An activatable fluorescence probe for rapid detection and in situ imaging of β-galactosidase activity in cabbage roots under heavy metal stress

β-Galactosidase (β-gal), an enzyme related to cell wall degradation, plays an important role in regulating cell wall metabolism and reconstruction. However, activatable fluorescence probes for the detection and imaging of β-gal fluctuations in plants have been less exploited. Herein, we report an activatable fluorescent probe based on intramolecular charge transfer (ICT), benzothiazole coumarin-bearing β-galactoside (BC-βgal), to achieve distinct in situ imaging of β-gal in plant cells. It exhibits high sensitivity and selectivity to β-gal with a fast response (8 min). BC-βgal can be used to efficiently detect the alternations of intracellular β-gal levels in cabbage root cells with considerable imaging integrity and imaging contrast. Significantly, BC-βgal can assess β-gal activity in cabbage roots under heavy metal stress (Cd2+, Cu2+, and Pb2+), revealing that β-gal activity is negatively correlated with the severity of heavy metal stress. Our work thus facilitates the study of β-gal biological mechanisms.

Plant growth promoting rhizobacteria induced metal and salt stress tolerance in Brassica juncea through ion homeostasis

Soil heavy metal contamination and salinity constitute a major environmental problem worldwide. The affected area and impact of these problems are increasing day by day; therefore, it is imperative to restore their potential using environmentally friendly technology. Plant growth-promoting rhizobacteria (PGPR) provides a better option in this context. Thirty-seven bacteria were isolated from the rhizosphere of maize cultivated in metal- and salt-affected soils. Some selected bacterial strains grew well under a wide range of pH (4-10), salt (5-50 g/L), and Cd (50-1000 mg/L) stress. Three bacterial strains, Exiguobacterium aestuarii (UM1), Bacillus cereus (UM8), and Bacillus megaterium (UM35), were selected because of their robust growth and high tolerance to both stress conditions. The bacterial strains UM1, UM8, and UM35 showed P-solubilization, whereas UM8 and UM35 exhibited 1-aminocyclopropane-1-carboxylate deaminase activity and indole acetic acid (IAA) production, respectively. The bacterial strains were inoculated on Brassica juncea plants cultivated in Cd and salt-affected soils due to the above PGP activities and stress tolerance. Plants inoculated with the bacterial strains B. cereus and B. megaterium significantly (p < 0.05) increased shoot fresh weight (17 ± 1.17-29 ± 0.88 g/plant), shoot dry weight (2.50 ± 0.03-4.40 ± 0.32 g/plant), root fresh weight (7.30 ± 0.58-13.30 ± 0.58 g/plant), root dry weight (0.80 ± 0.04-2.00 ± 0.01 g/plant), and shoot K contents (62.76 ± 1.80-105.40 ± 1.15 mg/kg dwt) in normal and stressful conditions. The bacterial strain B. megaterium significantly (p < 0.05) decreased shoot Na+ and Cd++ uptake in single and dual stress conditions. Both bacterial strains, E. aestuarii and B. cereus, efficiently reduced Cd++ translocation and bioaccumulation in the shoot. Bacterial inoculation improved the uptake of K+ and Ca++, while restricted Na+ and Cd++ in B. juncea shoots indicated their potential to mitigate the dual stresses of salt and Cd in B. juncea through ion homeostasis.

Citric acid and hydrogen sulfide cooperate to mitigate chromium stress in tomato plants by modulating the ascorbate-glutathione cycle, chromium sequestration, and subcellular allocation of chromium

The study aimed to investigate the role of hydrogen sulfide (H2S) in regulating chromium stress (Cr-S) tolerance of tomato plants treated with citric acid (CA). Prior to the Cr treatment, tomato plants were foliar-fed with CA (100 μM) daily for 3 days. Subsequently, the plants were grown for another ten days in a hydroponic system in a 50 μM Cr (VI) solution. Chromium treatment reduced photosynthetic pigments and plant biomass, but boosted the levels of hydrogen peroxide (H2O2) malondialdehyde (MDA), H2S, phytochelatins (PCs), and glutathione (GSH), electrolyte leakage (EL), and antioxidant enzyme activity in tomato plants. However, the foliar spray of CA mitigated the levels of H2O2, MDA, and EL, promoted plant growth and chlorophyll content, enhanced antioxidant enzymes' activities, and increased H2S production in Cr-S-tomato plants. CA also increased the levels of GSH and PCs, potentially reducing the toxicity of Cr through regulated sequestration. Additionally, the application of sodium hydrogen sulfide (NaHS), a donor of H2S, improved CA-induced Cr stress tolerance. The addition of CA promoted Cr accumulation in root cell wall and leaf vacuoles to suppress its toxicity. To assess the involvement of H2S in CA-mediated Cr-S tolerance, 0.1 mM hypotaurine (HT), an H2S scavenger, was provided to the control and Cr-S-plants along with CA and CA + NaHS. HT reduced the beneficial effects of CA by decreasing H2S production in tomato plants. However, the NaHS addition with CA + HT inverted the adverse impacts of HT, indicating that H2S is required for CA-induced Cr-S tolerance in tomato plants.

Nitric oxide mitigates vanadium toxicity in soybean (Glycine max L.) by modulating reactive oxygen species (ROS) and antioxidant system

Vanadium (V) induced hazardous effects posturing a serious concern on crop production as well as food security. However, the nitric oxide (NO)-mediated alleviation of V-induced oxidative stress in soybean seedlings is still unknown. Therefore, this research was designed to explore the effects of exogenous NO to mitigate the V-induced phytotoxicity in soybean plants. Our upshots disclosed that NO supplementation considerably improved the plant biomass, growth, and photosynthetic attributes by regulating the carbohydrates, and plants biochemical composition, which further improved the guard cells, and stomatal aperture of soybean leaves. Additionally, NO regulated the plant hormones, and phenolic profile which restricted the V contents absorption (65.6%), and translocation (57.9%) by maintaining the nutrient acquisition. Furthermore, it detoxified the excessive V contents, and upsurged the antioxidants defense mechanism to lower the MDA, and scavenge ROS production. The molecular analysis further verified the NO-based regulation of lipid, sugar production, and degradation as well as detoxification mechanism in the soybean seedlings. Exclusively, we elaborated very first time the behind mechanism of V-induced oxidative stress alleviation by exogenous NO, hence illustrating the NO supplementation role as a stress alleviating agent for soybean grown in V contaminated areas to elevate the crop development and production.

