Plant architecture, auxin homeostasis and phenol content in Arabidopsis thaliana grown in cadmium- and zinc-enriched media

Microbial mediated remediation of heavy metals toxicity: mechanisms and future prospects

Heavy metal pollution has become a serious concern across the globe due to their persistent nature, higher toxicity, and recalcitrance. These toxic metals threaten the stability of the environment and the health of all living beings. Heavy metals also enter the human food chain by eating contaminated foods and cause toxic effects on human health. Thus, remediation of HMs polluted soils is mandatory and it needs to be addressed at higher priority. The use of microbes is considered as a promising approach to combat the adverse impacts of HMs. Microbes aided in the restoration of deteriorated environments to their natural condition, with long-term environmental effects. Microbial remediation prevents the leaching and mobilization of HMs and they also make the extraction of HMs simple. Therefore, in this context recent technological advancement allowed to use of bioremediation as an imperative approach to remediate polluted soils. Microbes use different mechanisms including bio-sorption, bioaccumulation, bioleaching, bio-transformation, bio-volatilization and bio-mineralization to mitigate toxic the effects of HMs. Thus, keeping in the view toxic HMs here in this review explores the role of bacteria, fungi and algae in bioremediation of polluted soils. This review also discusses the various approaches that can be used to improve the efficiency of microbes to remediate HMs polluted soils. It also highlights different research gaps that must be solved in future study programs to improve bioremediation efficency.

Role of calcium signaling in cadmium stress mitigation by indol-3-acetic acid and gibberellin in chickpea seedlings

Given the adverse impacts of heavy metals on plant development and physiological processes, the present research investigated the protective role of indole-3-acetic acid (IAA) and gibberellic acid (GA3) against cadmium (Cd)-induced injury in chickpea seedlings. Therefore, seeds germinated for 6 days in a medium containing 200 μM Cd alone or combined with 10 μM GA3 or 10 μM IAA. Both GA3 and IAA mitigated Cd-imposed growth delays in roots and shoots (80% and 50% increase in root and shoot length, respectively). This beneficial effect was accompanied by a significant reduction in Cd2+ accumulation in both roots (74% for IAA and 38% for GA3) and shoots (68% and 35%, respectively). Furthermore, these phytohormones restored the cellular redox state by reducing the activity of NADPH oxidase and downregulating the transcription level of RbohF and RbohD genes. Likewise, hydrogen peroxide contents were reduced by GA3 and IAA supply. Additionally, GA3 and IAA countered the Cd-induced reduction in total phenols, flavonoids, and reducing sugars in both roots and shoots. The exogenous effectors enhanced the activities of catalase, ascorbate peroxidase, and thioredoxin, as well as the corresponding gene expressions. Interestingly, adding GA3 and IAA to the Cd-contaminated germination media corrected the level of calcium (Ca2+) ion within seedling tissues. This effect coincided with the upregulation of key genes associated with stress sensing and signal transduction, including auxin-binding protein (ABP19a), mitogen-activated protein kinase (MAPK2), calcium-dependent protein kinase (CDPK1), and calmodulin (CaM). Overall, the current results suggest that GA3 and IAA sustain the Ca2+ signaling pathway, resulting in metal phytotoxicity relief. Amendment of agricultural soils contaminated with heavy metals with GA3 or IAA could represent an effective practice to improve crop yield.

Auxin homeostasis in plant responses to heavy metal stress

Expeditious industrialization and anthropogenic activities have resulted in large amounts of heavy metals (HMs) being released into the environment. These HMs affect crop yields and directly threaten global food security. Therefore, significant efforts have been made to control the toxic effects of HMs on crops. When HMs are taken up by plants, various mechanisms are stimulated to alleviate HM stress, including the biosynthesis and transport of auxin in the plant. Interestingly, researchers have noted the significant potential of auxin in mediating resistance to HM stress, primarily by reducing uptake of metals, promoting chelation and sequestration in plant tissues, and mitigating oxidative damage. Both exogenous administration of auxin and manipulation of intrinsic auxin status are effective strategies to protect plants from the negative consequences of HMs stress. Regulation of genes and transcription factors related to auxin homeostasis has been shown to be related to varying degrees to the type and concentration of HMs. Therefore, to derive the maximum benefit from auxin-mediated mechanisms to attenuate HM toxicities, it is essential to gain a comprehensive understanding of signaling pathways involved in regulatory actions. This review primarily emphases on the auxin-mediated mechanisms participating in the injurious effects of HMs in plants. Thus, it will pave the way to understanding the mechanism of auxin homeostasis in regulating HM tolerance in plants and become a tool for developing sustainable strategies for agricultural growth in the future.

