Deciphering lead tolerance mechanisms in a population of the plant species Biscutella auriculata L. from a mining area: Accumulation strategies and antioxidant defenses

Exogenous methyl jasmonate enhanced kenaf (Hibiscus cannabinus) tolerance against lead (Pb) toxicity by improving antioxidant capacity and osmoregulators

In this study, the effects of exogenous methyl jasmonate (MeJA) on metal uptake and its ability to attenuate metal toxicity in kenaf plants under Pb stress were investigated. The experiment was conducted with five different MeJA concentrations (0, 40, 80, 160, and 320 μM) as a foilar application to kenaf plants exposed to 200 μM Pb stress. The results revealed that pretreatmen of MeJA significantly increased plant dry weight, plant height, and root architecture at all concentrations tested, with the most significant increase at 320 μM. Foliar application of MeJA at 160 μM and 320 μM increased the Pb concentrations in leaves and stems as well as the translocation factor (TF) from root to leaf. However, the bioaccumulation factor in the shoot initially decreased and then increased with increasing MeJA concentration. By increasing enzymatic (SOD, POD, and CAT) and non-enzymatic (AsA and non-protein thiols) antioxidants, MeJA pretreatment decreased lipid peroxidation, O2- and H2O2 accumulation and recovered photosynthetic pigment content under Pb stress. Increased osmolytes (proline, sugar, and starch) and protein content after MeJA pretreatment under Pb stress restore cellular homeostasis and improved kenaf tolerance. Our results suggest that MeJA pretreatment modifies the antioxidant machinery of kenaf and inhibits stress-related processes that cause lipid peroxidation, hence enhancing plant tolerance to Pb stress.

Responses to lead stress in Scrophularia striata: insights into antioxidative defence mechanisms and changes in flavonoids profile

Lead (Pb) induces oxidative stress in plants, which results in different responses, including the production of antioxidants and changes in the profile of secondary metabolites. In this study, the responses of Scrophularia striata exposed to 250mgL-1 Pb (NO3 )2 in a hydroponic environment were determined. Growth parameters, oxidative and antioxidative responses, redox status, and the concentration of Pb were analysed in roots and shoots. Malondialdehyde and hydrogen peroxide (H2 O2 ) levels in the roots were significantly increased and reached their highest value at 72h after Pb treatment. Superoxide dismutase, catalase, and peroxidase, as an enzymatic antioxidant system, were responsible for reactive oxygen species scavenging, where their activities were increased in the shoot and root of Pb-treated plants. Enzymatic antioxidant activities were probably not enough to remove a significant H2 O2 content in response to Pb treatment. Therefore, other defence responses were activated. The results stated that the flavonoid components of S. striata progressed towards the increase of isoflavone, flavanol, and stilbenoid contents under Pb treatment. In general, S. striata stimulates the enzymatic defence system and activates the non-enzymatic system by modulating the profile of flavonoids toward the production of flavonoids with high antioxidant activity, such as quercetin and myricetin in response to Pb stress.

Exploration of Cd transformations in Cd spiked and EDTA-chelated soil for phytoextraction by Brassica species

The study of soil cadmium (Cd) fractionation has become the need of the hour due to phytoextraction of Cd heavy metal by indigenous Brassica species of northwest India. The present study was conducted to explore the Cd speciation in soils treated with Cd (0, 5.0, 10.0, 20.0, 40.0, and 80.0 mg kg-1 soil) and synthetic chelate ethylene diamine tetraacetic acid (EDTA-0, 1.0 and 2.0 g kg-1 soil) planted under three Brassica species (Brassica juncea L., Brassica campestris L., and Brassica napus L). The studied Cd fractions viz. exchangeable and water-soluble (EX + WS), carbonate (CARB), organic matter (OM), Mn oxide (MnOX), amorphous Fe oxide (AFeOX), crystalline Fe oxide (CFeOX), and residual (RES) differed in their Cd content in soils under three investigated Brassica species. Among all plantations, B. juncea reduced the highest soil Cd content of EX + WS form which reflected its bioavailability. The Cd supplementation significantly enhanced the Cd concentration in all Cd forms with EX + WS Cd form exhibiting higher increase even at low Cd level (5.0 mg kg-1), whereas the EDTA addition did not influence Cd fractions. The application of EDTA @ 1.0 g kg-1 soil proved beneficial as it enhanced the metal mobility for plant extraction. All species positively significantly correlated (r = 0.648** to 0.747**) with all Cd fractions but except B. juncea all confronted reduction in their total biomass. In nutshell, it suggested that Brassica species having large plant biomass could be considered as a potential candidate for phytoremediation.

