Lead-induced changes in plant cell ultrastructure: an overview.

Lead (Pb) is one of the most harmful toxic metals and causes severe damage to plants even at low concentrations. Pb inhibits plant development, reduces photosynthesis rates, and causes metabolic disfunctions. Plant cells display these alterations in the form of abnormal morphological modifications resulting from ultrastructural changes in the cell wall, plasma membrane, chloroplast, endoplasmic reticulum, mitochondria, and nuclei. Depending on plant tolerance capacity, the ultrastructural changes could be either a sign of toxicity that limits plant development or an adaptive strategy to cope with Pb stress. This paper gathers data on Pb-induced changes in cell ultrastructure observed in many tolerant and hyperaccumulator plants and describes the ultrastructural changes that appear to be mechanisms to alleviate Pb toxicity. The different modifications caused by Pb in cell organelles are summarized and reinforced with hypotheses that provide an overview of plant responses to Pb stress and explain the physiological and morphological changes that occur in tolerant plants. These ultrastructural modifications could help assess the potential of plants for use in phytoremediation.

De Novo Transcriptome Assembly, Gene Annotations, and Characterization of Functional Profiling Reveal Key Genes for Lead Alleviation in the Pb Hyperaccumulator Greek Mustard (Hirschfeldia incana L.).

Lead (Pb) contamination is a widespread environmental problem due to its toxicity to living organisms. Hirschfeldia incana L., a member of the Brassicaceae family, commonly found in the Mediterranean regions, is characterized by its ability to tolerate and accumulate Pb in soils and hydroponic cultures. This plant has been reported as an excellent model to assess the response of plants to Pb. However, the lack of genomic data for H. incana hinders research at the molecular level. In the present study, we carried out RNA deep transcriptome sequencing (RNA-seq) of H. incana under two conditions, control without Pb(NO3)2 and treatment with 100 µM of Pb(NO3)2 for 15 days. A total of 797.83 million reads were generated using Illumina sequencing technology. We assembled 77,491 transcript sequences with an average length of 959 bp and N50 of 1330 bp. Sequence similarity analyses and annotation of these transcripts were performed against the Arabidopsis thaliana nr protein database, Gene Ontology (GO), and KEGG databases. As a result, 13,046 GO terms and 138 KEGG maps were created. Under Pb stress, 577 and 270 genes were differentially expressed in roots and aboveground parts, respectively. Detailed elucidation of regulation of metal transporters, transcription factors (TFs), and plant hormone genes described the role of actors that allow the plant to fine-tune Pb stress responses. Our study revealed that several genes related to jasmonic acid biosynthesis and alpha-linoleic acid were upregulated, suggesting these components' implication in Hirschfeldia incana L responses to Pb stress. This study provides data for further genomic analyses of the biological and molecular mechanisms leading to Pb tolerance and accumulation in Hirschfeldia incana L.

Native plant species growing on the abandoned Zaida lead/zinc mine site in Morocco: Phytoremediation potential for biomonitoring perspective.

This study aims to assess the level of metal contamination and the ecological risk index at the abandoned Zaida Pb/Zn mining site in eastern Morocco and identify native plant species found on the site that can be used in site rehabilitation through phytoremediation strategies. Samples from seven native and abundant plant species at the site, along with their rhizospheric soils, were collected and analyzed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to determine the concentrations of various metal(loid)s, including As, Cu, Ni, Cd, Sb, Zn, and Pb. Indicators of soil pollution and ecological risks were also assessed, including the enrichment factor (EF), pollution index (PI), and ecological risk index (ERI). The Biological Accumulation Coefficient (BAC), Translocation Factor (TF), and Biological Concentration Factor (BCF) of plant samples were calculated. The results reveal polymetallic soil contamination, with notably higher concentrations of Pb, Cu and Zn, reaching respectively 5568 mg kg-1 DW, 152 mg kg-1 DW, and 148 mg kg-1 DW, indicating a significant potential ecological risk. The enrichment factor (EF) was also assessed for each metal(loid)s, and the results indicated that the metal contamination was of anthropogenic origin and linked to intensive mining activities in Zaida. These findings are supported by the pollution index (PI) ranging from 1.6 to 10.01, which reveals an extremely high metal(loid)s pollution level. None of the plant species exhibited a hyperaccumulation of metal(loid)s. However, Artemisia herba alba demonstrated a strong capacity to accumulate Pb in its aboveground parts, with a concentration of 468 mg kg-1 DW. Stipa tenacissima, Retama spherocarpa, and Astragalus armatus, showed a significant Pb accumulation in their roots reaching 280, 260, and 256 mg kg-1 DW.respectively. Based on BAC, TF, and BCF, Stipa tenacissima exhibited potential for Ni and Cd phytostabilization, as well as the ability for Zn phytoextraction. Additionally, Artemisia herba alba displayed the capability to phytoextract Cd and had a high propensity to translocate all the studied metal(loid)s. Astragalus armatus has the potential to be used in the phytostabilization of Zn and Ni, as well as for the phytoextraction of As and Sb. These native species from the Zaida site, although not hyperaccumulators, have the potential to contribute significantly to the phytoextraction or phytostabilization of potentially toxic elements (PTEs). Moreover, they can serve as vegetative cover to mitigate the erosion and dispersion of metal(loid)s.

