International Journal of Aquaculture, 2024, Vol.14, No.3, 165-173 http://www.aquapublisher.com/index.php/ija 167 Figure 1 Cellular mechanisms of metal extraction/excretion and transportation through endocytosis - 1, exocytosis - 2, active transport - 3, diffusion - 4, and through ion channels (Photo credit: Skuza et al., 2022) Image caption: MBV - multivesicular body, Me2+ - divalent metal, GA - Golgi apparatus, Vs - transport vesicles, TGN - early endosomes, RE - recycling endosomes (Adopted from Skuza et al., 2022) 3.2 Antioxidant defense systems Heavy metal exposure often leads to the generation of reactive oxygen species (ROS), which can cause oxidative stress and damage cellular components. Aquatic plants have evolved robust antioxidant defense systems to counteract this oxidative stress. These systems include enzymes such as superoxide dismutase, catalase, and peroxidases, which neutralize ROS. The presence of antioxidants like glutathione and tocopherols further enhances the plant's ability to mitigate oxidative damage. The dynamic balance between ROS production and detoxification is crucial for maintaining cellular homeostasis and ensuring the plant's survival under heavy metal stress (Alsherif et al., 2021; Molina and Segura, 2021). 3.3 Role of phytochelatins and metallothioneins Phytochelatins (PCs) and metallothioneins (MTs) are pivotal in the detoxification of heavy metals in aquatic plants. PCs are synthesized enzymatically from glutathione and are involved in the chelation and sequestration of heavy metals into vacuoles. The synthesis of PCs is regulated by the enzyme phytochelatin synthase, which is activated in response to heavy metal exposure (Yadav, 2010). Metallothioneins, being gene-encoded proteins, also play a significant role in binding heavy metals and facilitating their detoxification. The coordinated expression of genes encoding PCs and MTs is essential for the effective detoxification of heavy metals, as demonstrated in studies on various plant species. These molecules not only help in detoxification but also contribute to the overall metal homeostasis within the plant cells.
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