Effects of iron nanoparticle on plants development and its environmental implications

Abstract

This study explores various effects of iron nanoparticle on plants development that have not revealed yet before. Iron nanoparticle used in the present study is nano zero-valent iron(nZVI) that widely has been applied in fields of groundwater remediation. nZVI has shown outstanding ability that can degrade not only some organic pollutants such as trichloroethylene, tetra chloride with redox reaction, but also generate OH radicals in aerobic conditions. This is the reason nZVI has been widely used more than 2 decades and the usage of it has been increasing still now in the environmental fields. Continuously increased release of the nanomaterials into our ecosystem has raised a need for examination of their unknown toxicity. nZVI is the only nano-material injected in subsurface intentionally, thereby its unknown impacts on ecosystem should be investigated. Herein, we report the effects of nZVI on plants regarding to development such as root elongation and stomatal opening, and its environmental implications.

1. We investigated the effect of nZVI on plant root elongation in Arabidopsis thaliana and showed for the first time, that nZVI enhanced root elongation by inducing OH radical induced cell wall loosening. Exposure of plants to 0.5 g/L nZVI enhanced root elongation by 150−200% over that in the control, and further mechanistic studies showed that this occurred via nZVI-mediated OH radical-induced cell wall loosening. The oxidation capacity of nZVI, leading to release of H2O2, allowed it to cause OH radical-induced cell wall loosening in roots. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometers (MALDITOFMS)-based analysis clearly revealed that pectin-polysaccharides in roots were degraded; they are one of the main matrix-polysaccharide-connecting and load-bearing polymers in cell walls. Rapid root elongation led to structural changes in root cell walls: reduction of cell wall thickness and a bias on the orientation of cellulose.

2. The aim of this study was to see whether nZVI-induced cell wall loosening occurred in another plant species and soil. We found that root was elongated in Alfalfa and soil, and that pectins in the root cell wall were degraded. From those result, germination index was evaluated using 5 days old seedlings. The nZVI-treated plants showed slightly higher value in germination index (1.4, control=1) due to its longer root length. Because the germination index comprises relative root length and germination index. In leaf, chlorophyll content were measured and the result confirmed that content of chlorophyll was increased by nZVI. Released iron from nZVI and the particles uptaken in the leaf possibly contributed the increased chlorophyll. Overall, these data suggest that these evaluated phytotoxicity parameters could be useful for investigations of phytotoxicity effects of nZVI on plants grown in soil as well as for researches aimed at identifying damages on plants induced by other nano materials exposed to soil.

3. In plants, there are reports that exposure to engineered nanomaterials produces effects distinct from those induced by their bulk counterparts, such as increase in biomass and chlorophyll, but few studies have addressed the mechanisms underlying such physiological effects. The current investigation found that the exposure of Arabidopsis thaliana to nano zero valent iron (nZVI) triggered high plasma membrane H+-ATPase activity. This increase in activity caused a decrease in apoplastic pH, an increase in leaf area and also wider stomatal aperture. Analysis of gene expression indicated that the level of the H+-ATPase isoform responsible for stomatal opening, AHA2, was 5-fold higher in plants exposed to nZVI than in the control plants unexposed to nZVI. This is the first study to show that nZVI enhances stomatal opening by inducing the activation of plasma membrane H+-ATPase, leading to the possibility of increased CO2 uptake.