Induction of Nickel Accumulation in Response to Zinc Deficiency in Arabidopsis thaliana

Azolla pinnata as a phytoremediator: improves germination, growth and yield of maize irrigated with Ni-polluted water

The removal of pollutants from the environment has become a global demand. The current study aimed to relieve the Ni toxicity effect on the germination, growth, and grain yield of maize by using Azolla pinnata as a phytoremediator. Azolla-treated and untreated nickel solutions [0 (control), 24, 70, 140 and 190 ppm] were applied for germination and pot experiments. Electron microscope examination cleared the Ni accumulation in Azolla's cell vacuole and its adsorption on the cell wall. The inhibition of the hydrolytic enzyme activity reduces maize germination; maximal inhibition was 57.1% at 190 ppm of Ni compared to the control (100%). During vegetative growth, Ni stimulated the generation of H2O2 (0.387 mM g-1 F Wt at 190 ppm of Ni), which induced maximal lipid peroxidation (3.913 µMDA g-1 F Wt) and ion leakage (74.456%) compared to control. Chlorophyll content and carbon fixation also showed significant reductions at all Ni concentrations; at 190 ppm, they showed maximum reductions of 56.2 and 63%, respectively. However, detoxification enzymes' activity such as catalase and antioxidant substances (phenolics) increased. The highest concentration of Ni (190 ppm) had the most effect on constraining yield, reaching zero for the weight of 100 grains at 190 ppm of Ni. Azolla-treated Ni solutions amended all determinant parameters, indicating a high percentage of changes in hydrolytic enzyme activity (125.2%) during germination, chlorophyll content (77.6%) and photosynthetic rate (120.1%). Growth measurements, carbon fixation, and yield components showed a positive association. Thus, we recommended using Azolla as a cost-effective and eco-friendly strategy to recover Ni-polluted water.

Comparative study on changes in Cd accumulation and ionome between rice and spinach: Impact of zinc ion activity

Excessive Cd accumulation in rice grain has caused chronic Cd diseases in humans. In most crops, 100 times more Zn than Cd strongly inhibits Cd uptake and translocation. However, this response is not found for rice (Oryza sativa L.), which was found to have an unusual Cd uptake pattern compared with other crops, such as spinach (Spinacia oleracea L.). Moreover, studies on shared transporters between Zn and Cd using normal solution experiments with traditional high concentrations of metal ions may result in irrelevant interactions. Therefore, we developed ethyleneglycoltetraacetate-buffered nutrient solutions in this work. Rice and spinach seedlings were grown under calibrated low Cd²⁺ activity and low to phytotoxic Zn²⁺ activity levels while buffering other micronutrient cations at sufficient levels. Results showed that as rice grew with pZn²⁺  = 8.1-5.4, root Cd and shoot Ni decreased significantly and gradually. However, shoot Cd and Mn in rice decreased slightly with the increase of solution Zn²⁺ from deficiency to sufficiency and then increased at toxic Zn²⁺ solution (pZn²⁺  = 5.4). The shoot/root ratios of Cd in rice under toxic pZn²⁺ (5.6 and 5.4 pZn²⁺ activity) were significantly increased (p < .05). It could be concluded that rice absorption of Cd is not inhibited by co-contaminating (toxic) Zn. For spinach, with Zn varying from pZn²⁺  = 8.1-5.7, both shoot and root Cd substantially decreased, as did shoot Ni. This work revealed that, to understand food chain Cd risks, one needs to consider the inhibitory role of Zn in limiting Cd absorption in all crops studied except rice.

