Jatropha integerrima, Duranta erecta and Hibiscus rosa-sinensis as Potential Lead Absorbent from Polluted Air in Dense Traffic Area

Lead (Pb) is one of the contaminants found in polluted air, especially in the area with dense traffic. Herbaceous plants are potentially used to reduce the Pb content in the polluted air. This study was designed to evaluate the potential of several plants, such as Jatropha integerrima, Duranta erecta and Hibiscus rosa-sinensis, as Pb absorbents from polluted air based on Pb accumulation and chlorophyll levels. The lead content was measured using atomic absorption spectrophotometry (AAS), while the chlorophyll content was tested using spectrophotometers at wavelengths of 649 nm and 665 nm. Results showed that the three plant species had potential as Pb absorbents from the air. The highest lead content was found from J. integerrima at 1.293 mg/kg, followed by H. rosa-sinensis at 1.232 mg/kg and D. erecta at 0.840 mg/kg. On the other hand, the highest level of leaf chlorophyll content was H. rosa-sinensis at 16.116 mg/kg, followed by D. erecta L. at 12.594 mg/kg and J. integerrima Jacq. of 10.297 mg/kg. No correlation was found between the Pb level and chlorophyll content of the three plants. It can be concluded that the three herbaceous plants have potential as Pb absorbents in the polluted air.

Physiological responses to lead and PEG-simulated drought stress in metallicolous and non-metallicolous Matthiola (Brassicaceae) species from Iran

Aims Plants growing on quarry tailings at the Irankouh Pb/Zn mine encounter both drought stress and high levels of Pb. To better understand role of drought and Pb in plant adaptation to Pb/Zn quarry tailings, we compared effects of drought stress (simulated by polyethylene glycol - PEG) and Pb, individually and in concert, to determine how these stressors affected two plant species: the metallicolous species Matthiola flavida and the non-metallicolous M. incana. Methods Plants were exposed to Pb (Pb(NO3)2) and three levels of PEG (0, -0.5, and − 0.75 MPa) in a complete factorial design. Results Lead had non-significant effects on growth and oxidative stress but enhanced levels of osmoprotectants and phenol compounds in the metallicolous M. flavida, whereas in the non-metallicolous M. incana Pb had non-significant or toxic effects on the same variables (except for the osmoprotectants proline and glycine betaine, and anthocyanins). In contrast to M. incana, the metallicolous species was hypertolerant of Pb, showing strongly reduced root-to-shoot translocation and enhanced Pb accumulation in the root, especially when under drought stress. Conclusion We conclude that enhanced Pb accumulation in the root and reduced translocation to the shoot, particularly when under high PEG exposure in the metallicolous species, reduced toxic effects of Pb in the shoot. This was aided by the accumulation of reducing sugars and phenolic compounds as well as greater catalase activity.

Genome-Wide Association Study Reveals Key Genes for Differential Lead Accumulation and Tolerance in Natural Arabidopsis thaliana Accessions

Soil contamination by lead (Pb) has become one of the major ecological threats to the environment. Understanding the mechanisms of Pb transport and deposition in plants is of great importance to achieve a global Pb reduction. We exposed a collection of 360 Arabidopsis thaliana natural accessions to a Pb-polluted soil. Germination rates, growth, and leaf Pb concentrations showed extensive variation among accessions. These phenotypic data were subjected to genome wide association studies (GWAs) and we found a significant association on chromosome 1 for low leaf Pb accumulation. Genes associated with significant SNP markers were evaluated and we selected EXTENSIN18 (EXT18) and TLC (TRAM-LAG1-CLN8) as candidates for having a role in Pb homeostasis. Six Pb-tolerant accessions, three of them exhibiting low leaf Pb content, and three of them with high leaf Pb content; two Pb-sensitive accessions; two knockout T-DNA lines of GWAs candidate genes (ext18, tlc); and Col-0 were screened under control and high-Pb conditions. The relative expression of EXT18, TLC, and other genes described for being involved in Pb tolerance was also evaluated. Analysis of Darwinian fitness, root and leaf ionome, and TEM images revealed that Pb-tolerant accessions employ two opposing strategies: (1) low translocation of Pb and its accumulation into root cell walls and vacuoles, or (2) high translocation of Pb and its efflux to inactive organelles or intracellular spaces. Plants using the first strategy exhibited higher expression of EXT18 and HMA3, thicker root cell walls and Pb vacuolar sequestration, suggesting that these genes may contribute to the deposition of Pb in the roots. On the other hand, plants translocating high amounts of Pb showed upregulation of TLC and ABC transporters, indicating that these plants were able to properly efflux Pb in the aerial tissues. We conclude that EXT18 and TLC upregulation enhances Pb tolerance promoting its sequestration: EXT18 favors the thickening of the cell walls improving Pb accumulation in roots and decreasing its toxicity, while TLC facilitates the formation of dictyosome vesicles and the Pb encapsulation in leaves. These findings are relevant for the design of phytoremediation strategies and environment restoration.

