Impacts of ZnO Nanoparticles on Plants: Cytotoxicity, Genotoxicity, Antioxidant Defense Deregulation, and Cell-Cycle Arrest -

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Impacts of ZnO Nanoparticles on Plants: Cytotoxicity, Genotoxicity, Antioxidant Defense Deregulation, and Cell-Cycle Arrest

Introduction:

ZnO nanoparticles (NPs) are widely used in various products due to their excellent UV absorption and reflective properties. However, their increasing production and wide range of applications raise concerns about their potential environmental exposure and toxicity. This study investigated the toxicity of ZnO NPs in three plant species, Allium cepa, Nicotiana tabacum, and Vicia faba. The results showed that ZnO NPs induced cytotoxicity, genotoxicity, oxidative stress, and altered antioxidant enzyme activities in all three plant species. These findings suggest that ZnO NPs have the potential to cause adverse effects on plants and should be handled with caution.

Methodology:-

Plant materials and treatment:

Used Allium cepa as the primary system, Vicia faba and Nicotiana  tabacum  for confirmation in some assays. Germinated seeds in appropriate media until reaching specific growth stages.

Exposed roots to ZnO NPs (0.2, 0.4, 0.8 g/L) in water for 24h in well-aerated vials. Used bulk ZnO particles as a control. Conducted experiments at room temperature.

Nanocharacterization:

Determined ZnO NP size and morphology using Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Measured  hydrodynamic diameter using dynamic light scattering (DLS). Quantified Zn2+ ion release from ZnO NPs using atomic absorption spectroscopy.

Cytotoxicity assay (Evans blue): Stained roots with Evans blue to assess cell viability loss.

Measured absorbance at 600 nm after N,N-dimethylformamide extraction. Used  Triton X-100 as a positive control.

Genotoxicity assays: Chromosome aberration and micronucleus assay: Analyzed root cells for chromosomal aberrations and micronuclei formation. Used maleic   hydrazide as a positive control.

Comet assay: Evaluated DNA damage in root and leaf cells using the alkaline comet assay. Analyzed tail DNA  percentage as a measure of damage. Used EMS as a positive control.

Oxidative stress and antioxidant enzyme activity: Measured H2O2 content, lipid peroxidation, and intracellular ROS production. Analyzed activities of antioxidant enzymes catalase  (CAT) and guaiacol peroxidase (GPOD). Determined  total thiol, protein thiol, and GSH content.

Cell cycle analysis: Used flow cytometry to analyze cell cycle distribution in root cells after propidium iodide staining.

Transmission Electron Microscopy (TEM): Analyzed root samples using TEM to visualize ZnO NP uptake and structural alterations.

Results: 

Cytotoxicity: Increased cell death observed in all three plant species (Allium cepa, Vicia faba, Nicotiana tabacum) with ZnO nanoparticle exposure.

Genotoxicity: DNA damage detected in all species, with higher sensitivity in A. cepa compared to N. tabacum. Micronucleus formation and chromosome aberrations also observed, suggesting both clastogenic and aneugenic effects.

Oxidative stress: Increased production of reactive oxygen species (ROS) and lipid peroxidation in A. cepa, indicating oxidative stress.

Antioxidant enzyme activity: Increased activity of guaiacol peroxidase (GPOD) but decreased activity of catalase, suggesting an adaptive response to oxidative stress. Cell cycle: ZnO nanoparticles disrupted cell cycle progression in A. cepa, leading to arrest at G2/M checkpoint and accumulation in sub G0 phase.

Uptake and cell death: Nanoparticles were observed along the plasma membrane and within vacuoles of A. cepa root cells. Extensive vacuolation and loss of nuclear organization suggested apoptotic/necrotic cell death.

Conclusion:

These findings strongly suggest that ZnO NPs disrupt the ROS-antioxidant balance in plants, leading to DNA damage, cell cycle arrest, and ultimately, cell death. Notably, A. cepa proved particularly sensitive and reliable for assessing nanoparticle toxicity. Overall, this study highlights the potential risks of ZnO NPs to plant health and emphasizes the need for further investigation into their environmental impact.

Reference:

GHOSH, M., JANA, A., SINHA, S., JOTHIRAMAJAYAM, M., NAG, A., CHAKRABORTY, A., MUKHERJEE, A. AND MUKHERJEE, A., 2016. Effects of ZnO nanoparticles in plants: cytotoxicity, genotoxicity, deregulation of antioxidant defenses, and cell-cycle arrest. Mutation Research/Genetic Toxicology and Environmental Mutagenesis, 807, pp.25-32.