MTM1 displays a new function in the regulation of nickel resistance in Saccharomyces cerevisiae

Nickel (Ni) is an essential yet toxic trace element. Although a cofactor for many metalloenzymes, nickel function and metabolism is not fully explored in eukaryotes. Molecular biology and metallomic methods were utilized to explore the new physiological functions of nickel in Saccharomyces cerevisiae. Here we showed that MTM1 knockout cells displayed much stronger nickel tolerance than wild-type cells and mitochondrial accumulations of Ni and Fe of mtm1Δ cells dramatically decreased compared to wild-type cells when exposed to excess nickel. Superoxide dismutase 2 (Sod2p) activity in mtm1Δ cells was severely attenuated and restored through Ni supplementation in media or total protein. SOD2 mRNA level of mtm1Δ cells was significantly higher than that in the wild-type strain but was decreased by Ni supplementation. MTM1 knockout afforded resistance to excess nickel mediated through reactive oxygen species levels. Meanwhile, additional Ni showed no significant effect on the localization of Mtm1p. Our study reveals the MTM1 gene plays an important role in nickel homeostasis and identifies a novel function of nickel in promoting Sod2p activity in yeast cells.

Heavy metal transporters: Functional mechanisms, regulation, and application in phytoremediation

Heavy metal pollution in soil is a global problem with serious impacts on human health and ecological security. Phytoextraction in phytoremediation, in which plants uptake and transport heavy metals (HMs) to the tissues of aerial parts, is the most environmentally friendly method to reduce the total amount of HMs in soil and has wide application prospects. However, the molecular mechanism of phytoextraction is still under investigation. The uptake, translocation, and retention of HMs in plants are mainly mediated by a variety of transporter proteins. A better understanding of the accumulation strategy of HMs via transporters in plants is a prerequisite for the improvement of phytoextraction. In this review, the biochemical structure and functions of HM transporter families in plants are systematically summarized, with emphasis on their roles in phytoremediation. The accumulation mechanism and regulatory pathways related to hormones, regulators, and reactive oxygen species (ROS) of HMs concerning these transporters are described in detail. Scientific efforts and practices for phytoremediation carried out in recent years suggest that creation of hyperaccumulators by transgenic or gene editing techniques targeted to these transporters and their regulators is the ultimate powerful path for the phytoremediation of HM contaminated soils.

Cellular biogenesis of metal nanoparticles by water velvet (Azolla pinnata): different fates of the uptake Fe3+ and Ni2+ to transform into nanoparticles

Plants can produce cellular metal nanoparticles (NPs) from the uptake of metal ions, but the mechanism remains unclear. This work reported the new insight into different fates of iron (Fe) and nickel (Ni) ions to transform into the metal NPs in Azolla pinnata roots. After exposing to ferric nitrate, nickel nitrate, and a combination of both for 12 h, the energy dispersive X-ray fluorescence analysis indicated the efficient uptakes of both metal ions in the roots and their transports into the shoots. Transmission electron microscope images revealed the accumulation of spherical FeNPs, but not NiNPs, near the cell wall and cell membrane, and inside vacuoles and multivesicular bodies in cortical and vascular cells at the root tips. The energy dispersive X-ray analysis suggested that the formation of metal NPs depended on the sufficient concentration of metal ions localized in the roots. FeNPs were identified to ɑ-Fe2O3 and Fe3O4 by selected area electron diffraction analysis. The formation of FeNPs might involve the increase of superoxide dismutase activity. This work is the first report about the cellular biogenesis of metal NPs in plant roots that likely depends on cellular metal content and involves the reducing activity of antioxidant enzymes.

Physiological and molecular mechanisms of metal accumulation in hyperaccumulator plants