Phosphorus and Serendipita indica synergism augments arsenic stress tolerance in rice by regulating secondary metabolism related enzymatic activity and root metabolic patterns

The multifarious problems created by arsenic (As), for collective environment and human health, serve a cogent case for searching integrative agricultural approaches to attain food security. Rice (Oryza sativa L.) acts as a sponge for heavy metal(loid)s accretion, specifically As, due to anaerobic flooded growth conditions facilitating its uptake. Acclaimed for their positive impact on plant growth, development and phosphorus (P) nutrition, 'mycorrhizas' are able to promote stress tolerance. Albeit, the metabolic alterations underlying Serendipita indica (S. indica; S.i) symbiosis-mediated amelioration of As stress along with nutritional management of P are still understudied. By using biochemical, RT-qPCR and LC-MS/MS based untargeted metabolomics approach, rice roots of ZZY-1 and GD-6 colonized by S. indica, which were later treated with As (10 µM) and P (50 µM), were compared with non-colonized roots under the same treatments with a set of control plants. The responses of secondary metabolism related enzymes, especially polyphenol oxidase (PPO) activities in the foliage of ZZY-1 and GD-6 were enhanced 8.5 and 12-fold, respectively, compared to their respective control counterparts. The current study identified 360 cationic and 287 anionic metabolites in rice roots, and the commonly enriched pathway annotated by Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis was biosynthesis of phenylalanine, tyrosine and tryptophan, which validated the results of biochemical and gene expression analyses associated with secondary metabolic enzymes. Particularly under As+S.i+P comparison, both genotypes exhibited an upregulation of key detoxification and defense related metabolites, including fumaric acid, L-malic acid, choline, 3,4-dihydroxybenzoic acid, to name a few. The results of this study provided the novel insights into the promising role of exogenous P and S. indica in alleviating As stress.

Nanovesicle and extracellular polymeric substance synthesis from the remediation of heavy metal ions from soil

Heavy metal toxicity affects aquatic plants and animals, disturbing biodiversity and ecological balance causing bioaccumulation of heavy metals. Industrialization and urbanization are inevitable in modern-day life, and control and detoxification methods need to be accorded to meet the hazardous environment. Microorganisms and plants have been widely used in the bioremediation of heavy metals. Sporosarcina pasteurii, a gram-positive bacterium that is widely known for its calcite precipitation property in bio-cementing applications has been explored in the study for its metal tolerance ability for the first time. S. pasteurii SRMNP1 (KF214757) can tolerate silver stress to form nanoparticles and can remediate multiple heavy metals to promote the growth of various plants. This astounding property of the isolate warranted extensive examinations to comprehend the physiological changes during an external heavy metal stress condition. The present study aimed to understand various physiological responses occurring in S. pasteuriiSRMNP1 during the metal tolerance phenomenon using electron microscopy. The isolate was subjected to heavy metal stress, and a transmission electron microscope examination was used to analyze the physiological changes in bacteria to evade the metal stress. S. pasteurii SRMNP1 was tolerant against a wide range of heavy metal ions and can withstand a broad pH range (5-9). Transmission Electron Microscopy (TEM) examination of S. pasteurii SRMNP1 followed by 5 mM nickel sulfate treatment revealed the presence of nanovesicles encapsulating nanosized particles in intra and extracellular spaces. This suggests that the bacteria evade the metal stress by converting the metal ions into nanosized particles and encapsulating them within nanovesicles to efflux them through the vesicle budding mechanism. Moreover, the TEM images revealed an excessive secretion of extracellular polymeric substances by the strain to discharge the metal particles outside the bacterial system. S. pasteurii can be foreseen as an effective bioremediation agent with the potential to produce nanosized particles, nanovesicles, and extracellular polymeric substances. This study provides physiological evidence that, besides calcium precipitation applications, S. pasteurii can further be explored for its multidimensional roles in the fields of drug delivery and environmental engineering.

Sublethal and lethal Cd toxicity in soybean roots specifically affects the metabolome, Cd binding to proteins and cellular distribution of Cd

Soybean (Glycine max (L.) Merr.) plants were exposed to various Cd concentrations from background and low non-toxic (0.5-50 nM) via sublethally toxic (< 550 nM) to highly, ultimately lethally toxic (3 µM) concentrations. Plants were cultivated hydroponically for 10 weeks until pod development stage of the control plants. The threshold and mechanism of sublethal Cd toxicity was investigated by metabolomics and metalloproteomics (HPLC-ICP-MS) measuring metal binding to proteins in the harvested roots. Spatial distribution of Cd was revealed by µXRF-CT. Specific binding of Cd to proteins already at 50 nM Cd revealed the likely high-affinity protein binding targets in roots, identified by protein purification from natural abundance. This revealed allantoinase, aquaporins, peroxidases and protein disulfide isomerase as the most likely high-affinity targets of Cd binding. Cd was deposited in cortex cell vacuoles at sublethal and bound to the cell walls of the outer cortex and the vascular bundle at lethal Cd. Cd binding to proteins likely inhibits them, and possibly induces detoxification mechanisms, as verified by metabolomics: allantoic acid and allantoate increased due to sublethal Cd toxicity. Changes of the Cd binding pattern indicated a detoxification strategy at lower Cd, but saturated binding sites at higher Cd concentrations.

Single and combined effects of Zn and Al on photosystem II of the green microalgae Raphidocelis subcapitata assessed by pulse-amplitude modulated (PAM) fluorometry

Increasing metal concentrations in aquatic environments are mainly due to anthropogenic actions, which is a matter of concern for the biodiversity of aquatic biota. It is known that metals coexist in environments, however environmental risk assessments do not usually take into account the effects of these mixtures. We aimed to test Zn and Al mixtures on the photosynthetic apparatus of a green microalga, for the first time, using PAM fluorometry. After 72 h exposure, single concentrations from 0.08 to 0.46 µM Zn and 22.24 to 37.06 µM Al affected the photosynthetic parameters of Raphidocelis subcapitata. Metals affected the efficiency of the oxygen-evolving complex - OEC (F₀/F_v), increasing it by 25% at 0.46 µM Zn and by 82% at 37.06 µM Al - concentrations where, 57% and 78% of growth inhibition occurred, respectively. We observed that the algal growth was more sensitive to infer Zn toxicity, while F₀/F_v was more affected by Al. Regarding quenching, there was an increase in passive energy dissipation ((Y(NO)) at 0.46 µM Zn, and we observed an increase in both regulated ((NPQ and Y(NPQ)) and non-regulated energy dissipation ((qN and (Y(NO)) at 37.06 µM Al. Our results showed synergism and antagonism at different concentrations in mixtures, the antagonism prevailing at higher metal concentrations and, in some cases, synergism at lower concentrations of Zn and Al. Since we observe more than additive and less than additive effects, it is of the utmost importance to take mixture toxicity tests into account when performing risk assessments on green algae.