Zinc oxide nano-flowers improve the growth and propagation of mulberry cuttings grown under different irrigation regimes by mitigating drought-related complications and enhancing zinc uptake

Silkworm larvae mainly consume mulberry leaves; therefore, mulberry cultivation is important for the production of raw silk. Drought stress and micronutrient deficiency (Zn) are known to affect the propagation of mulberry cuttings. In this purview, the current investigation attempted to inspect the efficacy of different concentrations of zinc oxide nano-flower (ZnNFs) applied through both soil admixture and foliar spray on the propagation of mulberry cuttings grown under deficit irrigation regimes. The overall results demonstrated that the ZnNF-treated plant cuttings were well-adapted to drought stress and performed better in comparison to the control set. Out of the tested concentrations - ZnNF-10 (applied as 10 mg/kg soil and 10 ppm as foliar spray thrice) was found to be optimum, showing relatively better initial root establishment, the emergence of leaves, and survival and sprouting percentage. Further studies also confirmed an improvement in the accumulation of photosynthetic pigments, carbohydrates, and protein content even under extreme drought conditions. Most importantly, the ZnNF-10 treatment contributed to ROS detoxification and cell membrane protection by enhancing the pool of antioxidant enzymes. The study further demonstrated that ZnNF-10 application enhanced zinc content by 147.50%, 179.49%, and 171.99% in root, shoot, and leaves of the treated cuttings; thereby, improving the bioaccumulation factor of the plant parts. All of these interactive phenomena led to an increment in shoot height, biomass, leaf area, and leaf number of cuttings. These findings, therefore, indicated that ZnNFs can be developed as a promising nano-fertilizer for mulberry growth facilitating Zn uptake and mitigation of drought-induced complications.

Nitrate enhances cadmium accumulation through modulating sulfur metabolism in sweet sorghum

Sweet sorghum deploys tremendous potential for phytoremediation of cadmium (Cd)-polluted soils. Nitrate increases Cd accumulation in sweet sorghum, but the mechanism underlying this is still elusive. Sulfur-containing metabolites have been corroborated to play important roles in Cd tolerance in plants. Thus, whether sulfur metabolism contributed to nitrate-increased Cd accumulation in sweet sorghum was investigated in the present study. Two-way ANOVA analysis showed that most sulfur-containing metabolites concentrations and relevant enzymes activities were regulated by nitrate, Cd and interplay of nitrate and Cd. By using grey correlation analysis and Pearson correlation coefficient, Cd accumulation in shoots as affected by nitrate was also mainly ascribed to sulfur metabolism. ATP sulfurylase (ATPS) activities and non-protein thiol (NPT) concentrations in leaves were the two prominent factors that positively correlated with Cd accumulation in shoots. Excess nitrate elevated ATPS activities in leaves which contributed to increased NPT and phytochelatins (PCs) concentrations in leaves. Nitrate enhanced Cd accumulation in shoots of sweet sorghum under a low level of Cd treatment. Intriguingly, Cd accumulation in shoots of sweet sorghum was similar between a low level and a high level of Cd treatment. Principal Components Analysis (PCA) based on 34 parameters failed to separate the low Cd treatment from the high Cd treatment either, suggesting sweet sorghum is exclusively suitable for phytoremediation of slight Cd-polluted arable lands. Taken together, enhanced Cd accumulation in shoots of sweet sorghum by excess nitrate application is closely correlated with sulfur metabolism containing elevated ATPS activities, NPT and PCs concentrations in leaves.