Cell selection to increase lawn grass resistance to lead pollution

Lead is one of the priority soil pollutants among heavy metals. To increase the species diversity of ecosystems, it is necessary to increase the resistance of plants to lead. The aim of the work was to obtain plants resistant to lead. The objects of our study were to lawn grasses. The effect of lead on the growth and regenerative ability of calli was determined. The results of this work showed that lead is less toxic to calli than copper. Biotechnological method for obtaining lead resistant plants has been developed. The effect of lead on the growth of regenerants and original plants was determined. Agrostis stolonifera plants that are obtained after cell selection have demonstrated a high degree of resistance to lead. Can the developed technology be used for other lawn grasses? We obtained lead resistant plants Festuca rubra. Therefore, using cell selection can increase the tolerance of lawn grasses to lead.

Physiological and biochemical bases of spermidine-induced alleviation of cadmium and lead combined stress in rice

Cadmium (Cd) and lead (Pb) pollution is a major environmental issue affecting plant production. Spermidine (Spd) is involved in plant response to abiotic stress. However, the role and associated mechanism of Spd under Cd + Pb combined stress are poorly understood. The potential protective role of Spd at different concentration on rice (Oryza sativa L.) seedlings exposed to Cd + Pb treatment was investigated by a hydroponic experiment in this study. The results showed that exogenous Spd enhanced the tolerance of rice seedlings to Cd + Pb stress, resulted in an increase in plant height, root length, fresh weight and dry weight of roots and shoots. Further, application of Spd decreased the contents of hydrogen peroxide, superoxide anion, malondialdehyde, and the accumulation of Cd and Pb, and increased the contents of mineral nutrient, carotenoids, chlorophyll, proline, soluble sugar, soluble protein, total phenol, flavonoid, anthocyanin, and antioxidant enzymes activities in roots and shoots of rice seedlings under Cd + Pb stress. Particularly, 0.5 mmol L^-1 Spd was the most effective to alleviate the adverse impacts on growth and physiological metabolism of rice seedlings under Cd + Pb stress. Principal component analysis and heat map clustering established correlations between physio-biochemical parameters and further revealed Spd alleviated Cd + Pb damage in rice seedling was associated with inhibition of accumulation and translocation of Cd and Pb, increasing the contents of photosynthetic pigments and mineral nutrient and stimulation of antioxidative response and osmotic adjustment. Overall, our findings provide an important prospect for use of Spd in modulating Cd + Pb tolerance in rice plants. Spd could help to alleviate Cd + Pb damage through inhibition of accumulation and translocation of Cd and Pb and stimulation of oxidant-defense system and osmotic adjustment.

Phytoremediation of contaminated industrial wastewater by duckweed (Lemna minor L.): Growth and physiological response under acetic acid application

Extensive usage of heavy metals (HMs) in chemical reactions and processes eventually contaminate the environmental segments and is currently a major environmental concern. HMs such as cadmium (Cd), copper (Cu), lead (Pb), chromium (Cr) and nickel (Ni) are considered the most harmful pollutants as they have adequate potential of bioaccumulation. The current research was carried out to assess the HMs toxicity of textile and tannery wastewater and effect of acetic acid (AA) on phytoextraction of HMs by duckweed (Lemna minor L.) in a hydroponic system. Plants were treated with different treatments having different hydroponic concentrations of AA (5 and 10 mM) and textile and tannery effluents, where these two effuents were equally mixed and then diluted with good quality water with different ratios (25, 50, 75, and 100%) along with three replications of each treatment. Results were recorded for growth attributes, chlorophylls, antioxidant enzymes, electrolytic leakage, reactive oxygen species and HMs accumulation in plants. HMs accumulation disrupts the growth parameters, chlorophyll contents and carotenoids contents along with increased activities of antioxidant enzyme such as catalases (CAT), superoxide dismutase (SOD), peroxidases (POD) and ascorbate peroxidase (APX). Addition of AA in the hydroponic experimental system significantly improves the antioxidant defense mechanism and alleviated the HM induced toxicity in plants. Cr, Cd, Pb, Cu and Ni concentrations were maximally increased up to 116 & 422%, 106 & 416%, 72 & 351%, 76 & 346%, and 41 & 328% respectively under AA (10 mM) application. The results revealed that duckweed can be applied as potential phyto-remedy to treat industrial wastewater.

