International Journal of Aquaculture, 2024, Vol.14, No.3, 165-173 http://www.aquapublisher.com/index.php/ija 168 4 Molecular and Genetic Mechanisms 4.1 Gene expression and regulation Plants have evolved various adaptive mechanisms to cope with heavy metal stress, including the regulation of gene expression. Key genes involved in heavy metal tolerance include those encoding for metallothioneins (MTs), phytochelatins (PCs), and various transporters. Metallothioneins and phytochelatins play crucial roles in chelating heavy metals, thereby reducing their toxicity (Kosakivska et al., 2020). Genes such as the ZIP (ZRT IRT related proteins) family, natural resistance-associated macrophage proteins (Nramp), and P1B-type ATPase family have been identified and cloned, demonstrating their roles in heavy metal uptake and sequestration. Additionally, the expression of genes encoding γ-glutamyl-cysteine synthetase, which is involved in the synthesis of phytochelatins, has been shown to contribute to heavy metal tolerance. 4.2 Molecular markers for tolerance Molecular markers are essential tools for identifying and breeding plants with enhanced heavy metal tolerance. These markers can be used to track the presence of specific genes associated with metal tolerance and accumulation. For instance, genes encoding ABC-type (ATP-binding cassette) transporters and cation diffusion facilitators (CDF) have been identified as key players in the sequestration of heavy metals in vacuoles (Kosakivska et al., 2020). The identification and use of these molecular markers can facilitate the development of plants with improved heavy metal tolerance through marker-assisted selection (Ovečka and Takáč, 2014). 4.3 Genetic engineering approaches Genetic engineering offers a promising approach to enhance heavy metal tolerance in plants. By introducing genes that encode for metal-binding proteins or transporters, plants can be engineered to improve their capacity for heavy metal uptake, translocation, and detoxification. For example, the genetic modification of metallothioneins has been shown to enhance the tolerance and bioaccumulation of heavy metals in Escherichia coli, suggesting similar strategies could be applied to plants (Li et al., 2021). Additionally, the heterologous expression of genes involved in sulfur metabolism and metal transport has been demonstrated to improve the phytoremediation potential of plants (Fasani et al., 2018). Recent advancements in genome editing technologies, such as CRISPR/Cas9, provide further opportunities to precisely modify plant genomes for enhanced heavy metal tolerance (Khan et al., 2021). 5 Case Studies of Heavy Metal Tolerance 5.1 Tolerance in freshwater plants Freshwater plants have evolved various mechanisms to tolerate and detoxify heavy metals, ensuring their survival in polluted environments. A notable example is Canadian waterweed (Elodea canadensis), which exhibits high tolerance to cadmium (Cd) by activating antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT). These enzymes mitigate oxidative stress caused by heavy metal accumulation. Additionally, the sequestration of Cd into vacuoles and binding to phytochelatins (PCs) further reduces its toxicity (Goyal et al., 2020). Another significant case is duckweed (Lemna minor), known for its rapid growth and ability to accumulate heavy metals like lead (Pb) and zinc (Zn). Ubuza et al. (2019) found that the small duckweed can effectively remove Pb from water (Figure 1). The production of metal-binding proteins, such as metallothioneins (MTs), plays a crucial role in its detoxification strategy. Figure from Ubuza et al. (2019) illustrates the findings on the use of duckweed for removing Pb from water. Panel B shows the highest bioaccumulation of 1.57 mg/L achieved at 3 d in the recirculated set-up. This result implies the efficient accumulation capabilities of duckweed as influenced by the recirculation mechanism. Panel C outlines the concentration of Pb in the effluent of 0.93 mg/L in the recirculated set-up with duckweed in 3 d was much lower compared to the initial concentration in the influent at 2.5 mg/L. This result underscores the potential of duckweed in phytoremediation, with recirculated systems enhancing metal removal efficiency.
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