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

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

Synthesis of a Lignin/Zinc Oxide Hybrid Nanoparticles System and Its Application by Nano-Priming in Maize

Nanotechnologies are attracting attention in various scientific fields for their technological and application potential, including their use as bio-activators and nanocarriers in agriculture. This work aimed to synthesize a hybrid material (ZnO@LNP) consisting of lignin nanoparticles containing zinc oxide (4 wt %). The synthesized ZnO hybrid material showed catalytic effect toward thermal degradation, as evidenced by the TGA investigation, while both spectroscopic and contact angle measurements confirmed a modification of surface hydrophilicity for the lignin nanoparticles due to the presence of hydrophobic zinc oxide. In addition, the antioxidant activity of the ZnO@LNP and the zinc release of this material were evaluated. At the application level, this study proposes for the first time the use of such a hybrid system to prime maize seeds by exploiting the release characteristics of this material. Concerning the dosage applied, ZnO@LNP promoted inductive effects on the early stages of seed development and plant growth and biomass development of young seedlings. In particular, the ZnO@LNP stimulated, in the primed seeds, a higher content of chlorophyll, carotenoids, anthocyanins, total phenols, and a better antioxidant activity, as supported by the lower levels of lipid peroxidation found when compared to the control samples.

Subtoxic levels of some heavy metals cause differential root-shoot structure, morphology and auxins levels in Arabidopsis thaliana

Contamination of soil by heavy metals severely affects plant growth and causes soil pollution. While effects on plant growth have been investigated for metals taken individually or in groups, less is known about their comparative effects. In this study Arabidopsis thaliana seedlings were grown for 14 days in Petri dishes containing medium contaminated by six common heavy metals (Hg, Cd, Pb, Cu, Ni and Zn), at the minimum concentrations defined as toxic by the most recent EU legislation on contamination of agricultural soils. (a) Root structure and morphology, (b) metal composition and translocation, and (c) the levels of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) were analyzed. Metals accumulated more in roots than in shoots, with concentrations that differed by several orders of magnitude depending on the metal: Cd (ca. 700 × and ca. 450 × in roots and shoots, respectively), Hg (150 × , 80 × ), Ni (50 × , 20 × ), Cu (48 × , 20 × ), Zn (23 × , 6 × ), and Pb (9 × , 4 × ). Responses were significant for at least nine of the ten root parameters (with the exception of Hg), and five of the six shoot parameters (with the exception of Zn). Cu and Zn induced respectively the strongest responses in root hormonal (up to ca. 240% the control values for IBA, 190% for IAA) and structural parameters (up to 210% for main root length, 330% for total lateral root length, 220% for number of root tips, 600% for total root surface, and from 2.5° to 26.0° of root growth angle). Regarding the shoots, the largest changes occurred for shoot height (down to 60% for Ni), rosette diameter (down to 45% for Hg), leaf number (up to 230% for Zn) and IBA (up to 240% for Pb and Cu). A microscope analysis revealed that shape and conformation of root hairs were strongly inhibited after Cd exposure, and enhanced under Hg and Pb. The results could have positive applications such as for defining toxicity thresholds (in phytoremediation) and acceptable concentration levels (for policies) for some of the most common heavy metals in agricultural soils.

A Comprehensive Review on the Heavy Metal Toxicity and Sequestration in Plants

Heavy metal (HM) toxicity has become a global concern in recent years and is imposing a severe threat to the environment and human health. In the case of plants, a higher concentration of HMs, above a threshold, adversely affects cellular metabolism because of the generation of reactive oxygen species (ROS) which target the key biological molecules. Moreover, some of the HMs such as mercury and arsenic, among others, can directly alter the protein/enzyme activities by targeting their –SH group to further impede the cellular metabolism. Particularly, inhibition of photosynthesis has been reported under HM toxicity because HMs trigger the degradation of chlorophyll molecules by enhancing the chlorophyllase activity and by replacing the central Mg ion in the porphyrin ring which affects overall plant growth and yield. Consequently, plants utilize various strategies to mitigate the negative impact of HM toxicity by limiting the uptake of these HMs and their sequestration into the vacuoles with the help of various molecules including proteins such as phytochelatins, metallothionein, compatible solutes, and secondary metabolites. In this comprehensive review, we provided insights towards a wider aspect of HM toxicity, ranging from their negative impact on plant growth to the mechanisms employed by the plants to alleviate the HM toxicity and presented the molecular mechanism of HMs toxicity and sequestration in plants.

