Enhancement of tolerance of Indian mustard (Brassica juncea) to mercury by carbon monoxide

24-epibrassinolide as a multidimensional regulator of rice (Oryza sativa) physiological and molecular responses under isoproturon stress

Brassinosteroids (BRs) can regulate various processes in plant development and defense against environmental stress. In this study, the contribution of BRs in the degradation of isoproturon (IPU) in rice has been established. IPU has a significant effect on rice growth, chlorophyll content, and membrane permeability. When treated with 1.0 μmol/L 24-epibrassinolide (EBR), a BR analogue, the associated symptoms of rice poisoning were alleviated as the IPU levels in the rice and growth media were decreased. In the presence of EBR, the activities of several IPU-related detoxification enzymes were enhanced to cope with the stress due to IPU. An RNA-sequencing (RNA-Seq) has been performed to determine the variation of transcriptomes and metabolic mechanisms in rice treated with EBR, IPU, or IPU+EBR. Some of the differentially expressed genes (DEGs) were Phase I-III reaction components of plants, such as cytochrome P450 (CYP450), glutathione S-transferase (GST), glycosyltransferases (GTs), and the ATP-binding cassette transporter (ABC transporter). The expression of some signal transduction genes was significantly up-regulated. The relative content of low-toxicity IPU metabolites increased due to the presence of EBR as determined by UPLC/Q-TOF-MS/MS. The IPU metabolic pathways include enzyme-catalyzed demethylation, hydroxylation, hydrolysis, glycosylation, and amino acid conjugation processes. The results suggest that EBR plays a key role in the degradation and detoxification of IPU. This study has provided evidence that BRs regulate the metabolism and detoxification of IPU in rice, and offers a new approach to ensuring cleaner crops by eliminating pesticide residues in the environment.

Appraisal of the Role of Gaseous Signaling Molecules in Thermo-Tolerance Mechanisms in Plants

A significant threat to the ongoing rise in temperature caused by global warming. Plants have many stress-resistance mechanisms, which is responsible for maintaining plant homeostasis. Abiotic stresses largely increase gaseous molecules' synthesis in plants. The study of gaseous signaling molecules has gained attention in recent years. The role of gaseous molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and ethylene, in plants under temperature high-temperature stress are discussed in the current review. Recent studies revealed the critical function that gaseous molecules play in controlling plant growth and development and their ability to respond to various abiotic stresses. Here, we provide a thorough overview of current advancements that prevent heat stress-related plant damage via gaseous molecules. We also explored and discussed the interaction of gaseous molecules. In addition, we provided an overview of the role played by gaseous molecules in high-temperature stress responses, along with a discussion of the knowledge gaps and how this may affect the development of high-temperature-resistant plant species.

Unveiling Innovative Approaches to Mitigate Metals/Metalloids Toxicity for Sustainable Agriculture

Due to anthropogenic activities, environmental pollution of heavy metals/metalloids (HMs) has increased and received growing attention in recent decades. Plants growing in HM-contaminated soils have slower growth and development, resulting in lower agricultural yield. Exposure to HMs leads to the generation of free radicals (oxidative stress), which alters plant morpho-physiological and biochemical pathways at the cellular and tissue levels. Plants have evolved complex defense mechanisms to avoid or tolerate the toxic effects of HMs, including HMs absorption and accumulation in cell organelles, immobilization by forming complexes with organic chelates, extraction via numerous transporters, ion channels, signaling cascades, and transcription elements, among others. Nonetheless, these internal defensive mechanisms are insufficient to overcome HMs toxicity. Therefore, unveiling HMs adaptation and tolerance mechanisms is necessary for sustainable agriculture. Recent breakthroughs in cutting-edge approaches such as phytohormone and gasotransmitters application, nanotechnology, omics, and genetic engineering tools have identified molecular regulators linked to HMs tolerance, which may be applied to generate HMs-tolerant future plants. This review summarizes numerous systems that plants have adapted to resist HMs toxicity, such as physiological, biochemical, and molecular responses. Diverse adaptation strategies have also been comprehensively presented to advance plant resilience to HMs toxicity that could enable sustainable agricultural production.

Carbon monoxide is involved in melatonin-enhanced drought resistance in tomato seedlings by enhancing chlorophyll synthesis pathway

BACKGROUND

Drought is thought to be a major abiotic stress that dramatically limits tomato growth and production. As signal molecule, melatonin (MT) and carbon monoxide (CO) can enhance plant stress resistance. However, the effect and underlying mechanism of CO involving MT-mediated drought resistance in seedling growth remains unknown. In this study, tomato (Solanum lycopersicum L. 'Micro-Tom') seedlings were used to investigate the interaction and mechanism of MT and CO in response to drought stress.

RESULTS

The growth of tomato seedlings was inhibited significantly under drought stress. Exogenous MT or CO mitigated the drought-induced impairment in a dose-dependent manner, with the greatest efficiency provided by 100 and 500 µM, respectively. But application of hemoglobin (Hb, a CO scavenger) restrained the positive effects of MT on the growth of tomato seedlings under drought stress. MT and CO treatment promoted chlorophyll a (Chl a) and chlorophyll a (Chl b) accumulations. Under drought stress, the intermediate products of chlorophyll biosynthesis such as protoporphyrin IX (Proto IX), Mg-protoporphyrin IX (Mg-Proto IX), potochlorophyllide (Pchlide) and heme were increased by MT or CO, but uroporphyrinogen III (Uro III) content decreased in MT-treated or CO-treated tomato seedlings. Meanwhile, MT or CO up-regulated the expression of chlorophyll and heme synthetic-related genes SlUROD, SlPPOX, SlMGMT, SlFECH, SlPOR, SlChlS, and SlCAO. However, the effects of MT on chlorophyll biosynthesis were almost reversed by Hb.

