International Journal of Aquaculture, 2024, Vol.14, No.3, 165-173 http://www.aquapublisher.com/index.php/ija 166 phytohormones and other plant defense systems in mitigating heavy metal toxicity, and identify future research directions and technological advancements needed to enhance the effectiveness of phytoremediation in heavy metal-contaminated water bodies. By addressing these objectives, this study seeks to contribute to the growing body of knowledge on phytoremediation and support the development of innovative and sustainable approaches for managing heavy metal pollution in aquatic environments. 2 Physiological Adaptations to Heavy Metals 2.1 Cellular mechanisms of tolerance Aquatic plants employ several cellular mechanisms to tolerate heavy metals. These mechanisms include the binding of metals to cell walls and extracellular exudates, reducing metal uptake or pumping metals out of cells via efflux transporters, chelation of metals in the cytosol by peptides such as phytochelatins, repairing stress-damaged proteins, and compartmentalizing metals in vacuoles through tonoplast-located transporters. Mycorrhizal associations also play a significant role in restricting heavy metal uptake by plants (Kushwaha et al., 2016). Additionally, the secretion of substances into the soil and metal immobilization are crucial for avoiding metal uptake (Skuza et al., 2022). 2.2 Biochemical pathways involved The biochemical pathways involved in heavy metal tolerance include the synthesis of metal-binding peptides like phytochelatins and metallothioneins, which chelate and sequester heavy metals, thereby reducing their toxicity (Kushwaha et al., 2016). Organic acids and amino acids also play a role in chelation and detoxification processes. The expression of genes encoding heavy metal transporters, such as the ZIP family, Nramp, and P1B-type ATPase, is crucial for regulating metal uptake and transport within plant cells. Additionally, the production of heat-shock proteins helps in the repair and stabilization of proteins damaged by heavy metal stress (Skuza et al., 2022). 2.3 Structural and morphological changes Aquatic plants exhibit various structural and morphological changes to mitigate the effects of heavy metal stress. These changes include modifications to the cell wall, which can bind and immobilize heavy metals, thereby preventing their entry into the cytoplasm (Figure 1) (Kosakivska et al., 2020; Skuza et al., 2022). The development of thicker cell walls and increased production of root exudates can alter the rhizosphere's pH and redox state, reducing metal bioavailability. Furthermore, the formation of symbioses with rhizosphere microorganisms can enhance metal tolerance by promoting metal sequestration and reducing metal uptake (Tiwari and Lata, 2018; Skuza et al., 2022). In summary, aquatic plants have evolved a range of physiological adaptations to tolerate heavy metal stress. These adaptations involve complex cellular mechanisms, biochemical pathways, and structural changes that work together to mitigate the toxic effects of heavy metals and ensure plant survival in contaminated environments. 3 Detoxification Strategies in Aquatic Plants 3.1 Chelation and sequestration mechanisms Chelation and sequestration are primary detoxification strategies employed by aquatic plants to mitigate heavy metal toxicity. Chelation involves the binding of heavy metals to organic molecules, rendering them less toxic. Phytochelatins (PCs) and metallothioneins (MTs) are key chelators in plants. PCs are synthesized from glutathione and play a significant role in binding heavy metals and facilitating their sequestration into vacuoles (Kushwaha et al., 2016). Metallothioneins, on the other hand, are gene-encoded proteins that also bind heavy metals, aiding in their detoxification and homeostasis. Additionally, the compartmentalization of heavy metals within vacuoles is a critical sequestration mechanism that prevents the metals from interfering with cellular processes (Kushwaha et al., 2016).
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