Salt stress responses and alleviation strategies in legumes: a review of the current knowledge.

Salinity is one of the most significant environmental factors limiting legumes development and productivity. Salt stress disturbs all developmental stages of legumes and affects their hormonal regulation, photosynthesis and biological nitrogen fixation, causing nutritional imbalance, plant growth inhibition and yield losses. At the molecular level, salt stress exposure involves large number of factors that are implicated in stress perception, transduction, and regulation of salt responsive genes' expression through the intervention of transcription factors. Along with the complex gene network, epigenetic regulation mediated by non-coding RNAs, and DNA methylation events are also involved in legumes' response to salinity. Different alleviation strategies can increase salt tolerance in legume plants. The most promising ones are Plant Growth Promoting Rhizobia, Arbuscular Mycorrhizal Fungi, seed and plant's priming. Genetic manipulation offers an effective approach for improving salt tolerance. In this review, we present a detailed overview of the adverse effect of salt stress on legumes and their molecular responses. We also provide an overview of various ameliorative strategies that have been implemented to mitigate/overcome the harmful effects of salt stress on legumes.

Zinc Hyperaccumulation in Plants: A Review.

Zinc is an essential microelement involved in many aspects of plant growth and development. Abnormal zinc amounts, mostly due to human activities, can be toxic to flora, fauna, and humans. In plants, excess zinc causes morphological, biochemical, and physiological disorders. Some plants have the ability to resist and even accumulate zinc in their tissues. To date, 28 plant species have been described as zinc hyperaccumulators. These plants display several morphological, physiological, and biochemical adaptations resulting from the activation of molecular Zn hyperaccumulation mechanisms. These adaptations can be varied between species and within populations. In this review, we describe the physiological and biochemical as well as molecular mechanisms involved in zinc hyperaccumulation in plants.

Screening of Native Plants Growing on a Pb/Zn Mining Area in Eastern Morocco: Perspectives for Phytoremediation.

Screening of native plant species from mining sites can lead to identify suitable plants for phytoremediation approaches. In this study, we assayed heavy metals tolerance and accumulation in native and dominant plants growing on abandoned Pb/Zn mining site in eastern Morocco. Soil samples and native plants were collected and analyzed for As, Cd, Cu, Ni, Sb, Pb, and Zn concentrations. Bioconcentration factor (BCF), translocation factor (TF), and biological accumulation coefficient (BAC) were determined for each element. Our results showed that soils present low organic matter content combined with high levels of heavy metals especially Pb and Zn due to past extraction activities. Native and dominant plants sampled in these areas were classified into 14 species and eight families. Principal components analysis separated Artemisia herba-alba with high concentrations of As, Cd, Cu, Ni, and Pb in shoots from other species. Four plant species, namely, Reseda alba, Cistus libanotis, Stipa tenacissima, and Artemisia herba-alba showed strong capacity to tolerate and hyperaccumulate heavy metals, especially Pb, in their tissues. According to BCF, TF, and BAC, these plant species could be used as effective plants for Pb phytoextraction. Stipa tenacissima and Artemisia herba-alba are better suited for phytostabilization of Cd/Cu and Cu/Zn, respectively. Our study shows that several spontaneous and native plants growing on Pb/Zn contaminated sites have a good potential for developing heavy metals phytoremediation strategies.

Down Regulation and Loss of Auxin Response Factor 4 Function Using CRISPR/Cas9 Alters Plant Growth, Stomatal Function and Improves Tomato Tolerance to Salinity and Osmotic Stress.