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

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

Potential local adaptation in populations of invasive reed canary grass (Phalaris arundinacea) across an urbanization gradient

Urban stressors represent strong selective gradients that can elicit evolutionary change, especially in non-native species that may harbor substantial within-population variability. To test whether urban stressors drive phenotypic differentiation and influence local adaptation, we compared stress responses of populations of a ubiquitous invader, reed canary grass (Phalaris arundinacea). Specifically, we quantified responses to salt, copper, and zinc additions by reed canary grass collected from four populations spanning an urbanization gradient (natural, rural, moderate urban, and intense urban). We measured ten phenotypic traits and trait plasticities, because reed canary grass is known to be highly plastic and because plasticity may enhance invasion success. We tested the following hypotheses: (a) Source populations vary systematically in their stress response, with the intense urban population least sensitive and the natural population most sensitive, and (b) plastic responses are adaptive under stressful conditions. We found clear trait variation among populations, with the greatest divergence in traits and trait plasticities between the natural and intense urban populations. The intense urban population showed stress tolerator characteristics for resource acquisition traits including leaf dry matter content and specific root length. Trait plasticity varied among populations for over half the traits measured, highlighting that plasticity differences were as common as trait differences. Plasticity in root mass ratio and specific root length were adaptive in some contexts, suggesting that natural selection by anthropogenic stressors may have contributed to root trait differences. Reed canary grass populations in highly urbanized wetlands may therefore be evolving enhanced tolerance to urban stressors, suggesting a mechanism by which invasive species may proliferate across urban wetland systems generally.

Zinc toxicity in plants: a review

This review highlights the most recent updated information available about Zn phytotoxicity at physiological, biochemical and molecular levels, uptake mechanisms as well as excess Zn homeostasis in plants. Zinc (Zn) is a natural component of soil in terrestrial environments and is a vital element for plant growth, as it performs imperative functions in numerous metabolic pathways. However, potentially noxious levels of Zn in soils can result in various alterations in plants like reduced growth, photosynthetic and respiratory rate, imbalanced mineral nutrition and enhanced generation of reactive oxygen species. Zn enters into soils through various sources, such as weathering of rocks, forest fires, volcanoes, mining and smelting activities, manure, sewage sludge and phosphatic fertilizers. The rising alarm in environmental facet, as well as, the narrow gap between Zn essentiality and toxicity in plants has drawn the attention of the scientific community to its effects on plants and crucial role in agricultural sustainability. Hence, this review focuses on the most recent updates about various physiological and biochemical functions perturbed by high levels of Zn, its mechanisms of uptake and transport as well as molecular aspects of surplus Zn homeostasis in plants. Moreover, this review attempts to understand the mechanisms of Zn toxicity in plants and to present novel perspectives intended to drive future investigations on the topic. The findings will further throw light on various mechanisms adopted by plants to cope with Zn stress which will be of great significance to breeders for enhancing tolerance to Zn contamination.

Jasmonates, Ethylene and Brassinosteroids Control Adventitious and Lateral Rooting as Stress Avoidance Responses to Heavy Metals and Metalloids

Developmental and environmental signaling networks often converge during plant growth in response to changing conditions. Stress-induced hormones, such as jasmonates (JAs), can influence growth by crosstalk with other signals like brassinosteroids (BRs) and ethylene (ET). Nevertheless, it is unclear how avoidance of an abiotic stress triggers local changes in development as a response. It is known that stress hormones like JAs/ET and BRs can regulate the division rate of cells from the first asymmetric cell divisions (ACDs) in meristems, suggesting that stem cell activation may take part in developmental changes as a stress-avoidance-induced response. The root system is a prime responder to stress conditions in soil. Together with the primary root and lateral roots (LRs), adventitious roots (ARs) are necessary for survival in numerous plant species. AR and LR formation is affected by soil pollution, causing substantial root architecture changes by either depressing or enhancing rooting as a stress avoidance/survival response. Here, a detailed overview of the crosstalk between JAs, ET, BRs, and the stress mediator nitric oxide (NO) in auxin-induced AR and LR formation, with/without cadmium and arsenic, is presented. Interactions essential in achieving a balance between growth and adaptation to Cd and As soil pollution to ensure survival are reviewed here in the model species Arabidopsis and rice.

