Heavy Metal Tolerance in Plants and Algae

Dear Colleagues,

Heavy metals represent an important constraint for living organisms in water and on land. Although some of them are useful as trace elements for plant and algae metabolism, they are very toxic when absorbed in large quantity. Heavy metal soil and water contamination due to natural and, above all, to anthropogenic activity have a strong impact both on crop production and on natural ecosystems, ultimately affecting the health of living organisms, food availability and life of whole ecosystems. Being sessile organisms, plants cannot escape unwanted changes in their environment and have evolved a series of mechanisms allowing to cope with heavy metal toxicity and to acquire tolerance toward them. Plants could adopt different strategies including lower accumulation, sequestration in inert compartments, chelation, and mitigation of negative effects through reduction of oxidative stress or chemical conversion of the stressor agents. Understanding how plants can tolerate heavy metals is crucial, especially in this period of important challenges driven by a strong requirement of environmental sustainability. Research in this area is driven by the hope to reduce the heavy metals uptake not only in crops, but also in wild plants, thereby decreasing the risk of contamination in animals and human beings. Understanding these mechanisms will open the way to the production of hypo-accumulator crops and hyper-accumulator plants to be addressed to phytodepuration. Currently, many studies have being carried out to address the onset of metal tolerance focused on tools taking into consideration transcriptomics (transcriptome), proteomics (proteome), ionomics (trace elements), and metabolomics (metabolome). In this Special Issue, articles (original research papers or reviews) that focus on heavy metal sensing, uptake, detoxification, involving biochemistry, physiology, genes, proteins, and metabolites and how these tolerance mechanisms evolved in different classes of plant organisms are welcome.

Prof. Dr. Anna Torelli
Dr. Matteo Marieschi
Guest Editors

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• heavy metal tolerance
• heavy metal sensing
• heavy metal uptake
• heavy metal sequestration
• phytochelatin
• glutathione and oxidative stress
• cysteine synthesis and degradation
• heavy metal tolerance evolution
• hyper and hypo accumulator plants
• environmental pollutants
• stress mitigation

BRASSICA NAPUS ASSISTED BIOREMEDIATION FOR RECOVERING A MULTI-CONTAMINATED SOIL: THE ROLE OF ORGANIC AMENDMENTS IN ENHANCING SOIL RESTORATION

Plant-assisted bioremediation (PABR) techniques for the recovery of areas contaminated by organic and inorganic pollutants have become an interesting, effective and eco-sustainable solution. In recent decades, the potential of some plant species to promote the processes of transformation and extraction of different types of contaminants has been explored. The Brassicaceae family is one of the most studied in the use of PABR techniques. Brassica napus L. has proved to be a particularly suitable species for this purpose, thanks to its high tolerance to high concentrations of contaminants and the ability to phyto-extract heavy metals. Furthermore, for the discourse of circular economy and recovery, the biomass of Brassica napus can be used to obtain valuable secondary products, such as biofuel and ethanol. In this study, the effectiveness of the removal of contaminants from historically contaminated soil, not only from heavy metals but also from persistent organic compounds such as polychlorinated biphenyls (PCBs), was investigated by Brassica napus. Two types of organic amendments, compost, and biochar, were also tested to observe their effectiveness in promoting those processes related to PABR techniques for the decontamination of polluted sites, as suggested by previous studies in which it was observed that these two organic compounds can improve the properties of the soil and stimulate both the growth of plants and microbial communities naturally present in the test soil. The experimental set-up involved the use of Brassica napus in three laboratory-scale conditions in the greenhouse: contaminated soil, contaminated soil and compost (10%), contaminated soil, and biochar (2.5%). Each condition also provided for controls without the plant. The analyses carried out at each sampling time (0 and 80 days) soil chemical (humidity, pH, electrical conductivity, assimilable phosphorus, organic carbon), microbiological, molecular, and contaminants analyses (PCBs and heavy metals), were performed on soil samples. The assessment of pollutants was also conducted on plant tissues (root system, stem, siliques, and seeds) to calculate the Translocation and Bioaccumulation Factor of pollutants in plants. The achieved results evidenced interesting PABR capabilities of Brassica Napus in recovering soil from heavy metal contamination, especially in the presence of organic amendments. With regard of PCB contamination, a decrease in dioxin-like congeners and an increase in markers and non-dioxin-like ones has been observed when biochar or compost is added in planted microcosms. Biochar has proved effective as it has a structure with a high surface area, an ideal niche for microbial communities, and is able to increase the bioavailability of nutrients for plant uptake. The results also showed a significant increase in the microbial community and its activity in all conditions compared to the initial time.

Bio-monitoring of metal(loid)s pollution in dry riverbeds affected by mining activity

The aim of this study was to evaluate the behavior of the most abundant native plant that could be used as bio-monitor of metal(loid)s concentration in dry riverbeds affected by mining activities. Three plants species that occur naturally in these polluted ecosystems and their respective rhizospheric soils were sampled from El Beal (Piptatherum miliaceum, 15 sampling points), La Carrasquilla (Foeniculum vulgare, 10 sampling points), and Ponce (Dittrichia viscosa, 12 sampling points) dry riverbeds from the mining district of Cartegena-La Unión (SE Spain).  Plants categorized as bio-monitor were established according to the bioaccumulation factor (BF) or mobility ratio (MR) and linear correlations between metal(loid)s concentrations in plants (root or stem)-rhizospheric soils. Other indices were also determined to corroborate: Contamination factor (Cf), Pollution load index (PLI), Potential ecological risk index (RI), and Translocation factor (TF). In the dry riverbeds, the rhizospheric soil was very highly contaminated for As, Cd, Pb, and Zn (Cf > 6), moderately contaminated for Mn (1 < Cf ≤ 3), and lowly contaminated for Cr and Ni (Cf < 1). Piptatherum miliaceum presented on Cd similar means concentrations on rhizospheric soil and root, BF=1.07 with a strong correlation soil-root (r=0.61, p=0.02).  Foeniculum vulgare showed the highest BF values for Cr (0.30) with no significant correlation between rhizospheric soil and root. Therefore, of the three species with the capacity to grow in anthropically altered areas, Piptatherum miliaceum showed characteristics to be categorized as bio-monitor for mine waste contamination-related metal(loid)s, with a BF above 1, and a positive-significant correlation between the rhizospheric soil -roots.

Phytochelatin synthase: a comparative analysis in cyanobacteria and eukaryotic microalgae.

Phytochelatins (PCs) are small cysteine-rich peptides involved in metal detoxification, not genetically encoded but enzymatically synthesized by Phytochelatin Synthase (PCS) starting from glutathione. The constitutive PCS expression even in the absence of metal contamination, the wide phylogenetic distribution and the similarity between PCS and the papain type cysteine proteases catalytic domain suggest a wide range of functions for PCS. These proteins, widely studied in land plants, have not been fully analyzed in algae and cyanobacteria although these organisms are the first to cope with heavy metal stress in aquatic environments and can be exploited for phytoremediation. To fill this gap, we compared the features of PCS proteins of different cyanobacteria and algal taxa by phylogenetic linkage. The analyzed sequences fall into two main, already known, groups of PCS-like proteins. Contrary to previous assumptions, they do not divide into prokaryotic and eukaryotic sequences, but rather into sequences characterized by the alternative presence of asparagine and aspartic/glutamic acid residues in proximity of the catalytic cysteine. The presence of these enzymes with peculiar features suggests differences in their post-translational regulation related to cell/environmental requirements or different cell functions rather than to differences due to their belonging to different phylogenetic taxa.