CONCLUSION

The results suggested that MT and CO can alleviate drought stress and facilitate the synthesis of Chl and heme in tomato seedlings. CO played an essential role in MT-enhanced drought resistance via facilitating chlorophyll biosynthesis pathway.

In situ imaging of signaling molecule carbon monoxide in plants with a fluorescent probe

Carbon monoxide (CO) is a recently discovered gasotransmitter. In animals, it has been found that endogenously produced CO participates in the regulation of various metabolic processes. Recent research has indicated that CO, acting as a signaling molecule, plays a crucial regulatory role in plant development and their response to abiotic stress. In this work, we developed a fluorescent probe, named COP (carbonic oxide Probe), for the in situ imaging of CO in Arabidopsis thaliana plant tissues. The probe was designed by combining malononitrile-naphthalene as the fluorophore and a typical palladium-mediated reaction mechanism. When reacted with the released CO, COP showed an obvious fluorescence enhancement at 575 nm, which could be observed in naked-eye conditions. With a linear range of 0-10 μM, the limit of detection of COP was determined as 0.38 μM. The detection system based on COP indicated several advantages including relatively rapid response within 20 min, steadiness in a wide pH range of 5.0-10.0, high selectivity, and applicative anti-interference. Moreover, with a penetration depth of 30 μm, COP enabled 3D imaging of CO dynamics in plant samples, whether it was caused by agent release, heavy metal stress, or inner oxidation. This work provides a fluorescent probe for monitoring CO levels in plant samples, and it expands the application field of CO-detection technology, assisting researchers in understanding the dynamic changes in plant physiological processes, making it an important tool for studying plant physiology and biological processes.

Bioavailability, transfer, toxicological effects, and contamination assessment of arsenic and mercury in soil-corn systems

Sewage irrigation has solved the shortage of agricultural water and increased the content of heavy metal(loid)s (HMs) in soil-crop systems, which harms human health via the food chain. In this study, 43 pairs of soil and corn samples (leaf, stem1, stem2, stem3, root, husk, grain, and corncob) were collected in the Dongdagou (DDG) and Xidagou (XDG) streams of Baiyin City. Fraction and transfer of As and Hg were investigated, and toxicological effects and contamination were assessed in soil-corn systems. The results showed that the mean values of As and Hg in soil were 33.79 mg/kg and 0.96 mg/kg, respectively, which exceeded the soil background values in Gansu Province. As and Hg are mainly dominated by the residual fraction. Total and bioavailability contributed significantly to As and Hg accumulation in corn, with root, stem3, and leaf accumulating more strongly. The results based on the bioavailability concentration soil-corn transfer factor indicated that As and Hg tended to accumulate more in the root, stem3, and leaf and less in grain, and further assessment of the human health effects of consuming contaminated cron is needed. Scanning electron microscope (SEM) and Fourier transform infrared (FTIR) results showed that As and Hg were not significantly toxic to corn parts, indicating morphology. As and Hg were bound to hydroxyl groups in the outer epidermal cell wall of the roots, thereby reducing upward translocation. The trinity assessment (TA) model results indicated that the most severe contamination was found in root and stem1. The TA provides a practical tool for soil-cron systems and helps develop management strategies to prevent ecological hazards.

microRNAs: Key Players in Plant Response to Metal Toxicity

Environmental metal pollution is a common problem threatening sustainable and safe crop production. Heavy metals (HMs) cause toxicity by targeting key molecules and life processes in plant cells. Plants counteract excess metals in the environment by enhancing defense responses, such as metal chelation, isolation to vacuoles, regulating metal intake through transporters, and strengthening antioxidant mechanisms. In recent years, microRNAs (miRNAs), as a small non-coding RNA, have become the central regulator of a variety of abiotic stresses, including HMs. With the introduction of the latest technologies such as next-generation sequencing (NGS), more and more miRNAs have been widely recognized in several plants due to their diverse roles. Metal-regulated miRNAs and their target genes are part of a complex regulatory network. Known miRNAs coordinate plant responses to metal stress through antioxidant functions, root growth, hormone signals, transcription factors (TF), and metal transporters. This article reviews the research progress of miRNAs in the stress response of plants to the accumulation of HMs, such as Cu, Cd, Hg, Cr, and Al, and the toxicity of heavy metal ions.

Mercury toxicity affects oxidative metabolism and induces stress responsive mechanisms in wheat (Triticum aestivum L.)

Mercury (Hg) toxicity is an increasing problem worldwide, with a negative impact on the environment and living organisms including both animals and plants. In this study, we analyzed molecular and biochemical changes related to Hg toxicity in wheat (Triticum aestivum L.) seedlings. Seven-day-old seedlings were exposed to various concentrations (5, 10, and 20 µM) of HgCl₂ for 24 and 48 h. Our results showed that HgCl₂ treatments led to an increase in the Hg content of wheat leaves in a time- and concentration-dependent manner. Furthermore, significant increases were observed in hydrogen peroxide, malondialdehyde, and proline contents in response to Hg toxicity. While all HgCl₂ treatments decreased the level of superoxide dismutase (SOD), the level of catalase (CAT) was reduced only in seedlings exposed to 5 µM of HgCl₂. Mercury stress caused a decline in the expression of Cu/Zn-SOD, Fe-SOD, TaWRKY19, and TaDREB1 genes at both exposure times. On the other hand, 10 and 20 µM HgCl₂ treatments caused significant induction (1.9 to 6.1-fold) in the expression of the CAT gene in wheat leaves. The mRNA level of the Mn-SOD and TaWRKY2 genes showed different patterns depending on the concentration and exposure period of HgCl₂. In conclusion, the findings of this work demonstrate that Hg toxicity causes oxidative damage in wheat seedlings and changes the expression of some genes encoding WRKY and DREB transcription factor families, which have important functions in abiotic stress response.