Auxins and Cytokinins Regulate Phytohormone Homeostasis and Thiol-Mediated Detoxification in the Green Alga Acutodesmus obliquus Exposed to Lead Stress

Phytohormones, such as auxins and cytokinins, take part in the integration of growth control and stress response, but their role in algal adaptation to heavy metal remains to be elucidated. The current research indicated that lead (Pb), one of the most toxic metals in nature, causes severe depletion of endogenous cytokinins, auxins, and gibberellin and an increase in abscisic acid content in the green alga Acutodesmus obliquus. Exogenous auxins and cytokinins alleviate Pb toxicity through the regulation of the endogenous phytohormones’ levels. Exogenously applied auxins provoked the coordinated activation metal tolerance mechanisms leading to the increase in phytochelatin synthase activity and accumulation of phytochelatins and their precursors, which are essential for Pb sequestration. On the other hand, phytochelatin synthesis decreased in algal cells treated with cytokinins. Significant changes in the levels of low molecular weight metabolites, mainly involved in metal chelation and glutathione synthesis pathway under the influence of phytohormones in algal cells growing in the presence of Pb stress, were observed. This is the first report showing that auxins and cytokinins are important regulatory factors in algal adaptation strategies to heavy metal stress based on thiol-mediated compounds and the maintenance of phytohormone homeostasis.

Lead-induced oxidative stress triggers root cell wall remodeling and increases lead absorption through esterification of cell wall polysaccharide

Tall fescue (Festuca arundinacea Schreb) shows remarkable tolerance to lead (Pb), but the mechanisms involved in metal tolerance are not yet well understood. Here, tall fescue were firstly cultivated hydroponically with Pb²⁺ (0, 50, 200 and 1000 mg/L) for 14 days. The results showed that remodeling of root architecture plays important roles in tolerance of tall fescue to Pb²⁺ stress. Increased cell wall (CW) components contribute to restrict high amount of Pb²⁺ in roots. Additionally, the uronic acid contents of pectin, hemicellulose 1 (HC1) and hemicellulose 2 (HC2) increased under Pb²⁺ stress. We further observed that tall fescue cultivated with H₂O₂ showed similar remodeling of root architecture as Pb²⁺ treatment. Furthermore, pectin, HC1 and HC2 fractions were sequentially extracted from 0 and 10 mM H₂O₂ treated roots, and Pb²⁺ adsorption capacity and contents of carboxyl groups of pectin and HC2 fractions were steadily increased under H₂O₂ treatment in vitro. Our results suggest that degrees of esterification of pectin and HC2 are regulated by H₂O₂. High amount of low-esterified pectin and HC2 offer more carboxyl groups, provide more Pb²⁺ binding sites, and restrict more Pb²⁺ in the CW, which may enhance tolerance of tall fescue to Pb²⁺ stress.

Modification of oxidative stress through changes in some indicators related to phenolic metabolism in Malva parviflora exposed to cadmium

This study was conducted to investigate the role of phenolic compounds in the antioxidant defense system in Malva parviflora L. plants treated with cadmium (Cd). After surface sterilization, the seeds were sown on seedling trays. Forty-day-old plants were then transferred to hydroponic cultures with Cd (40 μM) or without Cd (control). Some biochemical and physiological parameters were assayed on the sixth day after Cd treatment. Based on the results, the plants showed an increase in leaf soluble carbohydrates, total phenolic compounds, total flavonoids, and phenylalanine ammonia-lyase (PAL) activity at the end of the exposure period. However, length, fresh weight, chlorophyll (Chl) b, total Chl, stomatal conductance and starch content decreased under Cd treatment. There was no significant difference between the plants exposed to Cd and the control group for Chl a, SPAD index, carotenoids, and anthocyanins as well as the H₂O₂ content six days after treatment. The Cd content in the roots was considerably higher than that in the shoots. In assessing the antioxidant capacity of plant extracts, different results were observed using 1, 1-diphenyl-2-picryl-hydrazyl (DPPH) test and beta-carotene/linoleic acid bleaching assay. According to the results of this study, it seems that increased activity of PAL enzyme leads to an increase in biosynthesis of phenolic compounds in M. parviflora. This mechanism probably increases the antioxidant capacity of the plant to suppress Cd-induced toxicity and oxidative stress.