Auxin controls multiple aspects of plant growth and development. However, its role in stress responses remains poorly understood. Auxin acts on the transcriptional regulation of target genes, mainly through Auxin Response Factors (ARF). This study focuses on the involvement of SlARF4 in tomato tolerance to salinity and osmotic stress. Using a reverse genetic approach, we found that the antisense down-regulation of SlARF4 promotes root development and density, increases soluble sugars content and maintains chlorophyll content at high levels under stress conditions. Furthermore, ARF4-as displayed higher tolerance to salt and osmotic stress through reduced stomatal conductance coupled with increased leaf relative water content and Abscisic acid (ABA) content under normal and stressful conditions. This increase in ABA content was correlated with the activation of ABA biosynthesis genes and the repression of ABA catabolism genes. Cu/ZnSOD and mdhar genes were up-regulated in ARF4-as plants which can result in a better tolerance to salt and osmotic stress. A CRISPR/Cas9 induced SlARF4 mutant showed similar growth and stomatal responses as ARF4-as plants, which suggest that arf4-cr can tolerate salt and osmotic stresses. Our data support the involvement of ARF4 as a key factor in tomato tolerance to salt and osmotic stresses and confirm the use of CRISPR technology as an efficient tool for functional reverse genetics studies.

Transcriptome Changes in Hirschfeldia incana in Response to Lead Exposure.

Hirschfeldia incana, a pseudometallophyte belonging to the Brassicaceae family and widespread in the Mediterranean region, was selected for its ability to grow on soils contaminated by lead (Pb). The global comparison of gene expression using microarrays between a plant susceptible to Pb (Arabidopsis thaliana) and a Pb tolerant plant (H. incana) enabled the identification of a set of specific genes expressed in response to lead exposure. Three groups of genes were particularly over-represented by the Pb exposure in the biological processes categorized as photosynthesis, cell wall, and metal handling. Each of these gene groups was shown to be directly involved in tolerance or in protection mechanisms to the phytotoxicity associated with Pb. Among these genes, we demonstrated that MT2b, a metallothionein gene, was involved in lead accumulation, confirming the important role of metallothioneins in the accumulation and the distribution of Pb in leaves. On the other hand, several genes involved in biosynthesis of ABA were shown to be up-regulated in the roots and shoots of H. incana treated with Pb, suggesting that ABA-mediated signaling is a possible mechanism in response to Pb treatment in H. incana. This latest finding is an important research direction for future studies.

Effect of lead on root growth.

Lead (Pb) is one of the most widespread heavy metal contaminant in soils. It is highly toxic to living organisms. Pb has no biological function but can cause morphological, physiological, and biochemical dysfunctions in plants. Plants have developed a wide range of tolerance mechanisms that are activated in response to Pb exposure. Pb affects plants primarily through their root systems. Plant roots rapidly respond either (i) by the synthesis and deposition of callose, creating a barrier that stops Pb entering (ii) through the uptake of large amounts of Pb and its sequestration in the vacuole accompanied by changes in root growth and branching pattern or (iii) by its translocation to the aboveground parts of plant in the case of hyperaccumulators plants. Here we review the interactions of roots with the presence of Pb in the rhizosphere and the effect of Pb on the physiological and biochemical mechanisms of root development.

Lead tolerance and accumulation in Hirschfeldia incana, a Mediterranean Brassicaceae from metalliferous mine spoils.

Lead is a heavy metal of particular concern with respect to environmental quality and health. The lack of plant species that accumulate and tolerate Pb is a limiting factor to understand the molecular mechanisms involved in Pb tolerance. In this study we identified Hirschfeldia incana, a Brassicaceae collected from metalliferous mine spoils in Morocco, as a Pb accumulator plant. H. incana exhibited high Pb accumulation in mine soils and in hydroponic cultures. Major Pb accumulation occurred in the roots and a part of Pb translocated from the roots to the shoots, even to the siliques. These findings demonstrated that H. incana is a Pb accumulator species. The expression of several candidate genes after Pb-exposure was measured by quantitative PCR and two of them, HiHMA4 and HiMT2a, coding respectively for a P1B-type ATPase and a metallothionein, were particularly induced by Pb-exposure in both roots and leaves. The functional characterization of HiHMA4 and HiMT2a was achieved using Arabidopsis T-DNA insertional mutants. Pb content and primary root growth analysis confirmed the role of these two genes in Pb tolerance and accumulation. H. incana could be considered as a good experimental model to identify genes involved in lead tolerance and accumulation in plants.