Heme oxygenase-nitric oxide crosstalk-mediated iron homeostasis in plants under oxidative stress

Plant growth under abiotic stress conditions significantly enhances intracellular generation of reactive oxygen species (ROS). Oxidative status of plant cells is directly affected by the modulation of iron homeostasis. Among mammals and plants, heme oxygenase-1 (HO-1) is a well-known antioxidant enzyme. It catalyzes oxygenation of heme, thereby producing Fe²⁺, CO and biliverdin as byproducts. The antioxidant potential of HO-1 is primarily due to its catalytic reaction byproducts. Biliverdin and bilirubin possess conjugated π-electrons which escalate the ability of these biomolecules to scavenge free radicals. CO also enhances the ROS scavenging ability of plants cells by upregulating catalase and peroxidase activity. Enhanced expression of HO-1 in plants under oxidative stress accompanies sequestration of iron in specialized iron storage proteins localized in plastids and mitochondria, namely ferritin for Fe³⁺ storage and frataxin for storage of Fe-S clusters, respectively. Nitric oxide (NO) crosstalks with HO-1 at multiple levels, more so in plants under oxidative stress, in order to maintain intracellular iron status. Formation of dinitrosyl-iron complexes (DNICs) significantly prevents Fenton reaction during oxidative stress. DNICs also release NO upon dissociation in target cells over long distance in plants. They also function as antioxidants against superoxide anions and lipidic free radicals. A number of NO-modulated transcription factors also facilitate iron homeostasis in plant cells. Plants facing oxidative stress exhibit modulation of lateral root formation by HO-1 through NO and auxin-dependent pathways. The present review provides an in-depth analysis of the structure-function relationship of HO-1 in plants and mammals, correlating them with their adaptive mechanisms of survival under stress.

Interplay between gasotransmitters and potassium is a K+ey factor during plant response to abiotic stress

Carbon monoxide (CO), nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters known for their roles in plant response to (a)biotic stresses. The crosstalk between these gasotransmitters and potassium ions (K⁺) has received considerable attention in recent years, particularly due to the dual role of K⁺ as an essential mineral nutrient and a promoter of plant tolerance to abiotic stress. This review brings together what it is known about the interplay among NO, CO, H₂S and K⁺ in plants with focus on the response to high salinity. Some findings obtained for plants under water deficit and metal stress are also presented and discussed since both abiotic stresses share similarities with salt stress. The molecular targets of the gasotransmitters NO, CO and H₂S in root and guard cells that drive plant tolerance to salt stress are highlighted as well.

Regulatory hubs and strategies for improving heavy metal tolerance in plants: Chemical messengers, omics and genetic engineering

Heavy metal (HM) accumulation in the agricultural soil and its toxicity is a major threat for plant growth and development. HMs disrupt functional integrity of the plants, induces altered phenological and physiological responses and slashes down qualitative crop yield. Chemical messengers such as phytohormones, plant growth regulators and gasotransmitters play a crucial role in regulating plant growth and development under metal toxicity in plants. Understanding the intricate network of these chemical messengers as well as interactions of genes/metabolites/proteins associated with HM toxicity in plants is necessary for deciphering insights into the regulatory circuit involved in HM tolerance. The present review describes (a) the role of chemical messengers in HM-induced toxicity mitigation, (b) possible crosstalk between phytohormones and other signaling cascades involved in plants HM tolerance and (c) the recent advancements in biotechnological interventions including genetic engineering, genome editing and omics approaches to provide a step ahead in making of improved plant against HM toxicities.

Crosstalk among hydrogen sulfide (H2S), nitric oxide (NO) and carbon monoxide (CO) in root-system development and its rhizosphere interactions: A gaseous interactome

Root development in higher plants is achieved by a precise intercellular communication which determines cell fate in the primary embryonic meristem where the gasotransmitters H₂S, NO and CO participate dynamically. Furthermore, the rhizosphere interaction of these molecules with microbial and soil metabolism also affects root development. NO regulates root growth and architecture in association with several other biomolecules like auxin indole-3-acetic acid (IAA), ethylene, jasmonic acid (JA), strigolactones, alkamides and melatonin. The CO-mediated signal transduction pathway in roots is closely linked to the NO-mediated signal cascades. Interestingly, H₂S acts also as an upstream component in IAA and NO-mediated crosstalk during root development. Heme oxygenase (HO) 1 generates CO and functions as a downstream component in H₂S-mediated adventitious rooting and H₂S-CO crosstalk. Likewise, reactive oxygen species (ROS), H₂S and NO crosstalk are important components in the regulation of root architecture. Deciphering these interactions will be a potential biotechnological tool which could provide benefits in crop management in soils, especially under adverse environmental conditions. This review aims to provide a comprehensive update of the complex networks of these gasotransmitters during the development of roots.