Metal tolerance gene family in barley: an in silico comprehensive analysis

Metal-tolerance proteins (MTPs) are divalent cation transporters that play critical roles in metal tolerance and ion homeostasis in plants. However, a comprehensive study of MTPs is still lacking in crop plants. The current study aimed to comprehensively identify and characterize the MTP gene family in barley (Hordeum vulgare, Hv), an important crop. In total, 12 HvMTPs were identified in the barley genome in this study. They were divided into three phylogenetic groups (Zn-cation diffusion facilitator proteins [CDFs], Fe/Zn-CDFs, and Mn-CDFs) and further subdivided into seven groups (G1, G5, G6, G7, G8, G9, and G12). The majority of MTPs were hydrophobic proteins found in the vacuolar membrane. Gene duplication analysis of HvMTPs revealed one pair of segmental-like duplications in the barley genome. Evolutionary analysis suggested that barley MTPs underwent purifying natural selection. Additionally, the HvMTPs were analyzed in the pan-genome sequences of barley (20 accessions), which suggests that HvMTPs are highly conserved in barley evolution. Cis-acting regulatory elements, microRNA target sites, and protein-protein interaction analysis indicated the role of HvMTPs in a variety of biological processes. Expression profiling suggests that HvMTPs play an active role in maintaining barley nutrient homeostasis throughout its life cycle, and their expression levels were not significantly altered by abiotic stresses like cold, drought, or heat. The expression of barley HvMTP genes in the presence of heavy metals such as Zn²⁺, Cu²⁺, As³⁺, and Cd²⁺ revealed that these MTPs were induced by at least one metal ion, implying their involvement in metal tolerance or transportation. The identification and comprehensive investigation of MTP gene family members will provide important gene resources for the genetic improvement of crops for metal tolerance, bioremediation, or biofortification of staple crops.

Understanding the catalytic abilities of class IV sirtuin OsSRT1 and its linkage to the DNA repair system under stress conditions

The roles of sirtuins in plants are slowly unraveling. Regarding OsSRT1, there are only reports of its H3K9Ac deacetylation. Here we detect the other lysine deacetylation sites in histones, H3 and H4. Further, our studies shed light on its dual enzyme capability with preference for mono ADP ribosylation over deacetylation. OsSRT1 can specifically transfer the single ADP ribose group on its substrates in an enzymatic manner. This mono ADPr effect is not well known in plants, more so for deacetylases. The products of this reaction (NAM and ADP ribose) have a negative effect on this enzyme's action suggesting a tighter regulation. Resveratrol, a natural plant polyphenol proves to be a good activator of this enzyme at 150 ± 40 µM concentration. Under different abiotic stress conditions, we could link this ADP ribosylase activity to the DNA damage repair (DDR) pathway by activating the enzyme PARP1. There is also evidence of OsSRT1's interaction with the components of DDR machinery. Changes in the extent of different histone deacetylation by OsSRT1 is also related with these stress conditions. Metal stress in plants also influences these enzyme activities. Structurally there is a long C-terminal domain in OsSRT1 in comparison to other classes of plant sirtuins, which is required for its catalysis.

Mitigation of lead toxicity in Vigna radiata genotypes by silver nanoparticles

Heavy metal (HM) contamination of the soil through anthropogenic activities influences the living systems and drastically impacts food chain. This study examined the application of silver nanoparticles (AgNPs) in two genotypes (G1 and G2) of Mung bean (Vigna radiata) for ameliorating the Pb toxicity. Different doses of Pb (0, 25, 50 μM) were differentially tackled by AgNPs with the aim of ameliorating the plant attributes. Both genotypes displayed statistically significant quantitative and qualitative modulations for Pb tolerance. In G2, the most prominent increase in plant height (43.79%), fresh biomass (49.56%) and total chlorophyll (20%) was observed at L2 (AgNPs 10 mg/L) in comparison with the control. Overall, photosynthetic rate was increased by 26% in G2 at L6 (AgNPs 25 mg/L + Pb 25 μM). In addition, the results presented 78.5% increase in water use efficiency of G2 while G1 experienced a maximum internal CO₂ concentration (209.8%) at L8 (Pb 50 μM). AgNPs triggered balanced uptake of minerals and improved growth of Vigna genotypes. 50 μM Pb was most hazardous and caused maximum reduction in growth of Vigna plants along with a significant suppression in photosynthetic activity, increase in MDA (199.7%) in G1 and H₂O₂ (292.8%) in G2. In comparison to control, maximum superoxide dismutase (376%), peroxidase (659.8%) and catalase (9.3%) activity was observed in G2 at L11. The application of AgNPs substantially enhanced plant growth and helped them in surviving well in absence as well as presence of Pb. G2 genotype exhibited substantial tolerance capability and revealed less impairment in the studied attributes than G1 and treatment of AgNPs i.e. 25 mg/L was the best level that yielded best results in both genotypes. The results demonstrate that AgNPs mediate response(s) of plants under Pb stress and particularly contributed to HM tolerance of plants and thus showing great promise for use in phytoremediation.

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.

Differential expressions and enzymatic properties of malate dehydrogenases in response to nutrient and metal stresses in Stylosanthes guianensis

Malate dehydrogenase (MDH, EC 1.1.1.37) is a key enzyme that catalyzes a reversible NAD-dependent dehydrogenase reaction from oxaloacetate (OAA) to malate. Although MDH has been documented to participate in cellular metabolism and redox homeostasis in plants, the roles of MDH members in the tropical legume Stylosanthes guianensis (stylo) remain less definitive. In this study, except SgMDH1 that had been previously characterized, six novel MDH genes were isolated from stylo and were then designated as SgMDH2 to SgMDH7. All of the SgMDH proteins possessed the common features of NAD binding, dimerization interface and substrate binding sites. Expression analysis showed that three SgMDHs exhibited preferential expressions in leaves, and one SgMDH was mainly expressed in roots. Furthermore, SgMDHs were regulated by nutrient deficiencies in stylo roots, especially for phosphorus (-P) and potassium (-K) deficiencies. Differential responses of SgMDHs to trace metal stress and heavy metal toxicity were observed in stylo roots, suggesting the involvement of SgMDHs in the response of stylo to metal stresses. The six novel SgMDHs were subsequently expressed and purified from Escherichia coli to analyze their biochemical properties. Although SgMDHs exhibited variations in subcellular localizations, each SgMDH protein displayed a high level of catalytic efficiency towards OAA and NADH but a low level of catalytic efficiency towards malate and NAD⁺. In addition, the activities of recombinant SgMDH proteins were pH-dependent and temperature-sensitive, and exhibited differential regulations by various metal ions. These results together suggest the potential roles of SgMDHs in stylo coping with nutrient and metal stresses.

Elucidating silicon-mediated distinct morpho-physio-biochemical attributes and organic acid exudation patterns of cadmium stressed Ajwain (Trachyspermum ammi L.)

