proteomes (by 2-D electrophoresis coupled to LC-MS/MS), cell growth and morphometry (measured with an inverted microscope), in response to metal stress (addition of Zn²⁺, Pb²⁺, Cu²⁺ and Cd²⁺), in two Mediterranean A. pacificum strains: SG C10-3 and TAR C5-4F, respectively isolated from the Santa Giusta Lagoon (Sardinia, Italy) and from the Tarragona seaport (Spain), both metal-contaminated ecosystems. In the SG C10-3 cultures grown in a metal cocktail, cell growth was significantly delayed, and cell size increased (22% of 37.5 μm cells after 25 days of growth). Conversely, no substantial change was observed for cell growth or cell size in the TAR C5-4F cultures grown in a metal cocktail (P > 0.10), thus indicating intraspecific variability in the responses of A. pacificum strains to metal contamination. Regardless of the conditions tested, the total number of proteins constituting the membrane proteome was significantly higher for TAR C5-4F than for SG C10-3, which may help TAR C5-4F to thrive better in contaminated conditions. For both strains, the total number of proteins constituting the membrane proteomes was significantly lower in response to metal stress (29% decrease in the SG C10-3 proteome: 82 ± 12 proteins for controls, and 58 ± 12 in metal-contaminated cultures; 17% decrease in the TAR C5-4F proteome: 101 ± 8 proteins for controls, and 84 ± 5 in metal-contaminated cultures). Moreover, regardless of the strain, proteins with significantly modified expression in response to stress were mainly down-regulated (representing 45% of the proteome for SG C10-3 and 38% for TAR C5-4F), clearly showing the harmful effects of the metals. Protein down-regulation may affect cell transport (actin and phospholipid scramblase in SG C10-3), photosynthesis (RUBISCO in SG C10-3, light-harvesting protein in TAR C5-4F, and high-CO₂-inducing periplasmic protein in both strains), and finally energy metabolism (ATP synthase in both strains). However, other modifications in protein expression may confer to these A. pacificum strains a capacity for adaptation and/or resistance to metal stress conditions, for example by (i) limiting the metal entry through the plasma membrane of the SG C10-3 cells (via the down-regulation of scramblase) and/or (ii) reducing the oxidative stress generated by metals in SG C10-3 and TAR C5-4F cells (due to down-regulation of ATP-synthase).

Formation of iron plaque on roots of Iris pseudacorus and its consequence for cadmium immobilization is impacted by zinc concentration

The impact of iron plaque (IP) on bioavailability of heavy metals to plants has been well documented, but the role of zinc (Zn) in modulating the associated processes remains elusive. We took Iris pseudacorus used in wetland for remediating Cd-contaminated water as an example and systematically studied the combined influence of Cd and Zn concentration on formation of IP and its consequence for immobilization and plant uptake of Cd. The experiment was conducted in hydroponic culture and in each treatment, we measured the physiological traits, activity of antioxidant enzymes (SOD, POD, CAT), mass of the IP, as well as the Cd content in both plant tissues and IP. The results showed that increasing Cd concentration resulted in a steady reduction in IP while the impact of zinc on IP was complicated and appeared to be coupled with Cd. When the Cd concentration was low (0.5 mg L^-1 measured as CdCl₂ 2·5H₂O) increasing Zn concentration reduced IP, while when the Cd concentration was increased to 5 mg L^-1 increasing zinc concentration led to an increase in IP mass first followed by a decline after Zn concentration exceeded 100 mg L^-1 (measured as ZnSO₄·7H₂O). The change in IP as affected by Zn had a strong consequence for immobilization and plant uptake of Cd. When Cd concentration was low, the IP was comparatively abundant and hence adsorbed most Cd. In contrast, when Cd concentration was high, the IP reduced and the amount of Cd taken up by plant roots and translocated to shoots and leaves increased. Both Cd immobilization and its plant uptake were modulated by Zn concentration. At low Cd concentration the combined Cd immobilized and taken up by plant peaked when the Zn concentration was 50 mg L^-1, while at high Cd concentration the combined Cd reached maxima when theZn concentration was 100 mg L^-1. The activity of the antioxidant enzymes changed significantly with Zn rather than with Cd. Regardless of Cd concentration, the activity of all three antioxidant enzymes increased first with zinc concentration before declining when the Zn concentration exceeded approximately 100 mg L^-1 in all treatments, comparable with the change in immobilization and plant uptake of Cd as the Zn concentration increased. SEM analysis did prove the formation and variation of IP on the root surface of Iris pseudacorus in different treatments. We also found that the plant developed a survival strategy by scarifying its leaves with high Cd content. The results presented in this paper has wide implications as it revealed that care needs to be taken in applying Zn to enhance Cd immobilization and its plant uptake as exceeding the optimal application rate might reduce remediating efficiency rather than increase it.