Effect of soil mercury pollution on ginger (Zingiber officinale Roscoe): Growth, product quality, health risks and silicon mitigation

The mercury residue in soil not only poisons plants, but also bioaccumulates and biomagnifies through the food chain, causing a significant risk to human health. As an essential condiment on the table, the food safety of ginger should be focused on. Using soil culture experiments, this study aimed to identify the response of ginger growth to mercury pollution, assess the transmission and residue of mercury in different product organs and explore the mitigation mechanism of silicon on mercury toxicity. Effects of soil mercury pollution on ginger growth showed hormesis and time effect. Long-term mercury pollution led to growth inhibition and quality degradation of ginger, eventually reducing its yield by 25.96% (mercury = 9 mg kg-1). Contents of mercury and silicon in different organs both were the highest in root, followed by rhizome, less in stem and leaf, especially the mercury residue in rhizome manifested as Mother-ginger > Son-ginger > Grandson-ginger. At 6 mg kg-1 soil mercury level, the mercury residue of Mother-ginger exceeds the edible pollutant limit standard (China) by 10.7 times, which makes no obvious risk after being consumed by adults, but poses a potential health threat to children. Notably, it is safer to consume the newly sprouted and inflated tender ginger. Application of silicon fertilizer could alleviate mercury toxicity, mainly by promoting ginger root growth and leaf pigment synthesis, stimulating water-gas exchange system, fluorescence system and antioxidant system to make an anti-stress response. 2 mg kg-1 silicon fertilizer had the most significant mitigation effect on mercury stress, which increased the yield of ginger by 24.85% and reduced the mercury residue of ginger block by 44.44%-60.17%.

A critical review of mercury speciation, bioavailability, toxicity and detoxification in soil-plant environment: Ecotoxicology and health risk assessment

Environmental contamination by a non-essential and non-beneficial, although potentially toxic mercury (Hg), is becoming a great threat to the living organisms at a global scale. Owing to its various uses in numerous industrial processes, high amount of Hg is released into different environmental compartments. Environmental Hg contamination can result in food chain contamination, especially due to its accumulation in edible plant parts. Consumption of Hg-rich food is a key source of Hg exposure to humans. Since Hg does not possess any identified biological role and has genotoxic and carcinogenic potential, it is critical to monitor its biogeochemical behavior in the soil-plant system and its influence in terms of possible food chain contamination and human exposure. This review traces a plausible link among Hg levels, its chemical speciation and phytoavailability in soil, accumulation in plants, phytotoxicity and detoxification of Hg inside the plant. The role of different enzymatic (peroxidase, catalase, ascorbate peroxidase, superoxide dismutase, glutathione peroxidase) and non-enzymatic (glutathione, phytochelatins, proline and ascorbic acid) antioxidants has also been elucidated with respect to enhanced generation of reactive radicles and resulting oxidative stress. The review also outlines Hg build-up in edible plant tissues and associated health risks. The biogeochemical role of Hg in the soil-plant system and associated health risks have been described with well summarized and up-to-date data in 12 tables and 4 figures. We believe that this comprehensive review article and meta-analysis of Hg data can be greatly valuable for scientists, researchers, policymakers and graduate-level students.

Research Progress on the Functions of Gasotransmitters in Plant Responses to Abiotic Stresses

Abiotic stress is one of the major threats affecting plant growth and production. The harm of abiotic stresses includes the disruption of cellular redox homeostasis, reactive oxygen species (ROS) production, and oxidative stress in the plant. Plants have different mechanisms to fight stress, and these mechanisms are responsible for maintaining the required homeostasis in plants. Recently, the study of gasotransmitters in plants has attracted much attention, especially for abiotic stress. In the present review, abiotic stressors were mostly found to induce gasotransmitter production in plants. Meanwhile, these gasotransmitters can enhance the activity of several antioxidant enzymes, alleviate the harmfulness of ROS, and enhance plant tolerance under various stress conditions. In addition, we introduced the interaction of gasotransmitters in plants under abiotic stress. With their promising applications in agriculture, gasotransmitters will be adopted in the near future.

Hydrogen gas promotes the adventitious rooting in cucumber under cadmium stress

Hydrogen gas (H2) plays an important role in plant development and stress responses. Here, cucumber (Cucumis sativus L.) explants were used to investigate the roles of H2 in adventitious root development under cadmium (Cd) stress and its physiological mechanism. The results showed that hydrogen-rich water (HRW) promoted adventitious rooting under Cd stress and 50% HRW obtained the maximal biological response. Compared with Cd treatment, HRW + Cd treatment significantly reduced the content of malondialdehyde (MDA), hydrogen peroxide (H2O2), superoxide radical (O2-), thiobarbituric acid reactive substances (TBARS), ascorbic acid (AsA) and reduced glutathione (GSH), as well as relative electrical conductivity (REC), lipoxygenase (LOX) activity, AsA/docosahexaenoic acid (DHA) ratio, and GSH/oxidized glutathione (GSSG) ratio, while increasing DHA and GSSG content. HRW + Cd treatment also significantly increased in the activity and related gene expression of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR). Additionally, HRW + Cd treatment increased the contents of osmotic adjustment substances, as well as the activities of peroxidase (POD) and polyphenol oxidase (PPO), while significantly decreasing indoleacetic acid oxidase (IAAO) activity. In summary, H2 could induce adventitious rooting under Cd stress by decreasing the oxidative damage, increasing osmotic adjustment substance content and regulating rooting-related enzyme activity.