Soil contamination with toxic heavy metals [such as cadmium (Cd)] is becoming a serious global problem due to rapid development of social economy. Silicon (Si), being an important fertilizer element, has been found effective in enhancing plant tolerance against biotic and abiotic stresses. The present study investigated the extent to which different levels of Si modulated the Cd tolerance of Ajwain (Trachyspermum ammi L.) seedlings when maintained in artificially Cd spiked regimes. A pot experiment was conducted under controlled conditions for four weeks, by using sand, mixed with different levels of Cd i.e., 0, 1.5 and 3 mM together with the application of Si at 0, 1.5 and 3 mM levels to monitor different growth, gaseous exchange, oxidative stress, antioxidative responses, minerals accumulation, organic acid exudation patterns of T. ammi seedlings. Our results depicted that Cd addition to growth medium significantly decreased plant growth and biomass, gaseous exchange attributes and minerals uptake by T. ammi seedlings as compared to the plants grown without addition of Cd. However, Cd toxicity boosted the production of reactive oxygen species (ROS) by increasing the contents of malondialdehyde (MDA), which is the indication of oxidative stress in T. ammi seedlings and was also manifested by hydrogen peroxide (H₂O₂) contents and electrolyte leakage to the membrane bounded organelles. Although, activities of various antioxidative enzymes like superoxidase dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) initially increased up to a Cd level of 1.5 mM but were significantly diminished at the highest Cd level of 3 mM. Results revealed that the anthocyanin and soluble proteins contents were decreased in seedlings grown under elevating Cd levels but increased the Cd accumulation of T. ammi roots and shoots. The negative impacts of Cd injury were reduced by the application of Si which increased plant growth and biomass, improved photosynthetic apparatus, antioxidant enzymes, minerals uptake together with diminished exudation of organic acids as well as oxidative stress indicators in roots and shoots of T. ammi by decreasing Cd retention in different plant parts. Research findings, therefore, suggested that Si application can ameliorate Cd toxicity in T. ammi seedlings and resulted in improved plant growth and composition under metal stress as depicted by balanced exudation of organic acids.

Responses of the species complex Fallopia × bohemica to single-metal contaminations to Cd, Cr or Zn: growth traits, metal accumulation and secondary metabolism

Plant responses to heavy metals and their storage constitute a crucial step to understand the environmental impacts of metallic trace elements (MTEs). In controlled experiments, we previously demonstrated the tolerance and resilience of Japanese knotweed to soil artificial polymetallic contamination. Using the same experimental design, we tested here the effect of three individual MTEs on Fallopia × bohemica performance traits. Rhizome fragments from three different sites (considered as distinct morphotypes) were grown in a greenhouse for 1 month on a prairial soil artificially contaminated with either Cd, Cr (VI) or Zn at concentrations corresponding to relatively highly polluted soils. Our results confirmed the high tolerance of Bohemian knotweed to metal stress, though, plant response to MTE pollution was dependant on MTE identity. Bohemian knotweed was stimulated by Cr (VI) (increased root and aerial masses), did not display any measurable change in performance traits under Cd at the high dose of 10 mg kg^-1, and uptook all MTEs in its rhizome, but only Zn was transferred to its aerial parts. We also highlighted changes in root secondary metabolism that were more accentuated with Zn, including the increase of anthraquinone, stilbene and biphenyl derivatives. These results compared to multi-contamination experiments previously published suggest complex interactions between metals and plant, depending principally on metal identity and also suggest a potential role of soil microbes in the interactions.

Use of Zinc Carbonate Spiking to Obtain Phytotoxicity Thresholds Comparable to Those in Field-Collected Soils

Several studies have reported the presence of smithsonite (ZnCO₃ ) in soils polluted by zinc mining. The present study aimed to determine upper critical threshold values of Zn phytotoxicity in a substrate spiked with ZnCO₃ and to compare them with those obtained in field-collected soils. We studied Zn toxicity to perennial ryegrass (Lolium perenne L.) grown in pots with unpolluted peat treated with increasing concentrations of ZnCO₃ that produced nominal total Zn concentrations of 0, 0.7, 1.3, 2.0, 2.6, and 3.3%. To keep constant near-neutral pH value in all the treatments, we used decreasing concentrations of dolomitic lime. In the treatment with total soil Zn of 3.3% (pH 6.8), the foliar Zn concentration of L. perenne was 1914 ± 211 mg kg^-1 , falling into the range of 2400 ± 300 mg kg^-1 reported for Lolium species grown under similar laboratory conditions in a polluted soil (total soil Zn 5.4%, pH 7.3) collected near a Zn smelter. The value of 92 ± 98 mg kg^-1 was obtained for the median effective concentration (EC50) values of 0.01 M KNO₃ -extractable Zn using the responses of shoot dry biomass, shoot length, and total pigments. This value falls within the range of 95 ± 46 mg kg^-1 reported in other studies for the EC50 values of salt-extractable Zn using field-collected soils. The application of ZnCO₃ for spiking was able to mimic foliar Zn concentrations of Lolium species observed in field-collected soils. The effective concentrations of soil Zn obtained in the present study are comparable to those obtained in field-collected soils. Future research should determine effective concentrations of metals using soils spiked with metal-containing compounds that mimic a real source of contamination. Environ Toxicol Chem 2020;39:1790-1796. © 2020 SETAC.

Zinc and aluminum mixtures have synergic effects to the algae Raphidocelis subcapitata at environmental concentrations

A large number of metals is present in aquatic ecosystems, often occurring simultaneously, however, the isolated toxicity of them are better well known than their mixtures. Based on that, for the first time we aimed to test the effects of zinc (Zn) and aluminum (Al) mixtures to the microalgae Raphidocelis subcapitata. Regarding isolated toxicity, the 96 h IC₅₀ of Zn and Al based on specific growth rates occurred, respectively, at 0.40 and 27.40 μM, thus Zn was ≈70-fold more toxic than Al. Both Zn and Al altered the cell size and complexity of R. subcapitata at the highest concentrations, although only during Zn exposure was the chlorophyll a fluorescence significantly diminished. Microalgae exposed to Al produced more ROS than during Zn exposure. Moreover, algae produced less ROS at the highest Zn concentration than in the lower concentrations. According to species sensitivity curves (SSD), R. subcapitata was the most sensitive organism to Zn and one of the most sensitive to Al. With respect to mixture toxicity tests, there were significant deviations for both CA (concentration addition) and IA (independent action) models, although data best fitted the CA model and DL (dose level-dependence) deviation, in which metals showed synergic effects at low concentrations and antagonist effects at higher concentrations.

