The standard electrode potential (Eθ) predicts the prooxidant activity and the acute toxicity of metal ions

Biogenic zinc oxide nanoparticles: A viable agricultural tool to control plant pathogenic fungi and its potential effects on soil and plants

Fungal and bacterial pathogens represent some of the greatest challenges facing crop production globally and account for about 20-40 % crop losses annually. This review highlights the use of ZnO NPs as antimicrobial agents and explores their mechanisms of actions against disease causing plant fungal pathogens. The behavior of ZnO NPs in soil and their interactions with the soil components were also highlighted. The review discusses the potential effects of ZnO NPs on plants and their mechanisms of action on plants and how these mechanisms are related to their physicochemical properties. In addition, the reduction of ZnO NPs toxicity through surface modification and coating with silica is also addressed. Soil properties play a significant role in the dispersal, aggregation, stability, bioavailability, and transport of ZnO NPs and their release into the soil. The transport of ZnO NPs into the soil might influence soil components and, as a result, plant physiology. The harmful effects of ZnO NPs on plants and fungi are caused by a variety of processes, the most important of which is the formation of reactive oxygen species, lysosomal instability, DNA damage, and the reduction of oxidative stress by direct penetration/liberation of Zn2+ ions in plant/fungal cells. Based on these highlighted areas, this review concludes that ZnO NPs exhibit its antifungal activity via generations of reactive oxygen species, coupled with the inhibition of various metabolic pathways. Despite the numerous advantages of ZnO NPs, there is need to regulate its uses to minimize the harmful effects that may arise from its applications in the soil and plants.

Evaluation of potential ecological risk and prediction of zinc accumulation and its transfer in soil plants and ruminants: public health implications

Present work evaluated the zinc (Zn) concentration in soil, forage, blood plasma, hair, and feces samples of cows, buffaloes, and sheep taken from Mianwali, Punjab, Pakistan. The concentration of Zn was found in the ranged of 21.82-35.09mg/kg, 32.59-42.17mg/kg, 0.927-2.48mg/l, 1.03-2.84mg/kg, and 0.923-1.98mg/kg in soil, forage, blood plasma, hair, and feces samples, respectively. The Zn concentration in soil, forage, blood, hair, and feces was safer compared to standard limits. Statistical analysis described that values for BCF, PLI, EF, DIM, and HRI ranged 1.03-1.57mg/kg, 0.486-0.782mg/kg, 0.457-0.696mg/kg, 0.048-0.08mg/kg, and 0.160-0.272mg/kg, respectively. It can be concluded from the present work that Zn concentration was safe in soil, forages, and animal samples. BCF was noticed as greater than 1 while PLI, EF, DIM, and HRI were found less than 1, so regular heavy metal analysis was required to appraise the contamination level in environment.

Potential of ornamental monocot plants for rhizofiltration of cadmium and zinc in hydroponic systems

Cadmium (Cd) and zinc (Zn) accumulation and uptake ability have been investigated in three ornamental monocot plants (Heliconia psittacorum x H. spathocircinata, Echinodorus cordifolius, and Pontederia cordata) grown in hydroponic systems. All study plants in the highest heavy metal treatments were found to be excluders for Cd and Zn with translocation factor values < 1 and bioconcentration factor (BCF) values > 100. The highest Cd and Zn accumulations were found in roots of E. cordifolius (4766.6 mg Zn kg^-1 and 6141.6 mg Cd kg^-1), followed by H. psittacorum x H. spathocircinata (4313.5 mg Zn kg^-1) and P. cordata (3673.3 mg Cd kg^-1), respectively, whereas shoots had lower performances. However, P. cordata had the lowest dry biomass production compared to the other two plant species in this study. As a result of dilution effects, heavy metal accumulation for all study plants was lower in the combined heavy metal treatments than in solely Cd and Zn only treatments. At the end of experiments, the highest uptakes of Cd and Zn were found in H. psittacorum x H. spathocircinata (62.1% Zn²⁺ from 10 mg Zn L^-1 solution) and E. cordifolius (27.3% Cd²⁺ from 2 mg Cd L^-1 solution). Low percentage metal uptakes were found in P. cordata; therefore, E. cordifolius and H. psittacorum x H. spathocircinata are clearly better suited for removing Cd and/or Zn from contaminated waters and hydroponic systems.

A Role for Zinc in Plant Defense Against Pathogens and Herbivores

Pests and diseases pose a threat to food security, which is nowadays aggravated by climate change and globalization. In this context, agricultural policies demand innovative approaches to more effectively manage resources and overcome the ecological issues raised by intensive farming. Optimization of plant mineral nutrition is a sustainable approach to ameliorate crop health and yield. Zinc is a micronutrient essential for all living organisms with a key role in growth, development, and defense. Competition for Zn affects the outcome of the host-attacker interaction in both plant and animal systems. In this review, we provide a clear framework of the different strategies involving low and high Zn concentrations launched by plants to fight their enemies. After briefly introducing the most relevant macro- and micronutrients for plant defense, the functions of Zn in plant protection are summarized with special emphasis on superoxide dismutases (SODs) and zinc finger proteins. Following, we cover recent meaningful studies identifying Zn-related passive and active mechanisms for plant protection. Finally, Zn-based strategies evolved by pathogens and pests to counteract plant defenses are discussed.