Utilization of Mentha aquatica L. for removal of fecal pathogens and heavy metals from water of Bosna river, Bosnia and Herzegovina

The aim of the present study was to investigate the potential of Mentha aquatica L. for phytoremediation of water contaminated with heavy metals and fecal pathogens from Bosna river. The water was treated with M. aquatica for 5, 10, and 15 days consecutively after which it was analyzed for the various physicochemical and microbiological parameters. The initial concentration of cadmium (Cd) ranged from 3.644 to 6.108 µg/l, while lead (Pb) varied between 0.1 and 1.386 µg/l. After treatment, M. aquatica accumulated significant amounts of cadmium (Cd) and lead (Pb) with the highest removal rates of 96.49% for Cd and 45.72% for Pb. Values of several physicochemical parameters were decreased after 15 days treatment period. All water samples were analyzed for enumeration of aerobic heterotrophic bacteria, total coliforms, and fecal coliforms by the membrane filtration. Removal efficiency was greater than 80% for microbiological parameters. The concentration of heavy metals was determined in different plant parts and subsequently, the translocation factor was determined. In M. aquatica plant parts, concentrations of Pb and Cd were increased after 15 days of treatment. Our results demonstrated that M. aquatica could be good candidates for the removal of fecal pathogens and heavy metals present in surface water.

Haem oxygenase: A functionally diverse enzyme of photosynthetic organisms and its role in phytochrome chromophore biosynthesis, cellular signalling and defence mechanisms

Haem oxygenase (HO) is a universal enzyme that catalyses stereospecific cleavage of haem to BV IX α and liberates Fe⁺² ion and CO as by-product. Beside haem degradation, it has important functions in plants that include cellular defence, stomatal regulation, iron mobilization, phytochrome chromophore synthesis, and lateral root formation. Phytochromes are an extended family of photoreceptors with a molecular mass of 250 kDa and occur as a dimer made up of 2 equivalent subunits of 125 kDa each. Each subunit is made of two components: the chromophore, a light-capturing pigment molecule and the apoprotein. Biosynthesis of phytochrome (phy) chromophore includes the oxidative splitting of haem to biliverdin IX by an enzyme HO, which is the decisive step in the biosynthesis. In photosynthetic organisms, BVα is reduced to 3Z PΦB by a ferredoxin-dependent PΦB synthase that finally isomerised to PΦB. The synthesized PΦB assembles with the phytochrome apoprotein in the cytoplasm to generate holophytochrome. Thus, necessary for photomorphogenesis in plants, which has confirmed from the genetic studies, conducted on Arabidopsis thaliana and pea. Besides the phytochrome chromophore synthesis, the review also emphasises on the current advances conducted in plant HO implying its developmental and defensive role.

Assessing the difference of tolerance and phytoremediation potential in mercury contaminated soil of a non-food energy crop, Helianthus tuberosus L. (Jerusalem artichoke)

This study was conducted to evaluate the effects of mercury stress on growth, photosynthesis and mercury accumulation in different cultivars of a non-food energy crop, Jerusalem artichoke, and to screen appropriate cultivars for their efficacy in the phytoremediation of mercury (Hg²⁺) contaminated soil. Cultivars LZJ033 (high above-ground biomass and nutrient content, and strongly sexual reproduction) and LZJ119 (a long period of vegetative growth) exhibited more tolerance to mercury stress than LZJ047 (the highest tuber yield and total sugar content). The lines LZJ119 and LZJ047 showed delays in emergence time of about four weeks, and LZJ047 exhibited the highest mortality rate, 85.19%, under treatment with 10 mg kg^-1 mercury. The MDA (malondialdehyde) content increased whereas and the P_n (net photosynthetic rate), F_v∕F_m (the maximum quantum yield of PSII photochemistry) and chlorophyll content decreased in response to mercury stress. The stem diameter, stem biomass and photosynthetic rate of Jerusalem artichoke showed some modest increases in response to mercury stress and exhibited hormesis at least 1 mg kg^-1 mercury treatment. Overall, LZJ119 produced more biomass under mercury stress, whereas LZJ033 exhibited a greater capacity for mercury bioaccumulation. Accordingly, LZJ119 may be a good candidate cultivar for use in cases of moderate—low mercury contamination, whereas LZJ033 may be a better candidate under conditions of high mercury contamination. When Jerusalem artichoke was cultivated in mercury contaminated soil, it not only removed the mercury from soil but also produced large amounts of tubers and shoots which could be used as feedstock for the production of bioethanol.

Carbon Monoxide Is Involved in Hydrogen Gas-Induced Adventitious Root Development in Cucumber under Simulated Drought Stress