Morpho-physiological traits, gaseous exchange attributes, and phytoremediation potential of jute (Corchorus capsularis L.) grown in different concentrations of copper-contaminated soil

Jute (Corchorus capsularis L.) is the most commonly used natural fiber as reinforcement in green composites and, due to its huge biomass, deep rooting system, and metal tolerance in stressed environments, it is an excellent candidate for the phytoremediation of different heavy metals. Therefore, the present study was carried out to examine the growth, antioxidant capacity, gaseous exchange attributes, and phytoremediation potential of C. capsularis grown at different concentrations of Cu (0, 100, 200, 300, and 400 mg kg^-1) in a glass house environment. The results illustrate that C. capsularis can tolerate Cu concentrations of up to 300 mg kg^-1 without significant decreases in growth or biomass, but further increases in Cu concentration (i.e., 400 mg kg^-1) lead to significant reductions in plant growth and biomass. The photosynthetic pigments and gaseous exchange attributes in the leaves of C. capsularis decreased as the Cu concentration in the soil increased. Furthermore, high concentrations of Cu in the soil caused lipid peroxidation by increasing the malondialdehyde content in the leaves. This implies that elevated Cu levels cause oxidative damage in C. capsularis. Antioxidants, such as superoxidase dismutase and peroxidase, come into play to scavenge the reactive oxygen species which are generated as a result of oxidative stress. In the present study, the concentrations of Cu in different parts of the plant (the roots, leaves, stem core, and fibers) were also investigated at four different stages of the life cycle of C. capsularis, i.e., 30, 60, 90, and 120 days after sowing (DAS). The results of this investigation reveal that, in the earlier stages of the growth, Cu was highly accumulated in the belowground parts of the plant while little was transported to the aboveground parts. Contrastingly, at a fully mature stage of the growth (120 DAS), it was observed that the majority of Cu was transported to the aboveground parts of the plant and very little accumulated in the belowground parts. The results also show a progressive increase in Cu uptake in response to increasing Cu concentrations in the soil, suggesting that C. capsularis is a potential bio-resource for the phytoremediation of Cu in Cu-contaminated soil.

DE2009 Under Metal Stress

Phototrophic microorganisms are the dominant populations in microbial mats, which play an important role in stabilizing sediments, such as happens in the Ebro Delta. These microorganisms are exposed to low metal concentrations over a long period of time. Distinct methods have been used to evaluate their toxic effect on the preservation of these ecosystems. Nevertheless, most of these techniques are difficult to apply in isolated phototrophs because (i) they usually form consortia with heterotrophic bacteria, (ii) are difficult to obtain in axenic cultures, and (iii) do not grow on solid media.In this study, and for the first time, a combination of fast, non-invasive, and in vivo Confocal Laser Scanning Microscopy (CLSM) techniques were applied in a consortium of Scenedesmus sp. DE2009 to analyze its physiological state and viability under metal stress conditions. Microalga was more resistant to Pb followed by Cr and Cu. However, in multimetal combinations, the presence of Cu negatively affected microalga growth. Additionally, the inhibitory concentration (IC) values were also calculated by CLSM pigment analysis. The result determines a higher degree of toxicity for Cu and Cr in comparison to Pb. The high sensitivity of these CLSM-methods to detect low concentrations allows consideration of Scenedesmus sp. DE2009 as a good bioindicator of metal pollution in natural environments.

Proteomic and ecophysiological responses of soybean (Glycine max L.) root nodules to Pb and hg stress

BACKGROUND

Lead (Pb) and mercury (Hg) are persistent hazardous metals in industrially polluted soils which can be toxic in low quantities. Metal toxicity can cause changes at cellular and molecular level which should be studied for better understanding of tolerance mechanism in plants. Soybean (Glycine max L.) is an important oilseed crop of the world including India. Indian soils growing soybean are often contaminated by Pb and Hg. The aim of this study was to explore how soybean root nodule responds to Pb and Hg through proteomic and ecophysiological alterations in order to enhance tolerance to metal stress.

RESULTS

Soybean plants were exposed to Pb (30 ppm PbCl2) and Hg (0.5 ppm HgCl2) to study histological, histochemical, biochemical and molecular response of N2-fixing symbiotic nodules. Both Pb and Hg treatment increased the level of oxidative stress in leaves and nodules. Chlorosis in leaves and morphological/anatomical changes in nodules were observed. Activities of ascorbate peroxidase, glutathione reductase and catalase were also modulated. Significant changes were observed in abundance of 76 proteins by Pb and Hg. Pb and Hg influenced abundance of 33 proteins (17 up and 16 down) and 43 proteins (33 up and 10 down), respectively. MS/MS ion search identified 55 proteins which were functionally associated with numerous cellular functions. Six crucial proteins namely catalase (CAT), allene oxide synthase (AOS), glutathione S-transferase (GST), calcineurin B like (CBL), calmodulin like (CML) and rapid alkalinisation factor (RAF) were selected for transcript abundance estimation. The qRT-PCR based real time expression exhibited a positive correlation with proteomics expression except for GST and RAF.

CONCLUSION

Soybean root nodule responds to metal stress by increased abundance of defence, development and repair related proteins. An efficient proteomic modulation might lead to metal-induced stress tolerance in N2-fixing nodules. Although concentrations of Pb and Hg used in the study cannot be considered equimolar, yet Hg seems to induce more changes in nodule proteomic profile, and higher damage to both bacteroides and root anatomy.

Priming with ACC-utilizing bacterium attenuated copper toxicity, improved oxidative stress tolerance, and increased phytoextraction capacity in wheat

The major challenges for the plants growing in metal-contaminated soils are deficiency of nutrients, biomass reduction, and severe oxidative damages in the presence of heavy metals. In this regard, our aim was to overcome these challenges through the use of efficient microbial strains in metal-polluted soils and to assess its/their physiological and biochemical effects. In the current study, a copper (Cu)-resistant bacterium was isolated from the rhizospheric soil of 'Ziziphus nummularia' and evaluated for its ability to promote the wheat growth under the gradient stress of copper. Based on 16S rRNA gene sequencing, the isolate was identified as Pantoea sp. Among the plant growth promoting tests, the isolate showed the production of indole acetic acid, solubilization of inorganic phosphate, and ACC deaminase activity. Also, the isolate showed resistance to many heavy metals and antibiotics and increased the water-soluble copper in solution. The results of pot studies showed that bacterial application promoted various growth parameters of wheat plants and also enhanced the Cu uptake of wheat from the Cu-amended soil. The results showed that enhancement of Cu stress (100 to 300 mg kg^-1) resulted in a decrease in various compatible solutes such as proline, total soluble sugars, and total protein content, and increase in the level of malondialdehyde (MDA), latter of which is the indicator of oxidative stress. Bacterial treatment markedly increased the proline, soluble sugar, total protein content, and decreased the MDA content under Cu stress. In addition, bacterial inoculation significantly alleviated the harmful effect of metal toxicity by decreasing the activation of ROS molecules including superoxide (O₂^-) and hydrogen peroxide (H₂O₂). The activation of various antioxidative enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) was noted following bacterial inoculation under Cu stress. Therefore, the present study demonstrates the potential of the isolate Pantoea sp. ZNP-5 to improve the growth and phytoextraction of metal from the metal-polluted soil through the polyphasic mechanism of action.