Most of the heavy metals (HMs), and metals/metalloids are released into the nature either by natural phenomenon or anthropogenic activities. Being sessile organisms, plants are constantly exposed to HMs in the environment. The metal non-hyperaccumulating plants are susceptible to excess metal concentrations. They tend to sequester metals in their root vacuoles by forming complexes with metal ligands, as a detoxification strategy. In contrast, the metal-hyperaccumulating plants have adaptive intrinsic regulatory mechanisms to hyperaccumulate or sequester excess amounts of HMs into their above-ground tissues rather than accumulating them in roots. They have unique abilities to successfully carry out normal physiological functions without showing any visible stress symptoms unlike metal non-hyperaccumulators. The unique abilities of accumulating excess metals in hyperaccumulators partly owes to constitutive overexpression of metal transporters and ability to quickly translocate HMs from root to shoot. Various metal ligands also play key roles in metal hyperaccumulating plants. These metal hyperaccumulating plants can be used in metal contaminated sites to clean-up soils. Exploiting the knowledge of natural populations of metal hyperaccumulators complemented with cutting-edge biotechnological tools can be useful in the future. The present review highlights the recent developments in physiological and molecular mechanisms of metal accumulation of hyperaccumulator plants in the lights of metal ligands and transporters. The contrasting mechanisms of metal accumulation between hyperaccumulators and non-hyperaccumulators are thoroughly compared. Moreover, uses of different metal hyperaccumulators for phytoremediation purposes are also discussed in detail.

High-resolution genome-wide association study pinpoints metal transporter and chelator genes involved in the genetic control of element levels in maize grain

Despite its importance to plant function and human health, the genetics underpinning element levels in maize grain remain largely unknown. Through a genome-wide association study in the maize Ames panel of nearly 2,000 inbred lines that was imputed with ∼7.7 million SNP markers, we investigated the genetic basis of natural variation for the concentration of 11 elements in grain. Novel associations were detected for the metal transporter genes rte2 (rotten ear2) and irt1 (iron-regulated transporter1) with boron and nickel, respectively. We also further resolved loci that were previously found to be associated with one or more of five elements (copper, iron, manganese, molybdenum, and/or zinc), with two metal chelator and five metal transporter candidate causal genes identified. The nas5 (nicotianamine synthase5) gene involved in the synthesis of nicotianamine, a metal chelator, was found associated with both zinc and iron and suggests a common genetic basis controlling the accumulation of these two metals in the grain. Furthermore, moderate predictive abilities were obtained for the 11 elemental grain phenotypes with two whole-genome prediction models: Bayesian Ridge Regression (0.33–0.51) and BayesB (0.33–0.53). Of the two models, BayesB, with its greater emphasis on large-effect loci, showed ∼4–10% higher predictive abilities for nickel, molybdenum, and copper. Altogether, our findings contribute to an improved genotype-phenotype map for grain element accumulation in maize.

Nickel in terrestrial biota: Comprehensive review on contamination, toxicity, tolerance and its remediation approaches

Nickel (Ni) has been a subject of interest for environmental, physiological, biological scientists due to its dual effect (toxicity and essentiality) in terrestrial biota. In general, the safer limit of Ni is 1.5 μg g^-1 in plants and 75-150 μg g^-1 in soil. Litreature review indicates that Ni concentrations have been estimated up to 26 g kg^-1 in terrestrial, and 0.2 mg L^-1 in aquatic resources. In case of vegetables and fruits, mean Ni content has been reported in the range of 0.08-0.26 and 0.03-0.16 mg kg^-1. Considering, Ni toxicity and its potential health hazards, there is an urgent need to find out the suitable remedial approaches. Plant vascular (>80%) and cortical (<20%) tissues are the major sequestration site (cation exchange) of absorbed Ni. Deciphering molecular mechanisms in transgenic plants have immense potential for enhancing Ni phytoremediation and microbial remediation efficiency. Further, it has been suggested that integrated bioremediation approaches have a potential futuristic path for Ni decontamination in natural resources. This systematic review provides insight on Ni effects on terrestrial biota including human and further explores its transportation, bioaccumulation through food chain contamination, human health hazards, and possible Ni remediation approaches.