Evaluation of three wheat (Triticum aestivum L.) cultivars as sensitive Cd biomarkers during the seedling stage

Sensitive seedling crops have been developed to monitor Cadmium (Cd) contamination in agricultural soil. In the present study, 18 parameters involving growth conditions and physiological performances were assessed to evaluate Cd-responses of three wheat (Triticum aestivum L.) cultivars, Xihan1 (XH), Longzhong1 (LZ) and Dingfeng16 (DF). Principle component analysis illustrated that Factor 1, representing growth performance, soluble sugar content and catalase activity, responded to the Cd treatments in a dose dependent manner, while Factor 2 represented by chlorophyll content and germinating root growth was mainly dependent on cultivar differences. Higher inhibition rates were observed in growth performance than in physiological responses, with the highest inhibition rates of shoot biomasses (39.6%), root length (58.7%), root tip number (57.8%) and bifurcation number (83.2%), even under the lowest Cd treatment (2.5 mg·L⁻¹). According to the Cd toxicity sensitivity evaluation, DF exerted highest tolerance to Cd stress in root growth while LZ was more sensitive to Cd stress, suggesting LZ as an ideal Cd contaminant biomarker. This study will provide novel insight into the cultivar-dependent response during using wheat seedlings as Cd biomarkers.

Evidence for molecular antagonistic mechanism between mercury and selenium in rice (Oryza sativa L.): A combined study using 1, 2-dimensional electrophoresis and SR-XRF techniques

Mercury (Hg) is a hazardous chemical in the environment and can accumulate in the food chain. Selenium (Se) is a necessary element for human health and has antagonistic effects on Hg toxicity. In this work, we investigated the effect of Se on Hg containing and Hg-responsive proteins in rice using 1, 2-dimensional electrophoresis combined with SR-XRF techniques. Two weeks old rice seedlings were exposed to Hg and/or Se compounds. After 21days proteins in the rice roots were separated by electrophoresis and their metal contents were determined by X-ray fluorescence to identify Hg and Se responsive biomolecules. The results show that under Hg stress alone Hg is bound to proteins with molecular weights of 15-25kDa. With the addition of Se, a new Hg-containing protein band in the 55-70kDa range was also found, while the content of Hg in the 15-25kDa proteins decreased. Ten and nine new protein spots were identified after adding Se to inorganic Hg and methylmercury exposed roots, respectively. Adding Se regulates the abundance of proteins associated with carbohydrate and energy metabolism, stress response, cell cycle, and DNA replication indicating that these proteins mediate the antagonism of Se against Hg toxicity. This study helps us to better understand the molecular mechanism of Hg tolerance as well as the molecular antagonism between Hg and Se in rice plants.

Omics Approaches for Engineering Wheat Production under Abiotic Stresses

Abiotic stresses greatly influenced wheat productivity executed by environmental factors such as drought, salt, water submergence and heavy metals. The effective management at the molecular level is mandatory for a thorough understanding of plant response to abiotic stress. Understanding the molecular mechanism of stress tolerance is complex and requires information at the omic level. In the areas of genomics, transcriptomics and proteomics enormous progress has been made in the omics field. The rising field of ionomics is also being utilized for examining abiotic stress resilience in wheat. Omic approaches produce a huge amount of data and sufficient developments in computational tools have been accomplished for efficient analysis. However, the integration of omic-scale information to address complex genetics and physiological questions is still a challenge. Though, the incorporation of omic-scale data to address complex genetic qualities and physiological inquiries is as yet a challenge. In this review, we have reported advances in omic tools in the perspective of conventional and present day approaches being utilized to dismember abiotic stress tolerance in wheat. Attention was given to methodologies, for example, quantitative trait loci (QTL), genome-wide association studies (GWAS) and genomic selection (GS). Comparative genomics and candidate genes methodologies are additionally talked about considering the identification of potential genomic loci, genes and biochemical pathways engaged with stress resilience in wheat. This review additionally gives an extensive list of accessible online omic assets for wheat and its effective use. We have additionally addressed the significance of genomics in the integrated approach and perceived high-throughput multi-dimensional phenotyping as a significant restricting component for the enhancement of abiotic stress resistance in wheat.

