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Nanotoxicology studies the toxicity of nanomaterials or nanoparticles. Materials that are toxic at the nanoscale might not be toxic at the macroscale.
Plants are exposed to the air and soil. Nanoparticles are adsorbed to the plant surface and then translocated within the plant body.
Plant nanotoxicology is the study of the toxicity mechanisms and effects of nanoparticles in plants. Several researchers have reported both positive and negative effects of nanoparticles on plants, reflecting the variety of plant species, nanoparticle properties (e.g., size, shape, type, structure and defects), surface coating, and culture media.
Phytotoxicology Induced by Nanoparticles
The production of reactive oxygen species (ROS), which are induced by nanoparticles either directly or indirectly, plays a vital role in the phytotoxicity mechanism.
The creation of ROS are primarily a result of the physicochemical properties of nanoparticles as well as plant species. Various factors, including size and shape, solubility, metal ions released from metal, biotransformation of nanoparticles, and light, may instigate ROS production and phytotoxicity.
ROS generally consist of free radicals (hydroxyl radical and superoxide radical) and non-free radicals (singlet oxygen and hydrogen peroxide). ROS is produced during aerobic metabolism in an ordinary plant, and act as signaling molecules.
Excess ROS causes oxidative stress that poses a threat to cells by inducing DNA damage, electrolyte leakage, protein oxidation, membrane damage, and lipid peroxidation, resulting in cell death.
Detoxification Mechanism of Plants
Excess production of ROS can cause oxidative stress in plants under nanoparticle exposure. Plant cells and their organelles (mitochondria, peroxisomes, and chloroplasts) develop an antioxidant defense system to use up excess ROS production, to protect themselves against these toxic oxygen intermediates.
Antioxidant defense systems of plants consist of bot nonenzymatic antioxidants and enzymatic antioxidants.
Antioxidant Defense System – Nonenzymatic Agents
Thiols and ascorbate (AA) are two of the most important low molecular weight antioxidants. AA resists oxidative stress induced by excess ROS production and acts as the first barrier of defense against the potential negative external oxidants. This is due to the ability of AA to donate electrons, which can scavenge hydroxyl radicals and regenerate α-tocopherol from the tocopheroxyl radical to protect membranes directly.
Glutathione (GSH) is a nonprotein thiol with low molecular weight, which plays a key role in intercellular antioxidant defense against ROS-induced oxidative stress in plants.
GSH is commonly found in all cell compartments of plant tissues, such as chloroplasts, mitochondria, vacuole, cytosol, peroxisomes and endoplasmic reticulum. This metabolite plays an important role in preserving the normal state of cells by coping with the oxidative damage induced by ROS.
GSH, an antioxidant, is a proton donor in the organic free radicals. In the presence of ROS, it scavenges ROS and is reduced to a disulfide form – oxidized glutathione (GSSG). GSH also regenerates another water-soluble antioxidant product (AA) using the AA–GSH cycle.
Phenolic compounds also possess similar antioxidant properties and can chelate transition metal ions, scavenge ROS species, and inhibit lipid peroxidation. Carotenoids are lipophilic antioxidants that can detoxify many forms of ROS. Tocopherols (α-, β-, γ- and δ-tocopherol) also belong to the family of lipophilic antioxidants.
Antioxidant Defense System – Enzymatic Components
Some of the enzymes involved in the antioxidant defense mechanism in plants are guaiacol peroxidase (GPOX), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione S-transferases (GST), superoxide dismutase (SOD), and glutathione peroxidase (GPX).
Oxidized GSH contains the main antioxidant enzyme pathway that would be activated to detoxify hydrogen peroxide (H2O2). GR can participate in both enzymatic and nonenzymatic oxidation-reduction cycles as an antioxidant. GR is also an NADPH-dependent enzyme that catalyzes the oxidation of GSH to GSSG, maintaining a high ratio of GSH/GSSG in the cells.
GPX, another catalyzer, uses GSH to reduce lipid hydroperoxides and organic hydroperoxides, exhibiting a positive effect in plants against environmental stress.
GR and DHAR are involved in the AA–GSH, which can control excess levels of ROS or oxidative state in plants. Antioxidant enzymes can be activated by a multitude of nanoparticles, such as Fe3O4 nanoparticles, Co3O4 nanoparticles and CeO2 nanoparticles, Co3O4 nanoparticles, and CeO2 nanoparticles.
A thorough understanding of the response of the nonenzymatic and enzymatic defense system in plants, when exposed to nanoparticles, is vital for the accurate assessment of potential risks of nanoparticles to plants.
Researchers are seeking to identify the mechanisms of plant defense against nanomaterial-induced oxidative stress. Knowledge of the potentially harmful effects of nanoparticles is severely behind the development of nanotechnology. Due to the variety of experimental approaches taken, it is impossible to compare results across different studies.
To address this problem, the nanotoxicology community is discussing the provision of general guidelines for nanotoxicology research and establishing common parameters that should be addressed in all subsequent nanotoxicological research.
References and Further Reading
- Yang, J., Cao W., and Rui, Y. (2017) Interactions between nanoparticles and plants: phytotoxicity and defense mechanisms, Journal of Plant Interactions, 12:1, 158-169.
- Karl-Josef, D., and Simone, H. (2011). Plant nanotoxicology. Trends in plant science. 16. 582-9. DOI: 10.1016/j.tplants.2011.08.003.