9.4 Heavy metal toxicity and oxidative stress

Heavy metal toxicity poses a significant threat to plant health, triggering oxidative stress and cellular damage. Plants have evolved sophisticated mechanisms to combat these stressors, including metal-binding proteins, antioxidant systems, and vacuolar sequestration.

Understanding these defense mechanisms is crucial for developing strategies to enhance plant tolerance to heavy metals. This knowledge also has practical applications in phytoremediation, where plants are used to clean up contaminated soils and water bodies.

Metal-Binding Proteins

Phytochelatins and Metallothioneins

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• Phytochelatins are small, cysteine-rich peptides synthesized in plants in response to heavy metal stress
  • Bind to and sequester heavy metal ions, reducing their toxicity
  • Synthesized from glutathione, a tripeptide consisting of glutamate, cysteine, and glycine
  • Examples of heavy metals that induce phytochelatin synthesis include cadmium, lead, and mercury
• Metallothioneins are low molecular weight, cysteine-rich proteins that bind to heavy metals
  • Found in a wide range of organisms, including plants, animals, and fungi
  • Play a role in metal homeostasis and detoxification
  • In plants, metallothioneins are involved in the tolerance to heavy metals such as copper, zinc, and cadmium

Metal Chelation and Vacuolar Sequestration

• Metal chelation is the process by which metal ions are bound to organic compounds, forming stable complexes
  • Chelation reduces the bioavailability and toxicity of heavy metals
  • Organic acids, such as citrate and malate, can act as chelators in plants
  • Nicotianamine, a non-proteinogenic amino acid, is another important chelator in plants, particularly for iron and zinc
• Vacuolar sequestration involves the transport of heavy metal ions into the vacuole, a large, membrane-bound organelle in plant cells
  • Vacuoles act as a storage site for heavy metals, isolating them from sensitive cellular components
  • Transport of metal-phytochelatin complexes into the vacuole is mediated by ATP-binding cassette (ABC) transporters
  • Examples of plants that utilize vacuolar sequestration for heavy metal tolerance include Thlaspi caerulescens (cadmium) and Arabidopsis halleri (zinc)

Oxidative Stress Response

Reactive Oxygen Species (ROS) and Antioxidant Enzymes

• Heavy metal stress can lead to the production of reactive oxygen species (ROS), such as superoxide ($O_2^{•-}$), hydrogen peroxide ($H_2O_2$), and hydroxyl radicals ($OH^•$)
  • ROS are highly reactive molecules that can damage cellular components, including proteins, lipids, and DNA
  • Heavy metals can generate ROS through various mechanisms, such as Fenton reactions and displacement of essential metal cofactors in enzymes
• Plants have evolved antioxidant defense systems to combat ROS and maintain cellular redox homeostasis
• Superoxide dismutase (SOD) is an enzyme that catalyzes the conversion of superoxide to hydrogen peroxide and oxygen
  • SOD exists in multiple isoforms, each containing a specific metal cofactor (Cu/Zn-SOD, Mn-SOD, Fe-SOD)
  • Overexpression of SOD has been shown to enhance tolerance to heavy metal stress in various plant species (Arabidopsis thaliana, Nicotiana tabacum)
• Catalase is an enzyme that catalyzes the decomposition of hydrogen peroxide to water and oxygen
  • Catalase is located primarily in peroxisomes and is involved in the detoxification of hydrogen peroxide generated by photorespiration and fatty acid β-oxidation
• Peroxidases are a diverse group of enzymes that catalyze the reduction of hydrogen peroxide using various electron donors
  • Ascorbate peroxidase (APX) uses ascorbate as an electron donor and is a key enzyme in the ascorbate-glutathione cycle, a major antioxidant pathway in plants
  • Glutathione peroxidase (GPX) uses glutathione as an electron donor and is involved in the detoxification of lipid hydroperoxides

Antioxidant Molecules

Glutathione and Ascorbate

• Glutathione (GSH) is a tripeptide consisting of glutamate, cysteine, and glycine
  • Acts as a major redox buffer in plant cells and is involved in various cellular processes, including heavy metal detoxification and ROS scavenging
  • Reduced glutathione (GSH) can directly scavenge ROS or act as an electron donor for glutathione peroxidase (GPX) and glutathione S-transferase (GST)
  • Oxidized glutathione (GSSG) is reduced back to GSH by glutathione reductase (GR) using NADPH as an electron donor
• Ascorbate (vitamin C) is a water-soluble antioxidant molecule that plays a crucial role in plant stress responses
  • Acts as an electron donor for ascorbate peroxidase (APX) in the ascorbate-glutathione cycle
  • Can directly scavenge ROS, such as superoxide, hydrogen peroxide, and hydroxyl radicals
  • Regenerated from its oxidized form (dehydroascorbate) by dehydroascorbate reductase (DHAR) using glutathione as an electron donor
  • Examples of plants with high ascorbate levels include Arabidopsis thaliana, spinach, and kiwifruit

Phytoremediation

Using Plants to Clean Up Heavy Metal Contamination

• Phytoremediation is the use of plants to remove, stabilize, or detoxify contaminants in soil, water, or air
  • Phytoextraction: plants absorb and accumulate heavy metals in their above-ground biomass, which can then be harvested and disposed of safely
  • Phytostabilization: plants reduce the mobility and bioavailability of heavy metals in soil through root exudates and changes in soil chemistry
  • Phytovolatilization: plants convert heavy metals into volatile forms that are released into the atmosphere
• Hyperaccumulator plants are species that can accumulate exceptionally high levels of heavy metals in their tissues without exhibiting toxicity symptoms
  • Examples of hyperaccumulators include Noccaea caerulescens (cadmium), Pteris vittata (arsenic), and Alyssum murale (nickel)
  • These plants have evolved various mechanisms for heavy metal tolerance, such as enhanced metal uptake, translocation, and sequestration
• Genetic engineering approaches can be used to enhance the phytoremediation potential of plants
  • Transgenic plants expressing genes for metal-binding proteins (phytochelatins, metallothioneins) or metal transporters have shown improved heavy metal tolerance and accumulation
  • Examples include transgenic Arabidopsis expressing yeast metallothionein genes (CUP1, CRS5) and transgenic tobacco expressing a bacterial mercuric reductase gene (merA)