Hydrogen gas (H2) and carbon monoxide (CO) are involved in plant growth and developmental processes and may induce plant tolerance to several stresses. However, the independent roles and interaction effect of H2 and CO in adventitious root development under drought conditions have still not received the needed research attention. We hypothesize that there exists crosstalk between H2 and CO during adventitious root development under drought stress. The results of our current study revealed that 50% (v/v) hydrogen-rich water (HRW), 500 μM Hemin (the CO donor) and 30% (w/v) CO aqueous solution apparently promoted the development of adventitious roots in cucumber explants (Cucumis Sativus L.) under drought stress. H2 and CO increased relative water content (RWC), leaf chlorophyll content (chlorophyll a, b, and a+b), and chlorophyll fluorescence parameters [photochemical efficiency of photosystem II (PSII), PSII actual photochemical efficiency and photochemical quench coefficient] under drought condition. When the CO scavenger hemoglobin (Hb) or zinc protoporphyrin IX (ZnPPIX) was added to HRW/CO aqueous solution, the positive effect of HRW/CO aqueous solution on RWC, leaf chlorophyll content, and chlorophyll fluorescence parameters were reversed. Additionally, superoxide dismutases, peroxidase, catalase, and ascorbate peroxidase was significantly increased in the explants treated with HRW and CO aqueous solution under drought stress, thus alleviating oxidative damage, as indicated by decreases in thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H2O2), and superoxide radical (O2-) levels. H2 and CO also improved the levels of water soluble carbohydrate, total soluble protein, and proline content. However, the above CO/H2-mediated effects were reversed by CO scavenger Hb or CO specific synthetic inhibitor ZnPPIX. Therefore, CO may be involved in H2-induced adventitious rooting under drought stress and alleviate oxidative damage by enhancing RWC, leaf chlorophyll content, chlorophyll fluorescence parameters, metabolic constituent content, activating anti-oxidant enzymes and reducing TBARS, O2-, and H2O2 levels.

Hydrogen sulfide alleviates mercury toxicity by sequestering it in roots or regulating reactive oxygen species productions in rice seedlings

Soil mercury (Hg) contamination is a major factor that affects agricultural yield and food security. Hydrogen sulfide (H₂S) plays multifunctional roles in mediating a variety of responses to abiotic stresses. The effects of exogenous H₂S on rice (Oryza sativa var 'Nipponbare') growth and metabolism under mercuric chloride (HgCl₂) stress were investigated in this study. Either 100 or 200 μM sodium hydrosulfide (NaHS, a donor of H₂S) pretreatment improved the transcription of bZIP60, a membrane-associated transcription factor, and then enhanced the expressions of non-protein thiols (NPT) and metallothioneins (OsMT-1) to sequester Hg in roots and thus inhibit Hg transport to shoots. Meanwhile, H₂S promoted seedlings growth significantly even in the presences of Hg and superoxide dismutase (SOD, EC 1.15.1.1) or catalase (CAT, EC 1.11.1.6) inhibitors, diethyldithiocarbamate (DDC) or 3-amino-1,2,4-triazole (AT). H₂S might act as an antioxidant to inhibit or scavenge reactive oxygen species (ROS) productions for maintaining the lower MDA and H₂O₂ levels, and thereby preventing oxidative damages. All these results indicated H₂S effectively alleviated Hg toxicity by sequestering it in roots or by regulating ROS in seedlings and then thus significantly promoted rice growth.

Plant tolerance to mercury in a contaminated soil is enhanced by the combined effects of humic matter addition and inoculation with arbuscular mycorrhizal fungi

In a greenhouse pot experiment, lettuce plants (Lactuca sativa L.) were grown in a Hg-contaminated sandy soil with and without inoculation with arbuscular mycorrhizal fungi (AMF) (a commercial inoculum containing infective propagules of Rhizophagus irregularis and Funneliformis mosseae) amended with different rates of a humic acid (0, 1, and 2 g kg(-1) of soil), with the objective of verifying the synergistic effects of the two soil treatments on the Hg tolerance of lettuce plants. Our results indicated that the plant biomass was significantly increased by the combined effect of AMF and humic acid treatments. Addition of humic matter to soil boosted the AMF effect on improving the nutritional plant status, enhancing the pigment content in plant leaves, and inhibiting both Hg uptake and Hg translocation from the roots to the shoots. This was attributed not only to the Hg immobilization by stable complexes with HA and with extraradical mycorrhizal mycelium in soil and root surfaces but also to an improved mineral nutrition promoted by AMF. This work indicates that the combined use of AMF and humic acids may become a useful practice in Hg-contaminated soils to reduce Hg toxicity to crops.

Carbon Monoxide as a Signaling Molecule in Plants

Carbon monoxide (CO), a gaseous molecule, has emerged as a signaling molecule in plants, due to its ability to trigger a series of physiological reactions. This article provides a brief update on the synthesis of CO, its physiological functions in plant growth and development, as well as its roles in abiotic stress tolerance such as drought, salt, ultraviolet radiation, and heavy metal stress. CO has positive effects on seed germination, root development, and stomatal closure. Also, CO can enhance plant abiotic stress resistance commonly through the enhancement of antioxidant defense system. Moreover, CO shows cross talk with other signaling molecules including NO, phytohormones (IAA, ABA, and GA) and other gas signaling molecules (H2S, H2, CH4).

Antioxidant responses in the leaves of mercury-treated Eichhornia crassipes (Mart.) Solms