Spatial distribution and molecular speciation of copper in indigenous plants from contaminated mine sites: Implication for phytostabilization

Contaminated mining sites require ecological restoration work, of which phytoremediation using appropriate plant species is an attractive option. Our present study is focused on one typical contaminated mine site with indigenous plant cover. The X-ray absorption near edge structure (XANES) analysis indicated that Cu (the major contaminant) was primarily associated with goethite (adsorbed fraction), with a small amount of Cu oxalate-like species (organic fraction) in mine affected soil. With growth of plant species like Miscanthus floridulus and Stenoloma chusanum, the Cu-oxalate like organic species in rhizosphere soil significantly increased, with corresponding decrease in Cu-goethite. In the root cross-section of Miscanthus floridulus, synchrotron-based micro-X-ray fluorescence (μ-XRF) microscopy and micro-XANES results indicated that most Cu was sequestered around the root surface/epidermis, primarily forming Cu alginate-like species as a Cu-tolerance mechanism. From the root epidermis to the cortex and vascular bundle, more Cu(I)-glutathione was observed, suggesting reductive detoxification ability of Cu(II) to Cu(I) during the transport of Cu in the root. The observation of Cu-histidine in root internal cell layers showed another Cu detoxification pathway based on coordinating amino ligands. Miscanthus floridulus showed ability to accumulate phosphorous and nitrogen nutrients in rhizosphere and may be an option for in situ phytostabilization of metals in contaminated mining area.

MhNRAMP1 From Malus hupehensis Exacerbates Cell Death by Accelerating Cd Uptake in Tobacco and Apple Calli

Excessive cadmium (Cd) damages plants by causing cell death. The present study discusses the function of natural resistance-associated macrophage protein (NRAMP) on cell death caused by Cd in Malus hupehensis. MhNRAMP1 was isolated from M. hupehensis roots, and its protein was located in the cell membrane as a transmembrane protein characterized by hydrophobicity. MhNRAMP1 expression in the roots was induced by Cd stress and calcium (Ca) deficiency. MhNRAMP1 overexpression increased Cd concentration in yeasts and enhanced their sensitivity to Cd. Phenotypic comparisons of plants under Cd stress revealed that the growth of transgenic tobacco and apple calli overexpressing MhNRAMP1 was worse than that of the wild type (WT). The Cd²⁺ influx of transgenic tobacco roots and apple calli was higher, and the recovery time of the Cd²⁺ influx to a stable state in transgenic apple calli was longer than that of the WT. Cd accumulation and the percentage of apoptotic cells in transgenic lines were higher. Correspondingly, the caspase-1-like and vacuolar processing enzyme (VPE) activities and MdVPEγ expression were higher in transgenic apple calli, but the expression levels of genes that inhibit cell death were lower than those in the WT under Cd stress. Moreover, the Cd translocation from the roots to leaves was increased after MhNRAMP1 overexpression, but the Cd translocation from the leaves to seeds was not affected. These results suggest that MhNRMAP1 exacerbated Cd-induced cell death, which was accomplished by mediating Cd²⁺ uptake and accumulation, as well as stimulating VPE.

Response of transgenic tobacco overexpressing the CchGLP gene to cadmium and aluminium: phenotypic and microRNAs expression changes

Transgenic tobacco (N. tabacum cv. Xanthi nc) expressing Capsicum chinense CchGLP gene that encodes an Mn-SOD, constitutively produces hydrogen peroxide that increase endogenous ROS levels. Previous studies using these plants against geminivirus infections as well as drought stress confirmed that CchGLP expression conferred resistance against biotic and abiotic stresses. Cadmium (Cd) and Aluminium (Al) contamination in soils are a major ecological concern since they are two of the most widespread toxic elements in terrestrial environments. Trying to explore additional possible tolerance to another stresses in these plants, the aim of this work was to analyse the response to cadmium and aluminium salts during germination and early stages of plantlet development and a differential transcriptome of microRNAs (miRNAs) expression in expressing CchGLP transgenic lines and an azygote non-CchGLP expressing line. Plants were grown in vitro with addition of CdCl₂ and AlCl₃ at three different concentrations: 100, 300 and 500 μM and 50, 150 and 300 μM, respectively. The results showed higher tolerance to Cd and Al salts evaluated in two CchGLP-expressing transgenic lines L8 and L26 in comparison with the azygous non-CchGLP expressing line L1. Interestingly, L8 under Al stress presented vigorous roots and development of radicular hairs in comparison with azygous control (L1). Differentially expressed miRNAs in the comparison between L8 and L1 were associated with up and down-regulation of target genes related with structural molecule activity and ribosome constituents, as well as down-regulation in proton-transporting V-type ATPase (Vacuolar ATPase or V-ATPase). Moreover, KEGG analysis of the target genes for the differentially expressed miRNAs, led to identification of genes related with metabolic pathways and biosynthesis of secondary metabolites. One possible explanation of the tolerance to Cd and Al displayed in the transgenic tobaccos evaluated, might involve the fact that several down-regulated miRNAs, were found associated with target genes expressing V-ATPase. Specifically, miR7904-5p was down regulated and related with the up-regulation of one V-ATPase. The expression levels of these genes was confirmed by qRT-PCR assays, thus suggesting that a cation transport activity driven by the V-ATPases-dependent proton motive force, might significantly contribute as one mechanism for Cd and Al detoxification by vacuolar compartmentation in these transgenic tobacco plants.