Root hairs: the villi of plants

Strikingly, evolution shaped similar tubular structures at the µm to mm scale in roots of sessile plants and in small intestines of mobile mammals to ensure an efficient transfer of essential nutrients from 'dead matter' into biota. These structures, named root hairs (RHs) in plants and villi in mammals, numerously stretch into the environment, and extremely enlarge root and intestine surfaces. They are believed to forage for nutrients, and mediate their uptake. While the conceptional understanding of plant RH function in hydromineral nutrition seems clear, experimental evidence presented in textbooks is restricted to a very limited number of reference-nutrients. Here, we make an element-by-element journey through the periodic table and link individual nutrient availabilities to the development, structure/shape and function of RHs. Based on recent developments in molecular biology and the identification of mutants differing in number, length or other shape-related characteristics of RHs in various plant species, we present comprehensive advances in (i) the physiological role of RHs for the uptake of specific nutrients, (ii) the developmental and morphological responses of RHs to element availability and (iii) RH-localized nutrient transport proteins. Our update identifies crucial roles of RHs for hydromineral nutrition, mostly under nutrient and/or water limiting conditions, and highlights the influence of certain mineral availabilities on early stages of RH development, suggesting that nutritional stimuli, as deficiencies in P, Mn or B, can even dominate over intrinsic developmental programs underlying RH differentiation.

A comparative study of the factors affecting uptake and distribution of Cd with Ni in barley

Food crops often accumulate heavy metals above the recommended limits. Cadmium (Cd) is particularly harmful in terms of its potential dangers to human health. The effects of nutrient status and cation competition on Cd uptake and distribution in barley were investigated to analyse the main route for Cd entry into the plants. Cd uptake into whole plants was measured by radiotracer studies and elemental analysis using environmentally relevant concentrations. The nutrient status of the plants was altered by growing them hydroponically in micronutrient-deficient conditions (-Fe, -Mn, or -Zn). Fe and Zn were found to have a large effect on the uptake of Cd both via deficiencies and by the competition for uptake. However, Mn was found to have no effect on the uptake of Cd either via deficiency or by the competition for uptake. This strongly suggests that the main route for Cd uptake into the roots is via Fe and Zn transporters. The inhibition of Cd influx only by FeII (but not by FeIII) suggests that Cd uptake into the root occurs through divalent cation transporters. Since Cd is a non-essential metal in plants, the transport characteristics were compared with those of an essential micronutrient, Ni. At the same external concentration, more than twice as much Cd was absorbed as Ni in all of the different nutrient conditions. Ni translocation to the shoot was much lower than for Cd. The comparison of two metals showed some similarities in the root uptake processes but not in the shoot translocation.

Why Does the Halophyte Mesembryanthemum crystallinum Better Tolerate Ni Toxicity than Brassica juncea: Implication of Antioxidant Defense Systems

The implication of enzymatic and non-enzymatic antioxidative systems in response to Ni was evaluated in the halophyte Mesembryanthemum crystallinum in comparison with the metal tolerant glycophyte species Brassica juncea. Seedlings of both species were hydroponically subjected during 21 days to 0, 25, 50, and 100 µM NiCl2. Growth parameters showed that the halophyte M. crystallinum was more tolerant to Ni than B. juncea. Malondialdehyde (MDA) content increased to a higher extent in B. juncea than in M. crystallinum. Antioxidant enzymesactivities were differently affected by Ni in both species. Nickel increased shoot superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in B. juncea, whereas these activities were reduced in M. crystallinum when exposed to metal stress. The root SOD, APX and guaiacol peroxidase (GPX) activities increased upon Ni treatments for both species. The content of non-enzymatic antioxidative molecules such as glutathione, non-protein thiols and proline increased in Ni-treated plants, except for GSH content in the shoot of B. juncea. Based on the oxidative balance, our findings confirm the higher tolerance of the halophyte M. crystallinum to Ni-induced oxidative stress comparatively to B. juncea. We suggest that M. crystallinum is able to overcome the produced ROS using the non-enzymatic system, while Ni-induced oxidative stress was more acute in B. juncea, leading this species to mainly use the enzymatic system to protect against reactive oxygen species.