Accumulation, fractionation, and risk assessment of mercury and arsenic in the soil-wheat system from the wastewater-irrigated soil in Baiyin, northwest China

Wastewater irrigation can increase metal concentrations in soil and wheat, thereby posing metal-associated health risk via food ingestion. We investigated levels of mercury (Hg) and arsenic (As) in roots, husks, stems, leaves, and grains of wheat and their fractionations in farmland soil from Baiyin City, an industrial and mining city, northwest China. Results show that the mean concentrations of Hg in soils from Dongdagou and Xidagou stream in Baiyin were 8.5 times and three times higher than local soil background values, respectively. Those of As were 4.5 times and 1.6 times higher, respectively. Most Hg and As were mainly accumulated in wheat leaves. The spatial distributions of As in soils and grains exhibit a very similar pattern, which suggest that As pollution in soils might be predicted by its level in wheat grains. Residual fractions for Hg (RES-Hg) and As (RES-As) are the highest compared to other fractions, indicating weak mobility of Hg and As in soil. The crop oral intake hazard quotients of both Hg and As for children were approximately two times higher than that for adults, indicating that children have higher exposure risks to Hg- and As-contaminated wheat. The crop oral intake was the main route of exposure causing non-carcinogenic and carcinogenic risk for local residents.

Protein changes in Lepidium sativum L. exposed to Hg during soil phytoremediation

Some investigations have been carried out in this study to find the best technique of soil reclamation in mercurypolluted soil. In this study, we examined Lepidium sativum L. as a plant useful for Hg phytoextraction. The simultaneous application of compost and thiosulfate was explored as a possible method of enhancing the process of phytoextraction. The results of the investigations of plant protein changes during assisted Hg phytoextraction were also provided. The results of the study show that combined use of compost and thiosulfate significantly increased both the total Hg accumulation and its translocation to aerial plant tissues. Plant protein analysis showed that L. sativum L. has the ability to respond to environmental stress condition by the activation of additional proteins. The additional proteins, like homocysteine methyltransferase, ribulose bisphosphate carboxylases (long and short chains), 14-3-3-like protein, and biosynthesis-related 40S ribosomal protein S15, were activated in plant shoots only in experiments carried out in Hg-polluted soil. There were no protein changes observed in plants exposed to compost and thiosulfate. It suggests that the combined use of compost and thiosulfate decreased Hg toxicity.

Modifications of the soluble proteome of a mediterranean strain of the invasive neurotoxic dinoflagellate Alexandrium catenella under metal stress conditions

The soluble proteome of the mediterranean strain ACT03 of the invasive neurotoxic dinoflagellate Alexandrium catenella exposed to lead or zinc at 6, 12 or 18μM (total concentrations), or under control conditions, was characterized by two-dimensional gel electrophoresis (2-DE). Zinc reduced (P<0.05) the total number of protein spots (-41%, -52% and -60%, at 6, 12 or 18μM, respectively). Besides, most of the proteins constituting the soluble proteome were down-regulated in response to lead or zinc stresses. These proteins were involved mainly in photosynthesis (20-37% for lead; 36-50% for zinc) (ribulose-1,5-bisphosphate carboxylase/oxygenase: RUBISCO; ferredoxin-NADP⁺ reductase: FNR; peridinin-chlorophyll a-protein: PCP), and in the oxidative stress response (29-34% for lead; 17-36% for zinc) (superoxide dismutase: SOD; proteasome α/β subunits). These negative effects could be partly compensated by the up-regulation of specific proteins such as ATP-synthase β subunit (+16.3 fold after exposure to lead at 12μM). Indeed, an increase in the abundance of ATP-synthase could enrich the ATP pool and provide more energy available for the cells to survive under metal stress, and make the ATP-synthase transport of metal cations out of the cells more efficient. Finally, this study shows that exposure to lead or zinc have a harmful effect on the soluble proteome of A. catenella ACT03, but also suggests the existence of an adaptative proteomic response to metal stresses, which could contribute to maintaining the development of this dinoflagellate in trace metal-contaminated ecosystems.

miRNA-based heavy metal homeostasis and plant growth

Plants have been naturally gifted with mechanisms to adjust under very high or low nutrient concentrations. Heavy metal toxicity is considered as a major growth and yield-limiting factor for plants. This stress includes essential as well as non-essential metals. MicroRNAs (miRNAs) are known for mediating post-transcriptional regulation by cleaving transcripts or translational inhibition. It is commonly agreed that an extensive understanding of plant miRNAs will significantly help in the induction of tolerance against environmental stresses. With the introduction of the latest technology like next generation sequencing (NGS), a growing figure of miRNAs has been productively recognized in several plants for their diverse roles. These miRNAs are well-known modulators of plant responses to heavy metal (HM) stress. Data regarding metal-responsive miRNAs point out the vital role of plant miRNAs in supplementing metal detoxification by means of transcription factors (TF) or gene regulation. Acting as systemic signals, miRNAs also synchronize different physiological processes for plant responses to metal toxicities. In contrast to practicing techniques, using miRNA is a greatly helpful, pragmatic, and feasible approach. The earlier findings point towards miRNAs as a prospective target to engineer heavy metal tolerance in plants. Therefore, there is a need to augment our knowledge about the orchestrated functions of miRNAs during HM stress. We reviewed the deterministic significance of plant miRNAs in heavy metal tolerance and their role in mediating plant responses to HM toxicities. This review also summarized the topical developments by identification and validation of different metal stress-responsive miRNAs.