Reactive oxygen species, auxin and nitric oxide in metal-stressed roots: toxicity or defence

The presented review is a summary on the current knowledge about metal induced stress response in plants, focusing on the roles of reactive oxygen species, auxin and nitric oxide in roots. The article focuses mainly on the difference between defence and toxicity symptoms of roots during metal-induced stress. Nowadays, pollution of soils by heavy metals is a rapidly growing issue, which affects agriculture and human health. In order to deal with these problems, we must first understand the basic mechanisms and responses to environmental conditions in plants growing under such conditions. Studies so far show somewhat conflicting data, interpreting the same stress responses as both symptoms of defence and toxicity. Therefore, the aim of this review is to give a report about current knowledge of heavy metal-induced stress research, and also to differentiate between toxicity and defence, and outline the challenges of research, focusing on reactive oxygen and nitrogen species, auxin, and the interplay among them. There are still remaining questions on how reactive oxygen and nitrogen species, as well as auxin, can activate either symptoms of toxicity or defence, and adaptation responses.

Chitosan-elicited defense responses in Cucumber mosaic virus (CMV)-infected tomato plants

The control of plant diseases by inducing plant resistance responses represents an interesting solution to avoid yield losses and protect the natural environment. Hence, the intertwined relationships between host, pathogen and inducer are increasingly subject of investigations. Here, we report the efficacy of chitosan-elicited defense responses in Solanum lycopersicum var. cerasiforme plants against Cucumber mosaic virus (CMV). Chitosan was applied via foliar spray before the CMV inoculation to verify its effectiveness as a preventive treatment against the viral infection. Virus accumulation, photosynthetic performance, as well as genes encoding for proteins affecting resistance responses and biosynthetic pathways, were investigated. It was observed a significant reduction of CMV accumulation in chitosan-treated plants that were successively infected with CMV, compared to only CMV-infected ones (up to 100%). Similarly, a positive effect of chitosan on gas exchange dynamics was revealed. The analysis of gene expression (CEVI-1, NPR1, PSY2 and PAL5) suggested the occurrence of chitosan-induced, systemic acquired resistance-related responses associated with a readjustment of the plant's oxidative status. In addition, the absence of deleterious symptoms in chitosan-treated successively CMV-infected plants, confirmed that chitosan can be used as a powerful control agent. Our data indicate that chitosan, when preventively applied, is able to elicit defense responses in tomato to control CMV infection. Such finding may be recommended to protect the tomato fruit yields as well as other crops.

MhMAPK4 from Malus hupehensis Rehd. decreases cell death in tobacco roots by controlling Cd2+ uptake