Hormetic effects of metal ions upon V. fischeri and the application of a new parameter for the quantitative assessment of hormesis

Hormesis is an intriguing phenomenon that is characterized by low dose stimulation and high dose inhibition. Several traditional parameters, such as the concentration of the zero equivalent point (ZEP) and the maximal stimulatory effect (Y_max), have been used to characterize the zone of hormesis or the extent of the stimulatory effect. However, the characteristics of hormesis for chemicals cannot be quantified completely by one parameter, which is important to accurately compare the hormetic effects of chemicals and to describe the combined effects of chemical mixtures at low doses. In the present study, a novel parameter, termed the relative standard area of hormetic zone (Hor_Area^R), was developed and proposed to quantify the hormetic effects (24h exposure) of nine metal ions (Cr³⁺, Cu²⁺, Mg²⁺, Cd²⁺, Fe³⁺, Ni²⁺, Zn²⁺, Co²⁺, Cs⁺) towards Vibrio fischeri, both individually and as binary mixtures. The results indicate that Hor_Area^R can be used not only to accurately assess the hormetic effects and its relationship with structural characteristics but also to conveniently describe the combined effects of interactive mixtures at low dose. Thus, the Hor_Area^R parameter can quantitatively assess the hormetic effects and can offer a useful approach to perform environmental risk assessments of chemicals at low doses.

High salinity helps the halophyte Sesuvium portulacastrum in defense against Cd toxicity by maintaining redox balance and photosynthesis

NaCl alleviates Cd toxicity in Sesvium portulacastrum by maintaining plant water status and redox balance, protecting chloroplasts structure and inducing some potential Cd (2+) chelators as GSH and proline. It has been demonstrated that NaCl alleviates Cd-induced growth inhibition in the halophyte Sesuvium portulacastrum. However, the processes that mediate this effect are still unclear. In this work we combined physiological, biochemical and ultrastructural studies to highlight the effects of salt on the redox balance and photosynthesis in Cd-stressed plants. Seedlings were exposed to different Cd concentrations (0, 25 and 50 µM Cd) combined with low (0.09 mM) (LS), or high (200 mM) NaCl (HS) in hydroponic culture. Plant-water relations, photosynthesis rate, leaf gas exchange, chlorophyll fluorescence, chloroplast ultrastructure, and proline and glutathione concentrations were analyzed after 1 month of treatment. In addition, the endogenous levels of stress-related hormones were determined in plants subjected to 25 µM Cd combined with both NaCl concentrations. In plants with low salt supply (LS), Cd reduced growth, induced plant dehydration, disrupted chloroplast structure and functioning, decreased net CO2 assimilation rate (A) and transpiration rate (E), inhibited the maximum potential quantum efficiency (Fv/Fm) and the quantum yield efficiency (Φ PSII) of PSII, and enhanced the non-photochemical quenching (NPQ). The addition of 200 mM NaCl (HS) to the Cd-containing medium culture significantly mitigated Cd phytotoxicity. Hence, even at similar internal Cd concentrations, HS-Cd plants were less affected by Cd than LS-Cd ones. Hence, 200 mM NaCl significantly alleviates Cd-induced toxicity symptoms, growth inhibition, and photosynthesis disturbances. The cell ultrastructure was better preserved in HS-Cd plants but affected in LS-Cd plants. The HS-Cd plants showed also higher concentrations of reduced glutathione (GSH), proline and jasmonic acid (JA) than the LS-Cd plants. However, under LS-Cd conditions, plants maintained higher concentration of salicylic acid (SA) and abscisic acid (ABA) than the HS-Cd ones. We conclude that in S. portulacastrum alleviation of Cd toxicity by NaCl is related to the modification of GSH and proline contents as well as stress hormone levels thus protecting redox balance and photosynthesis.

How does NaCl improve tolerance to cadmium in the halophyte Sesuvium portulacastrum?

Sesuvium portulacastrum is a halophyte with considerable Cd tolerance and accumulation, especially under high salinity. The species seems a good candidate for phytoremediation of Cd-contaminated, saline soils. However, the mechanisms sustaining salt-induced alleviation of Cd toxicity remain unknown. Seedlings of S. portulacastrum were submitted hydroponically to different Cd concentrations (0, 25 and 50 μM Cd) in combination with low (0.09 mM), or high (200 mM) NaCl. Cadmium distribution within leaves and stems was assessed by total Cd, cell sap Cd, and Cd in different cell fractions. In plants with low salt supply (LS) Cd induced severe toxicity. The presence of 200 mM NaCl (HS) significantly alleviated Cd toxicity symptoms. HS drastically reduced both Cd-induced H2O2 production and membrane damage. In HS plants the reduced Cd uptake was only in part responsible for the lower Cd toxicity. Even at equal internal leaf Cd concentrations less Cd toxicity was observed in HS than in LS plants. In HS plants proportionally more Cd was bound in cell walls and proportionally less accumulated in the soluble fraction than in LS plants. Our results show that NaCl improves plant performance under Cd stress by both a decrease of Cd(2+) activity in the medium leading to less Cd uptake and a change of Cd speciation and compartmentation inside tissues. More efficient internal detoxification seems mainly brought about by preferential Cd binding to chloride and cell walls in plants treated with a high salt concentration.