The effects of rhenium accumulation on Indian mustard

Rhenium (Re) is one of Earth's scarcest and more largely scattered elements, with an estimate concentration of 0.4-0.6 μg kg^-1 in the upper crust. Still, considerable concentrations of bioavailable ReO₄^- ions are often found, particularly in copper-molybdenum mines, where their uptake by plants of these regions has been reported. Yet, the impact of Re on plants remains a question mark, as the only available knowledge derives from a limited investigation carried out over 60 years ago. The aim of this study was to evaluate the ecophysiological response of Brassica juncea, a species known to endure and accumulate various metals, to a broad range of Re concentrations. B. juncea plants were allowed to grow and on a substrate amended with KReO₄ to attain soil Re levels ranging from 0 to 80 mg kg^-1. Plants were collected 45 days after sowing for analysis. The results have shown that greater Re levels reduce growth, photosynthetic activity, soluble carbohydrate mobilization, and protein contents, and increase the plant's oxidative stress (anthocyanins, H₂O₂, lipid peroxidation) and corresponding response (ascorbic acid, superoxide dismutase activity). Nevertheless, B. juncea exhibited a remarkable ability to endure and uptake Re, featuring shoot Re concentrations that ranged from 1615 to 24,987 mg kg^-1 among the 5 and 80 mg kg^-1 treatments.

Chromium tolerance, bioaccumulation and localization in plants: An overview

In the current industrial scenario, chromium (Cr) as a metal is of great importance, but poses a major threat to the environment. Phytoremediation provides an environmentally sustainable, ecofriendly, cost effective approach for environmental cleanup of Cr. This review presents the current status of phytoremediation research with particular emphasis on cleanup of Cr contaminated soil and water systems. It gives a detailed account of the work done by different authors on the Cr bioavailability, uptake pathway, toxicity and storage in plants following the phytoextraction mechanism. This paper also describes recent findings related to Cr localization in hyperaccumulator plants. It gives an insight into the processes and mechanisms that allow plants to remove Cr from contaminated sites under varying conditions. These detailed knowledge of changes in plant metabolic pool in response to Cr stress would immensely help understand and improve the phytoextraction process. Further, this review provides a detailed understanding of Cr uptake and detoxification mechanism by plants that can be applied in developing a suitable approach for a better applicability of the process.

Different mechanisms of the metalliferous Zygophyllum fabago shoots and roots to cope with Pb toxicity

Lead (Pb) remains classified as a priority pollutant. Zygophyllum fabago is considered an early colonizer of heavy metal-polluted soils under semiarid conditions, but physiological mechanisms underlying this colonizing capacity remain unclear. In order to characterize Z. fabago plants' performance on Pb-contaminated soils, we evaluated how Pb influenced root and shoot growth, carbon metabolism, and oxidative status. For that, 30-day-old seedlings from one population colonizing a mine tailing ("Mercader") at Murcia (southeast Spain) were exposed to 500-μM Pb(NO₃)₂ for 1 week. Results showed that this high dose of Pb induced no plant mortality nor senescence, though promoting plant nanism. Besides the efficiency of roots to accumulate Pb, shoots also demonstrate a high efficiency to translocate and accumulate this metal. Pb exposure decreased Zn uptake to the aerial part and reduced net photosynthetic rate (A), RuBisCO activity, chlorophyll, and soluble sugar contents in shoots. Moreover, in shoots, Pb exposure increased the levels of O₂^- and decreased antioxidant capacity, culminating with a loss of cell membrane integrity (electrolyte leakage) and increased protein oxidation. Compared to controls, exposed roots had less Mn and Zn levels, and despite the rise in H₂O₂ levels, they were able to modulate non-protein thiols presenting a robust defense capacity. This capacity may support the roots' ability to maintain cell membrane integrity (electrolyte leakage) with regard to control. Principal component analysis (PCA) contributed to elucidate how this species adjusts physiological mechanisms to cope with Pb toxicity, showing that roots and shoots evolved different antioxidant defenses, which demonstrates the importance of organ specificity in the response of Z. fabago to heavy metals.

Diurnal Changes in Transcript and Metabolite Levels during the Iron Deficiency Response of Rice

BACKGROUND

Rice (Oryza sativa L.) is highly susceptible to iron (Fe) deficiency due to low secretion levels of the mugineic acid (MA) family phytosiderophore (PS) 2'-deoxymugineic acid (DMA) into the rhizosphere. The low levels of DMA secreted by rice have proved challenging to measure and, therefore, the pattern of DMA secretion under Fe deficiency has been less extensively studied relative to other graminaceous monocot species that secrete high levels of PS, such as barley (Hordeum vulgare L.).

RESULTS

Gene expression and metabolite analyses were used to characterise diurnal changes occurring during the Fe deficiency response of rice. Iron deficiency inducible genes involved in root DMA biosynthesis and secretion followed a diurnal pattern with peak induction occurring 3-5 h after the onset of light; a result consistent with that of other Strategy II plant species such as barley and wheat. Furthermore, triple quadrupole mass spectrometry identified 3-5 h after the onset of light as peak time of DMA secretion from Fe-deficient rice roots. Metabolite profiling identified accumulation of amines associated with metal chelation, metal translocation and plant oxidative stress responses occurring with peak induction 10-12 h after the onset of light.

CONCLUSION

The results of this study confirmed that rice shares a similar peak time of Fe deficiency associated induction of DMA secretion compared to other Strategy II plant species but has less prominent daily fluctuations of DMA secretion. It also revealed metabolic changes associated with the remediation of Fe deficiency and mitigation of damage from resulting stress in rice roots. This study complements previous studies on the genetic changes in response to Fe deficiency in rice and constitutes an important advance towards our understanding of the molecular mechanisms underlying the rice Fe deficiency response.

How innate immunity proteins kill bacteria and why they are not prone to resistance

Recent advances on antibacterial activity of peptidoglycan recognition proteins (PGRPs) offer some insight into how innate immunity has retained its antimicrobial effectiveness for millions of years with no frequent emergence of resistant strains. First, PGRP can bind to multiple components of bacterial envelope (peptidoglycan, lipoteichoic acid, and lipopolysaccharide). Second, PGRP simultaneously induces oxidative, thiol, and metal stress responses in bacteria, which individually are bacteriostatic, but in combination are bactericidal. Third, PGRP induces oxidative, thiol, and metal stress responses in bacteria through three independent pathways. Fourth, antibacterial effects of PGRP are enhanced by other innate immune responses. Thus, emergence of PGRP resistance is prevented by bacteriostatic effect and independence of each PGRP-induced stress response, as PGRP resistance would require simultaneous acquisition of three separate mechanisms disabling the induction of all three stress responses. By contrast, each antibiotic has one primary target and one primary antibacterial mechanism, and for this reason resistance to antibiotics can be generated by inhibition of this primary mechanism. Manipulating bacterial metabolic responses can enhance bacterial killing by antibiotics and elimination of antibiotic-tolerant bacteria, but such manipulations do not overcome genetically encoded antibiotic resistance. Pathogens cause infections by evading, inhibiting, or subverting host immune responses.