Eichhornia crassipes (Mart.) Solms. plantlets were grown in 0.1 and 1.0 ppm treatment solutions of Hoagland's hydroponic solutions modified with Hg(NO₃)2 in order to examine the specific cellular and biochemical mechanisms involved in the tolerance of this plant exposed to mercury. This study assessed the responses of chloroplast pigments, i.e., carotenoids and chlorophylls, and evaluated the enzymatic and nonenzymatic antioxidant systems. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) revealed varying Hg(2+) levels in the young and mature leaf tissues, with greater amounts of Hg(2+) found in the tissues of the young leaves. Total chlorophyll levels, notably those of chlorophyll a, chlorophyll b, and carotenoids, showed significant elevation in young leaf tissues, while a decrease in their levels was observed in mature leaf tissues in comparison to those of the control plants. These results lend support to the protective role of increased chlorophyll and carotenoid levels in the photosynthetic apparatus of young E. crassipes leaves in the presence of Hg(2+). The antioxidant responses of Hg-treated E. crassipes plants were also measured, revealing a highly significant increase in catalase units, catalase and ascorbate peroxidase activities, and mercury-binding thiols in leaves from Hg-treated plants. Moreover, substantial differences in the degree of oxidative injury between the cells in leaves from the control and Hg-treated plants were evidenced by the lipid peroxidation activities monitored. The Hg-treatment-induced significant decrease in malondialdehyde (MDA) levels was observed in 0.1-ppm Hg(NO₃)2-exposed plants, while a highly significant increase in MDA levels was noted in 1.0-ppm Hg(NO₃)2-exposed plants. The high degree of lipid peroxidation at 1.0-ppm Hg treatment was evidently counteracted by the compensatory protective mechanism brought about by the increased levels in chloroplast pigments and the enhanced activities of the antioxidant systems. E. crassipes responded to mercury treatments by enhancing the synthesis of chlorophyll and carotenoid pigments, enzymatic, and nonenzymatic antioxidant substances, concomitantly increasing the antioxidative activities, thus rendering E. crassipes capable of tolerating Hg-induced stress. The potential of E. crassipes as a phytoremediator is evident.

Physiological and biochemical changes in Brassica juncea plants under Cd-induced stress

Plants of Brassica juncea L. var. RLC-1 were exposed for 30 days to different concentrations (0, 0.2, 0.4, and 0.6 mM) of cadmium (Cd) to analyze the Cd uptake, H2O2 content, hormonal profiling, level of photosynthetic pigments (chlorophyll, carotenoid, and flavonoid), gaseous exchange parameters (photosynthetic rate, vapour pressure deficit, intercellular CO2 concentration, and intrinsic mesophyll rate), antioxidative enzymes (superoxide dismutase, polyphenol oxidase, glutathione-S transferase, and glutathione peroxidase), antioxidant assays (DPPH, ABTS, and total phenolic content), and polyphenols. Results of the present study revealed the increased H2O2 content and Cd uptake with increasing metal doses. UPLC analysis of plants showed the presence of various polyphenols. Gaseous exchange measurements were done by infrared gas analyzer (IRGA), which was negatively affected by metal treatment. In addition, LC/MS study showed the variation in the expression of plant hormones. Level of photosynthetic pigments and activities of antioxidative enzymes were altered significantly in response to metal treatment. In conclusion, the antioxidative defence system of plants got activated due to heavy metal stress, which protects the plants by scavenging free radicals.

Hydrogen-rich water confers plant tolerance to mercury toxicity in alfalfa seedlings

In this report, the effect of hydrogen-rich water (HRW), which was used to investigate the physiological roles of hydrogen gas (H2) in plants recently, on the regulation of plant adaptation to mercury (Hg) toxicity was studied. Firstly, we observed that the exposure of alfalfa seedlings to HgCl2 triggered production of reactive oxygen species (ROS), growth stunt and increased lipid peroxidation. However, such effects could be obviously blocked by HRW. Meanwhile, significant decreases in the relative ion leakage and Hg accumulation were observed. Hg-induced increases in total and isozymatic activities of superoxide dismutase (SOD) were significantly reversed by HRW. Further results suggested that HRW-induced the activities of guaiacol peroxidase (POD) and ascorbate peroxidase (APX), two hydrogen peroxide-scavenging enzymes, was at transcriptional levels. Meanwhile, obvious increases of the ratios of reduced/oxidized glutathione (GSH), homoglutathione (hGSH), and ascorbic acid (AsA) and corresponding gene expression were consistent with the decreased oxidative damage in seedling roots. In summary, the results of this investigation indicated that HRW was able to alleviate Hg toxicity in alfalfa seedlings by (i) alleviating growth stunt and reducing Hg accumulation, and (ii) avoidance of oxidative stress and reestablishment of redox homeostasis.

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.

miR395 is involved in detoxification of cadmium in Brassica napus

The toxic metal cadmium (Cd) constitutes one of the major inorganic contaminants in environments. microRNAs (miRNAs) are a class of endogenous non-coding small RNAs. miR395 is conserved and regulates sulfate assimilation and distribution in higher plants, but whether it is involved in detoxification of Cd in plants has not been described. In this study, transgenic rapeseed (Brassica napus) over-expressing miR395 was identified under Cd stress. miR395-over-expressing plants showed a lower degree of Cd-induced oxidative stress than wild type. By contrast, chlorophyll, glutathione and non-protein thiols contents were higher in the transformants than wild type. Determination of growth response showed that 35S::MIR395 plants accumulated higher levels of biomass and sulfur than wild type under Cd exposure. miR395 transgenic plants had higher levels of Cd in plants, particularly at the high supply of Cd in the medium, but they tended to repress Cd translocation from roots to shoots. Simultaneously, expression of metal-tolerance genes such as BnPCS1, BnHO1 and Sultr1;1 was up-regulated under Cd stress, and the expression of the genes was more pronounced in 35S::MIR395 plants than in wild type. These results suggest that miR395 would be involved in detoxification of Cd in B. napus.