Low-molecular-weight organic acid-mediated tolerance and Pb accumulation in centipedegrass under Pb stress

Lead (Pb) is one of the most harmful, toxic pollutants to the ecological environment and humans. Centipedegrass, a fast-growing warm-season turfgrass, is excellent for Pb pollution remediation. Exogenous low-molecular-weight organic acid (LMWOA) treatment is a promising approach for assisted phytoremediation. However, the effects of this treatment on the tolerance and Pb accumulation of centipedegrass are unclear. This study investigated these effects on the physiological growth response and Pb accumulation distribution characteristics of centipedegrass. Applications of 400 μM citric acid (CA), malic acid (MA) and tartaric acid (TA) significantly reduced membrane lipid peroxidation levels of leaves and improved biomass production of Pb-stressed plants. These treatments mainly increased peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) activities and enhanced free protein (Pro), ascorbic acid (AsA) and phytochelatins (PCs) contents, ultimately improving the Pb tolerance of centipedegrass. Their promoting effects decreased as follows: TA>CA>MA. All the treatments decreased root Pb concentrations and increased stem and leaf Pb concentrations, thus increasing total Pb accumulation and TF values. MA had the best and worst effects on Pb accumulation and Pb transportation, respectively. CA had the best and worst effects on Pb transportation and Pb accumulation, respectively. TA exhibited strong effects on both Pb accumulation and transport. Furthermore, all treatments changed the subcellular Pb distribution patterns and distribution models of the chemical forms of Pb in each tissue. The root Pb concentration was more highly correlated with the Pb subcellular fraction distribution pattern, while the stem and leaf Pb concentrations were more highly correlated with the distribution models of the chemical forms of Pb. Overall, TA improved plant Pb tolerance best and promoted both Pb absorption and transportation well and is considered the best candidate for Pb-contaminated soil remediation with centipedegrass. This study provides a new idea for Pb-contaminated soil remediation with centipedegrass combined with LMWOAs.

Lead, zinc tolerance mechanism and phytoremediation potential of Alcea rosea (Linn.) Cavan. and Hydrangea macrophylla (Thunb.) Ser. and ethylenediaminetetraacetic acid effect

In this study, we aimed to elucidate the defense mechanism of Alcea rosea (Linn.) Cavan. and Hydrangea macrophylla (Thunb.) Ser. against the single and compound toxicity of lead (Pb) and zinc (Zn) along with the synergistic effect of ethylenediaminetetraacetic acid (EDTA) in accumulation of metals in these two species. The two plant species were subjected to single metal treatment (Pb 1000 mg kg^-1, Zn 600 mg kg^-1) and compound metal treatment (Pb 1000 mg kg^-1 + Zn 600 mg kg^-1) in a greenhouse. Besides, different levels of EDTA were applied (2.5, 5.0, and 10.0 mmol kg^-1) with compound metal treatment. Several physiological and biochemical parameters, including plant photosynthetic parameters, enzymatic antioxidant system, accumulation concentration of metals, and subcellular distribution were estimated. The results showed that the antioxidative enzymes, proline, root morphological changes, and metal localization all played important roles in resisting Pb and Zn toxicity. A notable difference was that Zn was concentrated in the roots (58.5%) of H. macrophylla to reduce the damage but in the leaves (38.5%) of A. rosea to promote photosynthesis and resist the toxicity of metals. In addition, Zn reduced the toxicity of Pb to plants by regulating photosynthesis, Pb absorption and Pb distribution in subcells. The biological concentration factors (BCF) and translocation factors (TF) for Pb in two plants were less than 1, indicating that they could be considered as phytostabilizators in Pb-contaminated soils. Moreover, EDTA could enhance the enrichment and transport capacity of Pb and Zn to promote the phytoremediation effect. In summary, both plants have a certain application potential for repairing Pb-Zn-contaminated soil.