Efficient biodegradation of acephate by Pseudomonas pseudoalcaligenes PS-5 in the presence and absence of heavy metal ions [Cu(II) and Fe(III)], and humic acid

The present study was intended to investigate the biodegradation of acephate in aqueous media in the presence and in the absence of metal ions [Fe(III) and Cu(II)], and humic acid (HA). Biodegradations were performed using Pseudomonas pseudoalcaligenes PS-5 (PS-5) isolated from the heavy metal polluted site. Biodegradations were monitored by UV-Visible, FTIR, and electron spray ionization-mass spectrometry (ESI-MS) analyses. ESI-MS analysis revealed that PS-5 degraded acephate to two metabolites showing intense ions at mass-to-charge ratios (m/z) 62 and 97. The observed kinetic was the pseudo-first order, and half-life periods (t1/2) were 2.79 d-1 (of PS-5 + acephate), 3.45 d-1 [of PS-5 + acephate + Fe(III)], 3.16 d-1 [of PS-5 + acephate + Cu(II)], and 5.54 d-1 (of PS-5 + acephate + HA). A significant decrease in degradation rate of acephate was noticed in the presence of HA, and the same was confirmed by UV-Visible and TGA analyses. Strong aggregation behavior of acephate with humic acid in aqueous media was the major cause behind the slow degradation rate of acephate . New results on acephate metabolism by strain PS-5 in the presence and in the absence of metal ions [Fe(III) and Cu(II)] and humic acid were obtained. Results confirmed that Pseudomonas pseudoalcaligenes strain PS-5 was capable of mineralization of the acephate without formation of toxic metabolite methamidophos. More significantly, the Pseudomonas pseudoalcaligenes strain PS-5 could be useful as potential biological agents in effective bioremediation campaign for multi-polluted environments.

and associated rhizosphere bacterial communities

Plants adapt to metal stress by modifying their metabolism including the production of secondary metabolites in plant tissues. Such changes may impact the diversity and functions of plant associated microbial communities. Our study aimed to evaluate the influence of metals on the secondary metabolism of plants and the indirect impact on rhizosphere bacterial communities. We then compared the secondary metabolites of the hyperaccumulator Pteris vittata L. collected from a contaminated mining site to a non-contaminated site in Vietnam and identified the discriminant metabolites. Our data showed a significant increase in chlorogenic acid derivatives and A-type procyanidin in plant roots at the contaminated site. We hypothesized that the intensive production of these compounds could be part of the antioxidant defense mechanism in response to metals. In parallel, the structure and diversity of bulk soil and rhizosphere communities was studied using high-throughput sequencing. The results showed strong differences in bacterial composition, characterized by the dominance of Proteobacteria and Nitrospira in the contaminated bulk soil, and the enrichment of some potential human pathogens, i.e., Acinetobacter, Mycobacterium, and Cupriavidus in P. vittata's rhizosphere at the mining site. Overall, metal pollution modified the production of P. vittata secondary metabolites and altered the diversity and structure of bacterial communities. Further investigations are needed to understand whether the plant recruits specific bacteria to adapt to metal stress.

Arbuscular mycorrhizal fungi-assisted phytoremediation of a lead-contaminated site

Knowledge of the behavior of plant species associated with arbuscular mycorrhizal fungi (AMF) and the ability of such plants to grow on metal-contaminated soils is important to phytoremediation. Here, we evaluate the occurrence and diversity of AMF and plant species as well as their interactions in soil contaminated with lead (Pb) from the recycling of automotive batteries. The experimental area was divided into three locations: a non-contaminated native area, a coarse rejects deposition area, and an area receiving particulate material from the chimneys during the Pb melting process. Thirty-nine AMF species from six families and 10 genera were identified. The Acaulospora and Glomus genera exhibited the highest occurrences both in the bulk (10 and 6) and in the rhizosphere soils (9 and 6). All of the herbaceous species presented mycorrhizal colonization. The highest Pb concentrations (mgkg^-1) in roots and shoots, respectively, were observed in Vetiveria zizanoides (15,433 and 934), Pteris vitata (9343 and 865), Pteridim aquilinun (1433 and 733), and Ricinus communis (1106 and 625). The diversity of AMF seems to be related to the area heterogeneity; the structure communities of AMF are correlated with the soil Pb concentration. We found that plant diversity was significantly correlated with AMF diversity (r=0.645; P>0.05) in areas with high Pb soil concentrations. A better understanding of AMF communities in the presence of Pb stress may shed light on the interactions between fungi and metals taking place in contaminated sites. Such knowledge can aid in developing soil phytoremediation techniques such as phytostabilization.

Split-ubiquitin yeast two-hybrid interaction reveals a novel interaction between a natural resistance associated macrophage protein and a membrane bound thioredoxin in Brassica juncea

Natural resistance associated macrophage proteins (NRAMPs) are evolutionarily conserved metal transporters involved in the transport of essential and nonessential metals in plants. Fifty protein interactors of a Brassica juncea NRAMP protein was identified by a Split-Ubiquitin Yeast-Two-Hybrid screen. The interactors were predicted to function as components of stress response, signaling, development, RNA binding and processing. BjNRAMP4.1 interactors were particularly enriched in proteins taking part in photosynthetic or light regulated processes, or proteins predicted to be localized in plastid/chloroplast. Further, many interactors also had a suggested role in cellular redox regulation. Among these, the interaction of a photosynthesis-related thioredoxin, homologous to Arabidopsis HCF164 (High-chlorophyll fluorescence164) was studied in detail. Homology modeling of BjNRAMP4.1 suggested that it could be redox regulated by BjHCF164. In yeast, the interaction between the two proteins was found to increase in response to metal deficiency; Mn excess and exogenous thiol. Excess Mn also increased the interaction in planta and led to greater accumulation of the complex at the root apoplast. Network analysis of Arabidopsis homologs of BjNRAMP4.1 interactors showed enrichment of many protein components, central to chloroplastic/cellular ROS signaling. BjNRAMP4.1 interacted with BjHCF164 at the root membrane and also in the chloroplast in accordance with its proposed function related to photosynthesis, indicating that this interaction occurred at different sub-cellular locations depending on the tissue. This may serve as a link between metal homeostasis and chloroplastic/cellular ROS through protein-protein interaction.

x canadensis

Plants have divergent defense mechanisms against the harmful effects of heavy metals present in excess in soils and groundwaters. Poplars (Populus spp.) are widely cultivated because of their rapid growth and high biomass production, and members of the genus are increasingly used as experimental model organisms of trees and for phytoremediation purposes. Our aim was to investigate the copper and zinc stress responses of three outstanding biomass producer bred poplar lines to identify such transcripts of genes involved in the detoxification mechanisms, which can play an important role in the protection against heavy metals. Poplar cuttings were grown hydroponically and subjected to short-term (one week) mild and sublethal copper and zinc stresses. We evaluated the effects of the applied heavy metals and the responses of plants by detecting the changes of multiple physiological and biochemical parameters. The most severe cellular oxidative damage was caused by 30μM copper treatment, while zinc was less harmful. Analysis of stress-related transcripts revealed genotype-specific differences that are likely related to alterations in heavy metal tolerance. P. deltoides clones B-229 and PE 19/66 clones were clearly more effective at inducing the expression of various genes implicated in the detoxification process, such as the glutathione transferases, metallothioneins, ABC transporters, (namely PtGSTU51, PxMT1, PdABCC2,3), while the P. canadensis line M-1 accumulated more metal, resulting in greater cellular oxidative damage. Our results show that all three poplar clones are efficient in stress acclimatization, but with different molecular bases.