Distinct redox signalling and nickel tolerance in Brassica juncea and Arabidopsis thaliana

Despite of its essentiality, nickel (Ni) in excess is toxic for plants partly due to the overproduction of reactive oxygen species (ROS) and the consequent increase in oxidative stress signalling. However, in Ni-stressed plants little is known about the signal transduction of reactive nitrogen species (RNS) and protein tyrosine nitration as the protein-level consequence of increased RNS formation. Our experiments compared the nickel accumulation and tolerance, the redox signalling and the protein nitration in the agar-grown Arabidopsis thaliana and Brassica juncea exposed to Ni (50 μM nickel chloride). Studying GUS-tagged Arabidopsis lines (ARR5::GUS, ACS8::GUS and DR5::GUS) revealed that Ni-increased lateral root (LR) emergence, and concomitantly reduced LR initiation were accompanied by elevated levels of auxin, cytokinin, and ethylene in the LRs or in upper root parts, whereas Ni-induced primary root shortening is related to decreased auxin, and increased cytokinin and ethylene levels. These suggest the Ni-induced disturbance of hormonal balance in the root system. Results of the comparative study showed that weaker Ni tolerance of A. thaliana was coupled with a Ni-induced increase in RNS, ROS, and hydrogen sulfide levels, as well as with an increase in redox signalling and consequent increment of protein nitration. However, in relative Ni tolerant B. juncea, redox signalling (except for peroxynitrite) was not modified, and Ni-induced intensification of protein tyrosine nitration was less pronounced. Data collectively show that the better Ni tolerance of Brassica juncea may be related to the capability of preventing the induction of redox signalling and consequently to the slighter increase in protein nitration.

Interaction Between Zn Deficiency, Toxicity and Turnip Yellow Mosaic Virus Infection in Noccaea ochroleucum

Zinc is essential for the functioning of numerous proteins in plants. To investigate how Zn homeostasis interacts with virus infection, Zn-tolerant Noccaea ochroleucum plants exposed to deficient (Zn'0'), optimal (Zn10), and excess Zn (Zn100) concentrations, as well as Cd amendment, were infected with Turnip yellow mosaic virus (TYMV). Imaging analysis of fluorescence kinetics from the μs (OJIP) to the minutes (Kautsky effect, quenching analysis) time domain revealed strong patchiness of systemic virus-induced photosystem II (PSII) inhibition. That was more pronounced in Zn-deficient plants, while Zn excess acted synergistically with TYMV, in both cases resulting in reduced PSII reaction centers. Infected Cd-treated plants, already severely stressed, showed inhibited non-photochemical quenching and PSII activity. Quantitative in situ hybridization at the cellular level showed increased gene expression of ZNT5 and downregulation of HMA4 in infected Zn-deficient leaves. In Zn10 and Zn100 infected leaves, vacuolar sequestration of Zn increased by activation of HMA3 (mesophyll) and MTP1 (epidermis). This correlated with Zn accumulation in the mesophyll and formation of biomineralization dots in the cell wall (Zn100) visible by micro X-ray fluorescence tomography. The study reveals the importance of adequate Zn supply and distribution in the maintenance of photosynthesis under TYMV infection, achieved by tissue-targeted activation of metal transporter gene expression.

Elevated Expression of Vacuolar Nickel Transporter Gene IREG2 Is Associated With Reduced Root-to-Shoot Nickel Translocation in Noccaea japonica

