Metal-Based Nanotoxicity and Detoxification Pathways in Higher Plants

The potential risks from metal-based nanoparticles (NPs) in the environment have increased with the rapidly rising demand for and use of nanoenabled consumer products. Plant’s central roles in ecosystem function and food chain integrity ensure intimate contact with water and soil systems, both of which are considered sinks for NPs accumulation. In this review, we document phytotoxicity caused by metal-based NPs exposure at physiological, biochemical, and molecular levels. Although the exact mechanisms of plant defense against nanotoxicity are unclear, several relevant studies have been recently published. Possible detoxification pathways that might enable plant resistance to oxidative stress and facilitate NPs detoxification are reviewed herein. Given the importance of understanding the effects and implications of metal-based NPs on plants, future research should focus on the following: (1) addressing key knowledge gaps in understanding molecular and biochemical responses of plants to NPs stress through global transcriptome, proteome, and metablome assays; (2) designing long-term experiments under field conditions at realistic exposure concentrations to investigate the impact of metal-based NPs on edible crops and the resulting implications to the food chain and to human health; and (3) establishing an impact assessment to evaluate the effects of metal-based NPs on plants with regard to ecosystem structure and function

Physiological responses of wheat seedlings upon exposure to different levels of CO₂.

<p>Values are presented as mean±SD, error bars represent standard deviation (sample size, n = 64 under super-elevated CO₂ condition and n = 48 under normal CO₂ condition). Lower letters represent significant difference at p<0.05 between super-elevated and normal CO₂ treatments.</p

TEM image and particle size distribution of TiO₂ NPs.

<p>TEM image and particle size distribution of TiO₂ NPs.</p

Phenotypic images of wheat seedlings in different concentrations of TiO2 NPs treatments with or without super elevated CO₂.

<p>(I) Seedlings grown in different concentrations of TiO₂ NPs under normal CO₂ conditions in a plant growth chamber. (II) Seedlings grown in different concentrations of TiO₂ NPs under super-elevated CO₂ conditions.</p

Effects of TiO₂ NPs on seedling biomass and number of lateral roots.

<p>Values are mean±SD, error bars represent standard deviation (sample size, n = 12 for I and II, n = 16 for III, IV and V). Lower letters represent significant difference at p<0.05 among TiO₂ NPs treatments under the same CO₂ conditions; Upper letters represent significant difference at p<0.05 between super-elevated CO₂ and normal CO₂ conditions at the same TiO₂ NPs concentration.</p

Effects of TiO₂ nanoparticles on wheat (<i>Triticum aestivum L</i>.) seedlings cultivated under super-elevated and normal CO₂ conditions

<div><p>Concerns over the potential risks of nanomaterials to ecosystem have been raised, as it is highly possible that nanomaterials could be released to the environment and result in adverse effects on living organisms. Carbon dioxide (CO₂) is one of the main greenhouse gases. The level of CO₂ keeps increasing and subsequently causes a series of environmental problems, especially for agricultural crops. In the present study, we investigated the effects of TiO₂ NPs on wheat seedlings cultivated under super-elevated CO₂ conditions (5000 mg/L CO₂) and under normal CO₂ conditions (400 mg/L CO₂). Compared to the normal CO₂ condition, wheat grown under the elevated CO₂ condition showed increases of root biomass and large numbers of lateral roots. Under both CO₂ cultivation conditions, the abscisic acid (ABA) content in wheat seedlings increased with increasing concentrations of TiO₂ NPs. The indolepropioponic acid (IPA) and jasmonic acid (JA) content notably decreased in plants grown under super-elevated CO₂ conditions, while the JA content increased with increasing concentrations of TiO₂ NPs. Ti accumulation showed a dose-response manner in both wheat shoots and roots as TiO₂ NPs concentrations increased. Additionally, the presence of elevated CO₂ significantly promoted Ti accumulation and translocation in wheat treated with certain concentrations of TiO₂ NPs. This study will be of benefit to the understanding of the joint effects and physiological mechanism of high-CO₂ and nanoparticle to terrestrial plants.</p></div

Effects of TiO₂ NPs on phytohormone contents in wheat seedlings grown under elevated-and normal CO₂ conditions.