Toxicity responses of Cu and Cd: the involvement of miRNAs and the transcription factor SPL7

BACKGROUND

MicroRNAs are important posttranscriptional regulators of gene expression playing a role in developmental processes as well as in stress responses, including metal stress responses. Despite the identification of several metal-responsive miRNAs, the regulation and the role of these miRNAs and their targets remain to be explored. In this study, miRNAs involved in the response to Cd and Cu excess in Arabidopsis thaliana are identified. In addition, the involvement of the transcription factor SPL7, namely the key regulator of Cu homeostasis, in these metal stress responses is demonstrated by the use of an spl7 knockout mutant. Furthermore, more insight is given in the Cd-induced Cu deficiency response through determining the effects of adding supplemental Cu to Cd-exposed plants.

RESULTS

Thirteen miRNAs were identified in response to Cu and Cd excess in A. thaliana. Several of these miRNAs (miR397a, miR398b/c and miR857) were oppositely affected under Cu and Cd exposure. The induced expression of these miRNAs after Cd exposure was totally abolished in the spl7 mutant (SQUAMOSA promoter binding protein like7), indicating a major role for SPL7 in the Cd response. Plants exposed to Cd showed a higher Cu content in the roots, whereas the Cu content in the leaves of the spl7 mutant was reduced. Furthermore, the Cd-induced Cu deficiency response disappeared when supplemental Cu was added.

CONCLUSIONS

Copper- and Cd-responsive miRNAs were identified and several of them are SPL7-dependently regulated. SPL7 seems to be a shared component between both the Cu toxicity and the Cd toxicity response, yet oppositely regulated, that is inactivated after Cu exposure and activated after Cd exposure. Since SPL7 is the key regulator of Cu homeostasis, and Cd affects the Cu homeostasis, we hypothesize that SPL7 is activated in response to Cd possibly due to a Cd-induced Cu deficiency. Since adding additional Cu to Cd-exposed plants resulted in the disappearance of the Cu deficiency response, Cd possibly provokes Cu deficiency, thereby activating SPL7 and inducing subsequently the Cu deficiency response.

Thermal variation and factors influencing vertical migration behavior in Daphnia populations

The antipredator behavior diel vertical migration (DVM), common in aquatic keystone species Daphnia, involves daily migration from warmer surface waters before dawn to cooler deeper waters after dusk. Plasticity in Daphnia DVM behavior optimizes fitness via trade-offs between growth, reproduction, and predator avoidance. Migration behavior is affected by co-varying biotic and abiotic factors, including light, predator cues, and anthropogenic stressors making it difficult to determine each factor's individual contribution to the variation in this behavior. This study aims to better understand this ecologically significant behavior in Daphnia by: (1) determining how Daphnia pulicaria thermal preferences vary within and among natural populations; (2) distinguishing the role of temperature verses depth in Daphnia vertical migration; and (3) defining how two anthropogenic stressors (copper and nickel) impact Daphnia migratory behavior. Simulated natural lake stratification were constructed in 8L (0.5m tall, 14.5cm wide) water columns to monitor under controlled laboratory conditions the individual effects of temperature gradients, depth, and metal stressors on Daphnia vertical migration. Three major findings are reported. First, while no difference in thermal preference was found among the four populations studied, within lake populations variability among isolates was high. Second, decoupling temperature and depth revealed that depth was a better predictor of Daphnia migratory patterns over temperature. Third, exposure to environmentally relevant concentrations of copper or nickel inhibited classic DVM behavior. These findings revealed the high variability in thermal preference found within Daphnia populations, elucidated the individual roles that depth and temperature have on migratory behavior, and showed how copper and nickel can interfere with the natural response of Daphnia to fish predator cues. Thus contributing to the body of knowledge necessary to predict how natural populations of Daphnia will be affected by climate related changes in lake temperatures and increased presence of anthropogenic stressors.

Effects of calcium on rhizotoxicity and the accumulation and translocation of copper by grapevines

We assessed the effects of background concentrations of calcium (Ca) in solution on rhizotoxicity of copper (Cu) in and the accumulation and translocation of Cu by the grapevine, Vitis vinifera L. var. Kyoho. Grapevine cuttings in a hydroponic system were exposed to Cu-spiked solutions (0, 1, 2.5, 5, 10, and 25 μM) with two Ca backgrounds (0.5 and 5 mM) for 15 days. We found that when Cu exposure exceeded 5 μM, no new white roots were generated from the cuttings. When exposed to a Cu concentration of 25 μM, the lateral roots were sparse, appeared dark and exhibited malformed terminal swellings. The morphological phenomena of root response to an increase in Cu levels were relatively pronounced at a background concentration of 5 mM Ca; epidermal cell walls thickened, cortical cells remained intact and root terminal swelling was enhanced with Cu exposure. A 5 mM Ca background concentration enhanced the reduction in relative root elongation, but alleviated the reduction in relative root dry weight with increased Cu exposure. Moreover, there was a prominent increase in root Cu concentrations with increased Cu exposure, but the increases in leaf Cu concentrations were much less. The Cu profile of Cu exposure in a 5 mM Ca background concentration appeared higher in root, but lower in leaf than the Cu profile in a 0.5 mM Ca background; therefore, increase of Ca background concentrations would enhance Cu to be accumulated by root, but not translocated into the leaf.

Microtubule integrity and cell viability under metal (Cu, Ni and Cr) stress in the seagrass Cymodocea nodosa

The effects of increasing Cu, Ni and Cr concentrations (0.5, 5, 10, 20 and 40 mg L(-1)) on microtubule organization and the viability of leaf cells of the seagrass Cymodocea nodosa for 13 consecutive days were investigated under laboratory conditions. Increased oblique microtubule orientation, microtubule depolymerization at the 5-40 mg L(-1) Ni treatments after 3 d of exposure, and a complete microtubule depolymerization at all Ni treatments after 5 d were observed. Cu depolymerised microtubules after three to 7 d of exposure, while Cr caused an extensive microtubule bundling after 9 or 11 d of exposure, depending on metal dosage. Fluorescence intensity measurements further consolidated the above phenomena. Cell death, occurring at later time than microtubule disturbance, was also observed at all Cu and Ni treatments and at the 10-40 mg L(-1) Cr treatments and adding to the above quantification of the number of dead cells clearly showed that only a portion of the cell population studied died. The data presented, being the first assessment of microtubule disturbance in seagrasses, indicate that microtubules in seagrass leaf cells could be used as a valuable and early marker of metal-induced stress in biomonitoring programmes.