Cadmium minimization in wheat: A critical review

Cadmium (Cd) accumulation in wheat (Triticum aestivum L.) and its subsequent transfer to food chain is a major environmental issue worldwide. Understanding wheat response to Cd stress and its management for aiming to reduce Cd uptake and accumulation in wheat may help to improve wheat growth and grain quality. This paper reviewed the toxic effects, tolerance mechanisms, and management of Cd stress in wheat. It was concluded that Cd decreased germination, growth, mineral nutrients, photosynthesis and grain yield of wheat and plant response to Cd toxicity varies with cultivars, growth conditions and duration of stress applied. Cadmium caused oxidative stress and genotoxicity in wheat plants. Stimulation of antioxidant defense system, osmoregulation, ion homeostasis and over production of signalling molecules are important adaptive strategies of wheat under Cd stress. Exogenous application of plant growth regulators, inorganic amendments, proper fertilization, silicon, and organic, manures and biochar, amendments are commonly used for the reduction of Cd uptake in wheat. Selection of low Cd-accumulating wheat cultivars, crop rotation, soil type, and exogenous application of microbes are among the other agronomic practices successfully employed in reducing Cd uptake by wheat. These management practices could enhance wheat tolerance to Cd stress and reduce the transfer of Cd to the food chain. However, their long-term sustainability in reducing Cd uptake by wheat needs further assessment.

Wheat proteomics: proteome modulation and abiotic stress acclimation

Cellular mechanisms of stress sensing and signaling represent the initial plant responses to adverse conditions. The development of high-throughput "Omics" techniques has initiated a new era of the study of plant molecular strategies for adapting to environmental changes. However, the elucidation of stress adaptation mechanisms in plants requires the accurate isolation and characterization of stress-responsive proteins. Because the functional part of the genome, namely the proteins and their post-translational modifications, are critical for plant stress responses, proteomic studies provide comprehensive information about the fine-tuning of cellular pathways that primarily involved in stress mitigation. This review summarizes the major proteomic findings related to alterations in the wheat proteomic profile in response to abiotic stresses. Moreover, the strengths and weaknesses of different sample preparation techniques, including subcellular protein extraction protocols, are discussed in detail. The continued development of proteomic approaches in combination with rapidly evolving bioinformatics tools and interactive databases will facilitate understanding of the plant mechanisms underlying stress tolerance.

Problems inherent to a meta-analysis of proteomics data: a case study on the plants' response to Cd in different cultivation conditions

This meta-analysis focuses on plant-proteome responses to cadmium (Cd) stress. Initially, some general topics related to a proteomics meta-analysis are discussed: (1) obstacles encountered during data analysis, (2) a consensus in proteomic research, (3) validation and good reporting practices for protein identification and (4) guidelines for statistical analysis of differentially abundant proteins. In a second part, the Cd responses in leaves and roots obtained from a proteomics meta-analysis are discussed in (1) a time comparison (short versus long term exposure), and (2) a culture comparison (hydroponics versus soil cultivation). Data of the meta-analysis confirmed the existence of an initial alarm phase upon Cd exposure. Whereas no metabolic equilibrium is established in hydroponically exposed plants, an equilibrium seems to be manifested in roots of plants grown in Cd-contaminated soil after long term exposure. In leaves, the carbohydrate metabolism is primarily affected independent of the exposure time and the cultivation method. In addition, a metabolic shift from CO2-fixation towards respiration is manifested, independent of the cultivation system. Finally, some ideas for the improvement of proteomics setups and for comparisons between studies are discussed.

BIOLOGICAL SIGNIFICANCE

This meta-analysis focuses on the plant responses to Cd stress in leaves and roots at the proteome level. This meta-analysis points out the encountered obstacles when performing a proteomics meta-analysis related to inherent technologies, but also related to experimental setups. Furthermore, the question is addressed whether an extrapolation of results obtained in hydroponic cultivation towards soil-grown plants is possible.