Joint ecotoxicology of cadmium and metsulfuron-methyl in wheat (Triticum aestivum)

Herbicide is indispensable for crop production. However, substantial usage of herbicide has led to its increasing accumulation in soils and crops. In addition, cadmium has become one of the widely occurring contaminants in soils due to its significant release into environment via anthropogenic activities. In this study, ecotoxicological investigations were made by exposing the food crop wheat to joint contaminations of Cd and metsulfuron-methyl, a sulfonylurea herbicide. We analyzed growth and physiological and molecular responses in wheat exposed to 0.5 mg kg(-1) Cd and 0.02 mg kg(-1) metsulfuron-methyl (MSM). Soils contaminated with Cd and MSM complex caused significantly detrimental effect on wheat growth and physiological process. Combinative treatments with Cd and MSM damage more severely the plant cells as compared with Cd or MSM treatment alone. Compared with the growth parameter, the biochemical and molecular responses of wheat appeared more pronounced to Cd and MSM complex. Furthermore, compared with control, wheat plants exposed to Cd + MSM generated more O(2-.)and H2O2, both of which were shown to be the cause of enhanced activity of several antioxidant enzymes. Native polyacrylamide gel eletrophoresis and molecular response analyses were performed to validate the results indicated above. Our results indicated that joint contamination with Cd and MSM was more toxic to wheat than a single contamination. These sensitive biological parameters can be used as biomarkers monitoring the ecotoxicological process in plants.

BjHO-1 is involved in the detoxification of heavy metal in India mustard (Brassica juncea)

Heme oxygenase-1 (HO-1) is a stress-responsive gene coding for an enzyme catalyzing the catabolism of heme to yield biliverdin IXα, carbon monoxide (CO) and iron. However, its biological role in regulating metal homeostasis, particularly the tolerance to toxic heavy metals is poorly understood. In this study, a novel gene encoding a Brassica juncea heme oxygenase-1 (designated as BjHO-1) was cloned and functionally identified. Spatial expression of BjHO-1 showed that it was differentially expressed in cotyledon, hypocotyl, leaf and root. BjHO-1 was found to be induced significantly by heavy metal Hg. To understand whether BjHO-1 is able to regulate plant tolerance to Hg, we constructed transgenic B. juncea plants overexpressing HO-1, and showed that 35S::BjHO1 plants confer the plant resistance to Hg toxicity by improving plant dry mass, reducing Hg accumulation, and attenuating Hg-induced oxidative stress. We further cloned a 1,099 bp promoter sequence upstream of BjHO-1 using genome walking approach. Multiple stress-responsive elements were detected in the BjHO-1 promoter regions. The promoter can be activated by Zn, Cd, Hg and Pb exposure. Our results indicate that up-regulation of BjHO-1 is beneficial for limiting the uptake or accumulation of heavy metals into plants. This work also provides a new example for molecular breeding designed for plants that do not accumulate or minimizing accumulation of toxic trace metals growing on heavy metal-contaminated soils.

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.

Regulation of tolerance of Chlamydomonas reinhardtii to heavy metal toxicity by heme oxygenase-1 and carbon monoxide

Investigation of heavy metal tolerance genes in green algae is of great importance because heavy metals have become one of the major contaminants in the aquatic ecosystem. In plants, accumulation of heavy metals modifies many aspects of cellular functions. However, the mechanism by which heavy metals exert detrimental effects is poorly understood. In this study, we identified a role for HO-1 (encoding heme oxygenase-1) in regulating the response of Chlamydomonas reinhardtii, a unicellular green alga, to mercury (Hg). Transgenic algae overexpressing HO-1 showed high tolerance to Hg exposure, with a 48.2% increase in cell number over the wild type, but accumulated less Hg. Physiological analysis revealed that expression of HO-1 suppressed the Hg-induced generation of reactive oxygen species. We further identified the effect of carbon monoxide (CO), a product of HO-1-mediated heme degradation, on growth and physiological parameters. Interestingly, administration of exogenous CO at non-toxic levels also conferred the tolerance of algae to Hg exposure. The CO-mediated alleviation of Hg toxicity was closely related to the lower accumulation of Hg and free radical species. These results indicate that functional identification of HO-1 is useful for molecular breeding designed to improve plant tolerance to heavy metals and reduce heavy metal accumulation in plant cells.

Expression of a Brassica napus heme oxygenase confers plant tolerance to mercury toxicity

Plant heme oxygenases (HOs) regulate biosynthesis of phytochrome which accounts for photo-acceptance and -morphogenesis. Recent studies have demonstrated that plant HOs also regulate many other physiological processes including response to environmental stimuli. To elucidate the mechanism by which HOs regulate plant adaptation to heavy metal exposure, three novel HOs genes were isolated from rapeseed (Brassica napus) and their expression patterns were analysed. Alignment of deduced protein sequences revealed that the three BnHOs share high identity with their corresponding orthologos (AtHO1-3) from Arabidopsis. To investigate whether the BnHO regulates plant tolerance to Hg toxicity, we constructed B. napus transgenic plants overexpressing BnHO-1. Under Hg stress, the transgenic plants had 1.41-1.59 folds higher biomass than the untransformants. However, overexpression of BnHO-1 resulted in less accumulation of Hg in some lines of transformants than in untransformants. The transgenic plants show lower abundance of reactive oxygen species and attenuated oxidative injury compared with the untransgenic plants. We cloned the promoter sequences of BnHO-1 from B. napus. Analysis revealed that the 1119 bp fragment contains a conserved Cd responsive element (CdRE) and others responding to multiple environmental stimuli. Transient expression in tobacco leaves showed differential responses to heavy metals (Zn, Cu, Pb, Hg and Cd).