Speciation of metals in indigenous plants growing in post-mining areas: Dihydroxynicotianamine identified as the most abundant Cu and Zn ligand in Hypericum laricifolium

Ag, As, Cu, Pb and Zn were found to be the principal metallic contaminants of a post-mining area of Peru (Hualgayoc, Cajamarca). Study of metal distribution amongst roots, stems, and leaves of four indigenous hypertolerant plant species, Arenaria digyna, Puya sp., Hypericum laricifolium, Nicotiana thyrsiflora indicated significant translocation of Zn (0.6 < TF ≤ 10.0) and Cu (0.4 < TF ≤ 6.5) into aerial plant organs and substantial water-extractable fraction (20-60%) of these metals, except for A. digyna (root and stems). A study of the metal speciation by ultrahigh-performance size-exclusion (fast-SEC) and hydrophilic ion interaction (HILIC) liquid chromatography with dual ICP (inductively coupled plasma) and electrospray (ESI) Orbitrap MS detection revealed the presence of nicotianamine and deoxymugineic acid copper and zinc complexes in roots, stem and leaves of N. thyrsiflora and Puya sp., and nicotianamine alone in A. digyna. A previously unreported compound, dihydroxy-nicotianamine was identified as the most abundant Cu and Zn ligand in H. laricifolium. The presence of arsenobetaine and an arsenosugar was confirmed by ESI MS. Ag and Pb were hardly translocated to leaves and were found as high molecular species; one of the Pb-containing species co-eluted in fast-SEC-ICP MS with rhamnogalacturonan-II-Pb complex commonly found in in the walls of plants.

Heteroauxin-producing bacteria enhance the plant growth and lead uptake of Miscanthus floridulus (Lab.)

Soil lead (Pb) contamination has caused severe environmental threats and is in urgent need of remediation. This study was aimed to explore the feasibility of using the Miscanthus-microbe combination to reduce Pb pollution in the farmland surrounding a lead-zinc mining area. We have screened three heteroauxin (IAA)-producing microbes (Lelliottia jeotgali MR2, Klebsiella michiganensis TS8, and Klebsiella michiganensis ZR1) with high Pb tolerance. The IAA-producing ability of the mixed-species was stronger than that of the single bacterium. In pot experiments, the mixed-species of MR2-ZR1 and MR2-TS8 had better performance in enhancing the weight of Miscanthus grass (increased by 22.2-53.6% compared to the control group without inoculating microbes). The remediation efficiency of Pb was significantly higher in the MR2 (30.79%), MR2-TS8 (24.96%), and TS8-ZR1 (21.10%) groups than that in the control group (6.75%). We speculated that MR2 and mixed species of MR2-TS8 and TS8-ZR1 could promote the percentages of activated Pb fractions in soils and increase the Pb uptake of M. floridulus (Lab.). These results implied that the MR2-TS8 mixed-species might be selected as the effective microbial agent to simultaneously enhance the remediation efficiency of Pb-contaminated soils and the biomass of M. floridulus (Lab.).

Histochemical Techniques in Plant Science: More Than Meets the Eye

Histochemistry is an essential analytical tool interfacing extensively with plant science. The literature is indeed constellated with examples showing its use to decipher specific physiological and developmental processes, as well as to study plant cell structures. Plant cell structures are translucent unless they are stained. Histochemistry allows the identification and localization, at the cellular level, of biomolecules and organelles in different types of cells and tissues, based on the use of specific staining reactions and imaging. Histochemical techniques are also widely used for the in vivo localization of promoters in specific tissues, as well as to identify specific cell wall components such as lignin and polysaccharides. Histochemistry also enables the study of plant reactions to environmental constraints, e.g. the production of reactive oxygen species (ROS) can be traced by applying histochemical staining techniques. The possibility of detecting ROS and localizing them at the cellular level is vital in establishing the mechanisms involved in the sensitivity and tolerance to different stress conditions in plants. This review comprehensively highlights the additional value of histochemistry as a complementary technique to high-throughput approaches for the study of the plant response to environmental constraints. Moreover, here we have provided an extensive survey of the available plant histochemical staining methods used for the localization of metals, minerals, secondary metabolites, cell wall components, and the detection of ROS production in plant cells. The use of recent technological advances like CRISPR/Cas9-based genome-editing for histological application is also addressed. This review also surveys the available literature data on histochemical techniques used to study the response of plants to abiotic stresses and to identify the effects at the tissue and cell levels.