Membrane transporters mediating root signalling and adaptive responses to oxygen deprivation and soil flooding

This review provides a comprehensive assessment of a previously unexplored topic: elucidating the role that plasma- and organelle-based membrane transporters play in plant-adaptive responses to flooding. We show that energy availability and metabolic shifts under hypoxia and anoxia are critical in regulating membrane-transport activity. We illustrate the high tissue and time dependence of this regulation, reveal the molecular identity of transporters involved and discuss the modes of their regulation. We show that both reduced oxygen availability and accumulation of transition metals in flooded roots result in a reduction in the cytosolic K(+) pool, ultimately determining the cell's fate and transition to programmed cell death (PCD). This process can be strongly affected by hypoxia-induced changes in the amino acid pool profile and, specifically, ϒ-amino butyric acid (GABA) accumulation. It is suggested that GABA plays an important regulatory role, allowing plants to proceed with H2 O2 signalling to activate a cascade of genes that mediate plant adaptation to flooding while at the same time, preventing the cell from entering a 'suicide program'. We conclude that progress in crop breeding for flooding tolerance can only be achieved by pyramiding the numerous physiological traits that confer efficient energy maintenance, cytosolic ion homeostasis, and reactive oxygen species (ROS) control and detoxification.

Aluminium in biological environments: a computational approach

The increased availability of aluminium in biological environments, due to human intervention in the last century, raises concerns on the effects that this so far “excluded from biology” metal might have on living organisms. Consequently, the bioinorganic chemistry of aluminium has emerged as a very active field of research. This review will focus on our contributions to this field, based on computational studies that can yield an understanding of the aluminum biochemistry at a molecular level. Aluminium can interact and be stabilized in biological environments by complexing with both low molecular mass chelants and high molecular mass peptides. The speciation of the metal is, nonetheless, dictated by the hydrolytic species dominant in each case and which vary according to the pH condition of the medium. In blood, citrate and serum transferrin are identified as the main low molecular mass and high molecular mass molecules interacting with aluminium. The complexation of aluminium to citrate and the subsequent changes exerted on the deprotonation pathways of its tritable groups will be discussed along with the mechanisms for the intake and release of aluminium in serum transferrin at two pH conditions, physiological neutral and endosomatic acidic. Aluminium can substitute other metals, in particular magnesium, in protein buried sites and trigger conformational disorder and alteration of the protonation states of the protein's sidechains. A detailed account of the interaction of aluminium with proteic sidechains will be given. Finally, it will be described how alumnium can exert oxidative stress by stabilizing superoxide radicals either as mononuclear aluminium or clustered in boehmite. The possibility of promotion of Fenton reaction, and production of hydroxyl radicals will also be discussed.

The rhizotoxicity of metal cations is related to their strength of binding to hard ligands

Mechanisms whereby metal cations are toxic to plant roots remain largely unknown. Aluminum, for example, has been recognized as rhizotoxic for approximately 100 yr, but there is no consensus on its mode of action. The authors contend that the primary mechanism of rhizotoxicity of many metal cations is nonspecific and that the magnitude of toxic effects is positively related to the strength with which they bind to hard ligands, especially carboxylate ligands of the cell-wall pectic matrix. Specifically, the authors propose that metal cations have a common toxic mechanism through inhibiting the controlled relaxation of the cell wall as required for elongation. Metal cations such as Al(3+) and Hg(2+), which bind strongly to hard ligands, are toxic at relatively low concentrations because they bind strongly to the walls of cells in the rhizodermis and outer cortex of the root elongation zone with little movement into the inner tissues. In contrast, metal cations such as Ca(2+), Na(+), Mn(2+), and Zn(2+) , which bind weakly to hard ligands, bind only weakly to the cell wall and move farther into the root cylinder. Only at high concentrations is their weak binding sufficient to inhibit the relaxation of the cell wall. Finally, different mechanisms would explain why certain metal cations (for example, Tl(+), Ag(+), Cs(+), and Cu(2+)) are sometimes more toxic than expected through binding to hard ligands. The data presented in the present study demonstrate the importance of strength of binding to hard ligands in influencing a range of important physiological processes within roots through nonspecific mechanisms.

Transition metals: a double edge sward in ROS generation and signaling

Transition metals such as Iron (Fe) and Copper (Cu) are essential for plant cell development. At the same time, due their capability to generate hydroxyl radicals they can be potentially toxic to plant metabolism. Recent works on hydroxyl-radical activation of ion transporters suggest that hydroxyl radicals generated by transition metals could play an important role in plant growth and adaptation to imbalanced environments. In this mini-review, the relation between transition metals uptake and utilization and oxidative stress-activated ion transport in plant cells is analyzed, and a new model depicting both apoplastic and cytosolic mode of ROS signaling to plasma membrane transporters is suggested.