Cadmium (Cd) induces cell death in plant roots. Mitogen-activated protein kinase (MAPK) plays a role in the regulation of cell death induced by Cd in plant roots. In this study, MhMAPK4 was isolated from the roots of Malus hupehensis. Subcellular localization showed that the MhMAPK4 protein was located in the cell membrane and cytoplasm and is a transmembrane protein that is characterized by hydrophily. The expression of MhMAPK4 in the roots of M. hupehensis was up-regulated by Cd sulfate and Cd chloride. Phenotypic comparison under Cd stress showed that the growth of wild-type (WT) tobacco was lower than the transgenic lines overexpressing MhMAPK4. The fresh weight and the root length of WT also was lower than that of the transgenic tobacco. The net Cd2+ influx in the tobacco roots was decreased by the overexpression of MhMAPK4, as was root Cd accumulation. The recovery time of the Cd2+ influx to stable state in the transgenic tobacco was also shorter than the WT. The expression of iron-regulated transporter 1 (NtIRT1) and natural resistance associated macrophage protein 5 (NtNRAMP5) was relatively low in the transgenic lines under Cd stress. Cell death and apoptosis in the tobacco roots was reduced following the overexpression of MhMAPK4. The activity of vacuolar processing enzyme (VPE) and the transcript level of VPE in the transgenic tobacco was lower than that of WT under Cd stress. In addition, the electrolyte leakage and malondialdehyde and hydrogen peroxide contents in the transgenic tobacco were lower than those of WT, whereas the antioxidant enzyme activity and expression were higher. These results suggest that MhMAPK4 regulates Cd accumulation by mediating Cd2+ uptake by the roots, and controls Cd-caused cell death by adjusting VPE activity.

Antioxidant responses of edible and model plant species subjected to subtoxic zinc concentrations

Zinc (Zn) is a common heavy metal in polluted soils, as it is a widespread pollutant deriving both from natural sources and anthropogenic activities. The antioxidant tolerance/defence mechanisms against oxidative stress induced by subtoxic concentrations of Zn (50 and 150 μM ZnSO4) were studied in a widespread edible plant (lettuce; Lactuca sativa L.) and in an important model plant (Arabidopsis thaliana (L.) Heynh.). After 10 days (Arabidopsis) and 20 days (lettuce) of Zn exposure, Zn uptake/translocation was evaluated in both roots and shoots, while indicators of oxidative stress and stress intensity, total antioxidant capacity, and enzymatic and non-enzymatic antioxidative defence were measured in leaves. From an overall comparison of the two species, Zn root uptake in Arabidopsis subjected to 50 and 150 μM ZnSO4 was approximately 3- and 5-fold lower than in lettuce, while Zn translocation from roots to apical leaves was more efficient in Arabidopsis (23.7 vs 21.3% at 50 μM ZnSO4 and 19.3 vs 12.9% at 150 μM ZnSO4). Generally, a higher degree of Zn-induced oxidative stress (863.8 vs 21.3 μg g-1 FW H2O2 and 1.33 vs 0.75 μM g-1 FW MDAeq at 150 μM ZnSO4) and antioxidant response (441.2 vs 258.5 mM g-1 FW TEAC and 91.0 vs 54.9% RSA at 150 μM ZnSO4) were found in lettuce. The aim of this study is understanding (a) if subtoxic Zn levels can affect Zn uptake and translocation in the studied species and (b) if this eventual Zn absorption can influence plant oxidative status/antioxidant response. Considering that soil contamination by Zn can affect crop production and quality, the results of this research could be important for environmental, nutritional and human health issues.

NaCl impact on Kosteletzkya pentacarpos seedlings simultaneously exposed to cadmium and zinc toxicities

Data regarding NaCl impact on halophyte plant species exposed to a polymetallic contamination remain scarce. Seedlings of the salt marsh species Kosteletzkya pentacarpos were simultaneously exposed to cadmium (10 μM) and zinc (100 μM) in the absence or presence of 50 mM NaCl. Heavy metal exposure reduced plant growth and increased Cd and Zn concentrations in all organs. Cd and Zn accumulation reduced net photosynthesis in relation to stomatal closure, decreased in chlorophyll concentration and alteration in chlorophyll fluorescence-related parameters. Salinity reduced Cd and Zn bioaccumulation and translocation, with a higher impact on Cd than Zn. It mitigated the deleterious impact of heavy metals on photosynthetic parameters. NaCl reduced the heavy metal-induced oxidative stress assessed by malondialdehyde, carbonyl, and H₂O₂ concentration. Subcellular distribution revealed that Cd mainly accumulated in the cell walls, but NaCl increased it in the cytosol fraction in the leaf and in the metal-rich granule fraction in the roots. It had no impact on Zn subcellular distribution. The additional NaCl contributed to a higher sequestration of Cd on phytochelatins and stimulated glutathione synthesis. The positive impact of NaCl on K. pentacarpos response to polymetallic pollution made this species a promising candidate for revegetation of heavy metal-contaminated salt areas.