Evaluation of phytoremediation potential of native dominant plants and spatial distribution of heavy metals in abandoned mining area in Southwest China

A field investigation on the content of heavy metals in soils and dominant plants was conducted in three sites (A<0.5 km, B<1.0 km, C<1.5 km) with different distances of mine tailings. The spatial distribution of heavy metals and the accumulation in plants were compared, and the candidate species for ecosystem restoration were selected. The results indicated that the soil was polluted by chromium (Cr), Cadmium (Cd), copper (Cu), nickel (Ni) in varying degrees, which is 2.07, 2.60, 1.79, and 4.49 times higher than the Class-Ⅱ standard in China. The concentrate of Ni, Cd, and Zinc (Zn) increased, while Cr, Lead (Pb), and Cu decreased with the distance from the mine tailings. 73 species (34 families) were found and mainly herbaceous plants. The concentrate of Cd, Cu, Cr, and Ni in 29 dominant plants were measured and 66.67%, 21.43%, 100%, 47.62% plants exceeded the normal concentration range. Based on the comparative analysis of heavy metal content, bioconcentration factor, and translocation factor in plants, Polygonum capitatum has good phytoextraction ability, Boehmeria nivea, Chrysanthemum indicum, Miscanthus floridulus, Conyza canadensis, Rubus setchuenensis, Senecio scandens, and Arthraxon hispidus showed remarkable phytostabilization abilities of Cr, Cd, Ni, and Cu, which can be used as potential phytoremediation candidate.

Characterization of the biochemical basis for copper homeostasis and tolerance in Biscutella auriculata L

Biscutella auriculata L. is a plant that belongs to the Brassicaceae family and it has been found growing in a metal-contaminated area of the San Quíntín mine (Ciudad Real, Spain). The purpose of this work was to evaluate the mechanisms that allow this plant to tolerate high concentrations of copper. Seedlings were grown in a semi-hydroponic system for 15 days under 125 μM of Cu (NO₃ )₂ . Exposure to copper resulted in growth inhibition and reduction in the photosynthetic parameters. Copper was mainly accumulated in vascular tissue and vacuoles of the roots and only a minor proportion was transferred to the shoot. Biothiol analysis showed a greater enhancement of reduced glutathione in leaves and increases of phytochelatins (PC2 and PC3) in both leaves and roots. Copper treatment induced oxidative stress, which triggered a response of the enzymatic and non-enzymatic antioxidant mechanisms. The results show that B. auriculata is able to tolerate high metal levels through the activation of specific mechanisms to neutralize the oxidative stress produced and also by metal sequestration through phytochelatins. The preferential accumulation of copper in roots provides clues for further studies on the use of this plant for phytostabilization and environmental recovery purposes in Cu-contaminated areas.

Abandoned Mine Lands Reclamation by Plant Remediation Technologies

Abandoned mine lands (AMLs), which are considered some of the most dangerous anthropogenic activities in the world, are a source of hazards relating to potentially toxic elements (PTEs). Traditional reclamation techniques, which are expensive, time-consuming and not well accepted by the general public, cannot be used on a large scale. However, plant-based techniques have gained acceptance as an environmentally friendly alternative over the last 20 years. Plants can be used in AMLs for PTE phytoextraction, phytostabilization, and phytovolatilization. We reviewed these phytoremediation techniques, paying particular attention to the selection of appropriate plants in each case. In order to assess the suitability of plants for phytoremediation purposes, the accumulation capacity and tolerance mechanisms of PTEs was described. We also compiled a collection of interesting actual examples of AML phytoremediation. On-site studies have shown positive results in terms of soil quality improvement, reduced PTE bioavailability, and increased biodiversity. However, phytoremediation strategies need to better characterize potential plant candidates in order to improve PTE extraction and to reduce the negative impact on AMLs.