MicroRNA mediated regulation of metal toxicity in plants: present status and future perspectives

The human population is increasing at an alarming rate, whereas heavy metals (HMs) pollution is mounting serious environmental problem, which could lead to serious concern about the future sufficiency of global food production. Some HMs such as Mn, Cu, and Fe, at lower concentration serves as an essential vital component of plant cell as they are crucial in various enzyme catalyzed biochemical reactions. At higher concentration, a vast variety of HMs such as Mn, Cu, Cd, Fe, Hg, Al and As, impose toxic reaction in the plant system which greatly affect the crop yield. Recently, microRNAs (miRNAs) that are small class of non-coding riboregulator have emerged as central regulator of numerous abiotic stresses including HMs. Increasing reports indicate that plants have evolved specialized inbuilt mechanism viz. signal transduction, translocation and sequestration to counteract the toxic response of HMs. Combining computational and wet laboratory approaches have produced sufficient evidences concerning active involvement of miRNAs during HMs toxicity response by regulating various transcription factors and protein coding genes involved in plant growth and development. However, the direct role of miRNA in controlling various signaling molecules, transporters and chelating agents of HM metabolism is poorly understood. This review focuses on the latest progress made in the area of direct involvement of miRNAs in signaling, translocation and sequestration as well as recently added miRNAs in response to different HMs in plants.

Could an abandoned mercury mine area be cropped?

The Almadén area (Spain) is known for its high natural mercury background as well as for the anthropogenic impact due to mining activities. After the end of these activities, appropriate alternative use of the soil has to be found, and agricultural activities stand out as an environmentally-friendly and potentially profitable alternative, giving to the soil a sustainable use without risks for human or animal health according to current legislation. Experiments performed at different scales (involving hydroponics, growth in pots and lysimeters) allow recommendations to be made regarding the adequacy of cultivation of different crops for animal or human consumption before they are sown in the field. Regarding crops for animal feeding, mercury accumulation in vegetative organs represents a higher potential risk for animals. Nevertheless, seeds and fruits can be used, both for human and animal consumption. Finally, this work will lead the way to obtain a scientific basis for elaborating a list of recommendations on sustainable and safe alternative land use, according to current international legislation.

Oligomerization of the reversibly glycosylated polypeptide: its role during rice plant development and in the regulation of self-glycosylation

A multigenic family of self-glycosylating proteins named reversibly glycosylated polypeptides, designated as RGPs, have been usually associated with carbohydrate metabolism, although they are an enigma both at the functional, as well as at the structural level. In this work, we used biochemical approaches to demonstrate that complex formation is linked to rice plant development, in which class 1 Oryza sativa RGP (OsRGP) would be involved in an early stage of growing plants, while class 2 OsRGP would be associated with a late stage linked to an active polysaccharide synthesis that occurs during the elongation of plant. Here, a further investigation of the complex formation of the Solanum tuberosum RGP (StRGP) was performed. Results showed that disulfide bonds are at least partially responsible for maintaining the oligomeric protein structure, so that the nonreduced StRGP protein showed an apparent higher molecular weight and a lower radioglycosylation of the monomer with respect to its reduced form. Hydrophobic cluster analysis and secondary structure prediction revealed that class 2 RGPs no longer maintained the Rossman fold described for class 1 RGP. A 3D structure of the StRGP protein resolved by homology modeling supports the possibility of intercatenary disulfide bridges formed by exposed cysteines residues C79, C303 and C251 and they are most probably involved in complex formation occurring into the cell cytoplasm.

Mercury toxicity, molecular response and tolerance in higher plants

Mercury (Hg) contamination in soils has become a great concern as a result of its natural release and anthropogenic activities. This review presents broad aspects of our recent understanding of mercury contamination and toxicology in plants including source of Hg contamination, toxicology, tolerant regulation in plants, and minimization strategy. We first introduced the sources of mercury contamination in soils. Mercury exists in different forms, but ionic mercury (Hg(2+)) is the predominant form in soils and readily absorbed by plants. The second issue to be discussed is the uptake, transport, and localization of Hg(2+) in plants. Mercury accumulated in plants evokes severe phytotoxicity and impairs numerous metabolic processes including nutrient uptake, water status, and photosynthesis. The mechanisms of mercury-induced toxicology, molecular response and gene networks for regulating plant tolerance will be reviewed. In the case of Hg recent much progress has been made in profiling of transcriptome and more importantly, uncovering a group of small RNAs that potentially mediates plant tolerance to Hg. Several newly discovered signaling molecules such as nitric oxide and carbon monoxide have now been described as regulators of plant tolerance to Hg. A recently emerged strategy, namely selection and breeding of plant cultivars to minimize Hg (or other metals) accumulation will be discussed in the last part of the review.