Inoculation With Indigenous Rhizosphere Microbes Enhances Aboveground Accumulation of Lead in Salix integra Thunb. by Improving Transport Coefficients

The application of plant–microbial remediation of heavy metals is restricted by the difficulty of exogenous microbes to form large populations and maintain their long-term remediation efficiency. We therefore investigated the effects of inoculation with indigenous heavy-metal-tolerant rhizosphere microbes on phytoremediation of lead (Pb) by Salix integra. We measured plant physiological indexes and soil Pb bioavailability and conducted widespread targeted metabolome analysis of strains to better understand the mechanisms of enhance Pb accumulation. Growth of Salix integra was improved by both single and co-inoculation treatments with Bacillus sp. and Aspergillus niger, increasing by 14% in co-inoculated plants. Transfer coefficients for Pb, indicating mobility from soil via roots into branches or leaves, were higher following microbial inoculation, showing a more than 100% increase in the co-inoculation treatment over untreated plants. However, Pb accumulation was only enhanced by single inoculation treatments with either Bacillus sp. or Aspergillus niger, being 10% greater in plants inoculated with Bacillus sp. compared with uninoculated controls. Inoculation mainly promoted accumulation of Pb in aboveground plant parts. Superoxide dismutase and catalase enzyme activities as well as the proline content of inoculated plants were enhanced by most treatments. However, soil urease and catalase activities were lower in inoculated plants than controls. Proportions of acid-soluble Pb were 0.34 and 0.41% higher in rhizosphere and bulk soil, respectively, of plants inoculated with Bacillus sp. than in that of uninoculated plants. We identified 410 metabolites from the microbial inoculations, of which more than 50% contributed to heavy metal bioavailability; organic acids, amino acids, and carbohydrates formed the three major metabolite categories. These results suggest that both indigenous Bacillus sp. and Aspergillus niger could be used to assist phytoremediation by enhancing antioxidant defenses of Salix integra and altering Pb bioavailability. We speculate that microbial strains colonized the soil and plants at the same time, with variations in their metabolite profiles reflecting different living conditions. We also need to consider interactions between inocula and the whole microbial community when applying microbial inoculation to promote phytoremediation.

Cu and Pb Accumulation and Removal From Aqueous Medium by Enydra Fluctuans DC. (Asteraceae) - A Medicinal Plant With Potential for Phytoremediation

Enydra fluctuans DC. (Asteraceae) is an edible semi-aquatic floating or trailing herbaceous plant widely distributed in tropical Africa, South and South East Asia, and Australia. Its leaves, which are consumed as a vegetable, are also used in traditional medicine to treat several diseases. The efficacy of this plant in removal of copper and lead from aqueous medium was tested in the present study. Accumulation of both Cu and Pb was significantly higher in root than that in leaf and stem. Though all the bioconcentration factor (BCF) values were greater than unity, none of the translocation factor (TF) values was greater than unity, indicating that this plant could not be considered a hyperaccumulator of these metals. Nevertheless, E. fluctuans could remove Cu from aqueous medium at rates ranging from 98.8–99.7 %, with a mean reduction of 99.2 % after 96 h exposure at various concentrations. The removal of Pb ranged from 97.1–99.1 %, with a mean reduction of 98.2 %. Thus, E. fluctuans showed high potential in removal of Cu and Pb from aqueous medium, and has the prospect of being used in phytoremediation of these metals.

Melatonin-induced DNA demethylation of metal transporters and antioxidant genes alleviates lead stress in radish plants

Melatonin (MT) is a tryptophan-derived natural product that plays a vital role in plant response to abiotic stresses, including heavy metals (HMs). However, it remains elusive how exogenous MT mediates lead (Pb) accumulation and detoxification at the methylation and transcriptional levels in radish. In this study, decreased Pb accumulation and increased antioxidant enzyme activity were detected under MT treatment in radish. Single-base resolution maps of DNA methylation under Pb stress (Pb200) and Pb plus MT treatment (Pb_50MT) were first generated. The genome-wide methylation level was increased under Pb stress, while an overall loss of DNA methylation was observed under MT treatment. The differentially methylated region (DMR)-associated genes between Pb_50MT and Pb200 were uniquely enriched in ion binding terms, including cation binding, iron ion binding, and transition metal ion binding. Hyper-DMRs between Pb200 and Control exhibited a decreasing trend of methylation under Pb_50MT treatment. A few critical upregulated antioxidant genes (e.g., RsAPX2, RsPOD52 and RsGST) exhibited decreased methylation levels under MT treatment, which enabled the radish plants to scavenge lead-induced reactive oxygen species (ROS) and decrease oxidative stress. Notably, several MT-induced HM transporter genes with low methylation (e.g., RsABCF5, RsYSL7 and RsHMT) and transcription factors (e.g., RsWRKY41 and RsMYB2) were involved in reducing Pb accumulation in radish roots. These findings could facilitate comprehensive elucidation of the molecular mechanism underlying MT-mediated Pb accumulation and detoxification in radish and other root vegetable crops.