A number of metal hyperaccumulator plants, including nickel (Ni) hyperaccumulators, have been identified in the genus Noccaea. The ability to accumulate Ni in shoots varies widely among species and ecotypes in this genus; however, little is known about the molecular mechanisms underlying this intra- and inter-specific variation. Here, in hydroponic culture, we compared Ni accumulation patterns between Noccaea japonica, which originated in Ni-enriched serpentine soils in Mt. Yubari (Hokkaido, Japan), and Noccaea caerulescens ecotype Ganges, which originated in zinc/lead-mine soils in Southern France. Both Noccaea species showed extremely high Ni tolerance compared with that of the non-accumulator Arabidopsis thaliana. But, following treatment with 200 μM Ni, N. caerulescens showed leaf chlorosis, whereas N. japonica did not show any stress symptoms. Shoot Ni concentration was higher in N. caerulescens than in N. japonica; this difference was due to higher efficiency of root-to-shoot Ni translocation in N. caerulescens than N. japonica. It is known that the vacuole Ni transporter IREG2 suppresses Ni translocation from roots to shoots by sequestering Ni in the root vacuoles. The expression level of the IREG2 gene in the roots of N. japonica was 10-fold that in the roots of N. caerulescens. Moreover, the copy number of IREG2 per genome was higher in N. japonica than in N. caerulescens, suggesting that IREG2 expression is elevated by gene multiplication in N. japonica. The heterologous expression of IREG2 of N. japonica and N. caerulescens in yeast and A. thaliana confirmed that both IREG2 genes encode functional vacuole Ni transporters. Taking these results together, we hypothesize that the elevation of IREG2 expression by gene multiplication causes the lower root-to-shoot Ni translocation in N. japonica.

Towards an Understanding of the Molecular Basis of Nickel Hyperaccumulation in Plants

Metal hyperaccumulation is a rare and fascinating phenomenon, whereby plants actively accumulate high concentrations of metal ions in their above-ground tissues. Enhanced uptake and root-to-shoot translocation of specific metal ions coupled with an increased capacity for detoxification and sequestration of these ions are thought to constitute the physiological basis of the hyperaccumulation phenotype. Nickel hyperaccumulators were the first to be discovered and are the most numerous, accounting for some seventy-five percent of all known hyperaccumulators. However, our understanding of the molecular basis of the physiological processes underpinning Ni hyperaccumulation has lagged behind that of Zn and Cd hyperaccumulation, in large part due to a lack of genomic resources for Ni hyperaccumulators. The advent of RNA-Seq technology, which allows both transcriptome assembly and profiling of global gene expression without the need for a reference genome, has offered a new route for the analysis of Ni hyperaccumulators, and several such studies have recently been reported. Here we review the current state of our understanding of the molecular basis of Ni hyperaccumulation in plants, with an emphasis on insights gained from recent RNA-Seq experiments, highlight commonalities and differences between Ni hyperaccumulators, and suggest potential future avenues of research in this field.

Comparative RNA-seq analysis of nickel hyperaccumulating and non-accumulating populations of Senecio coronatus (Asteraceae)

Most metal hyperaccumulating plants accumulate nickel, yet the molecular basis of Ni hyperaccumulation is not well understood. We chose Senecio coronatus to investigate this phenomenon as this species displays marked variation in shoot Ni content across ultramafic outcrops in the Barberton Greenstone Belt (South Africa), thus allowing an intraspecific comparative approach to be employed. No correlation between soil and shoot Ni contents was observed, suggesting that this variation has a genetic rather than environmental basis. This was confirmed by our observation that the accumulation phenotype of plants from two hyperaccumulator and two non-accumulator populations was maintained when the plants were grown on a soil mix from these four sites for 12 months. We analysed the genetic variation among 12 serpentine populations of S. coronatus, and used RNA-seq for de novo transcriptome assembly and analysis of gene expression in hyperaccumulator versus non-accumulator populations. Genetic analysis revealed the presence of hyperaccumulators in two well supported evolutionary lineages, indicating that Ni hyperaccumulation may have evolved more than once in this species. RNA-Seq analysis indicated that putative homologues of transporters associated with root iron uptake in plants are expressed at elevated levels in roots and shoots of hyperaccumulating populations of S. coronatus from both evolutionary lineages. We hypothesise that Ni hyperaccumulation in S. coronatus may have evolved through recruitment of these transporters, which play a role in the iron-deficiency response in other plant species.