<p>Data are mean±SD, error bars represent standard deviation (sample size, n = 16 for treatments under super-elevated CO₂ condition and n = 12 for treatments under normal CO₂ condition). Lower letters represent significant difference at p<0.05 among TiO₂ NPs treatments under the same CO₂ conditions; Upper letters represent significant difference at p<0.05 between elevated CO₂ and normal CO₂ conditions at the same TiO₂ NPs concentration.</p

Differentially Charged Nanoplastics Induce Distinct Effects on the Growth and Gut of Benthic Insects (Chironomus kiinensis) via Charge-Specific Accumulation and Perturbation of the Gut Microbiota

Nanoplastics (NPs), as an emerging contaminant, have usually been found charged in the environment, posing threats to aquatic animals. However, the underlying mechanisms governing the gut toxicity of differentially charged NPs to benthic insects are not well understood. In this study, the gut toxicity in larvae of Chironomus kiinensis exposed to negatively charged NPs (PS-COOH, 50 nm) and positively charged NPs (PS-NH2, 50 nm) at 0.1 and 1 g/kg was investigated through fluorescence imaging, histopathology, biochemical approaches, and 16S rRNA sequencing. The results showed that PS-NH2 caused more adverse effect on the larval growth performance and induced more severe oxidative stress, epithelial damage, and inflammatory responses in the gut than PS-COOH. The stronger impact caused by PS-NH2 was because the gut accumulated PS-NH2 more readily than PS-COOH for its negatively charged cell membrane. In addition, PS-NH2 were less agglomerated compared with PS-COOH, leading to an increased interaction with gut cell membranes and microbiota. Furthermore, alpha diversity and relative abundance of the keystone microbiota related to gut barrier and nutrient absorption were markedly lower exposed to PS-NH2 than PS-COOH, indirectly exacerbating stronger gut and growth damage. This study provides novel insights into the effect mechanisms underlying differentially charged NPs on benthic insects

Nanoplastics Affect the Bioaccumulation and Gut Toxicity of Emerging Perfluoroalkyl Acid Alternatives to Aquatic Insects (<i>Chironomus kiinensis</i>): Importance of Plastic Surface Charge

Persistent organic pollutants (POPs) have been widely suggested as contributors to the aquatic insect biomass decline, and their bioavailability is affected by engineered particles. However, the toxicity effects of emerging ionizable POPs mediated by differentially charged engineered nanoparticles on aquatic insects are unknown. In this study, 6:2 chlorinated polyfluoroalkyl ether sulfonate (F-53B, an emerging perfluoroalkyl acid alternative) was selected as a model emerging ionizable POP; the effect of differentially charged nanoplastics (NPs, 50 nm, 0.5 g/kg) on F-53B bioaccumulation and gut toxicity to Chironomus kiinensis were investigated through histopathology, biochemical index, and gut microbiota analysis. The results showed that when the dissolved concentration of F-53B remained constant, the presence of NPs enhanced the adverse effects on larval growth, emergence, gut oxidative stress and inflammation induced by F-53B, and the enhancement caused by positively charged NP-associated F-53B was stronger than that caused by the negatively charged one. This was mainly because positively charged NPs, due to their greater adsorption capacity and higher bioavailable fraction of associated F-53B, increased the bioaccumulation of F-53B in larvae more significantly than negatively charged NPs. In addition, positively charged NPs interact more easily with gut biomembranes and microbes with a negative charge, further increasing the probability of F-53B interacting with gut biomembranes and microbiota and thereby aggravating gut damage and key microbial dysbacteriosis related to gut health. These findings demonstrate that the surface charge of NPs can regulate the bioaccumulation and toxicity of ionizable POPs to aquatic insects

Physiological and Molecular Response of <i>Arabidopsis thaliana</i> (L.) to Nanoparticle Cerium and Indium Oxide Exposure

The effects of cerium oxide (CeO₂) and indium oxide (In₂O₃) nanoparticles (NPs) exposure on <i>Arabidopsis thaliana</i> (L.) Heynh. were investigated. After inoculation in half strength MS medium amended with 0–2000 ppm CeO₂ and In₂O₃ NPs for 25 days, both physiological and molecular responses were evaluated. Exposure at 250 ppm CeO₂ NPs significantly increased plant biomass, but at 500–2000 ppm, plant growth was decreased by up to 85% in a dose-dependent fashion. At 1000 and 2000 ppm CeO₂ NPs, chlorophyll production was reduced by nearly 60% and 85%, respectively, and anthocyanin production was increased 3–5-fold. Malondialdehyde (MDA) production, a measure of lipid peroxidation, was unaffected by exposure to 250–500 ppm CeO₂ NPs, but at 1000 ppm, MDA formation was increased by 2.5-fold. Exposure to 25–2000 ppm In₂O₃ NPs had no effect on <i>A. thaliana</i> biomass and only minor effects (15%) on root elongation. Total chlorophyll and MDA production were unaffected by In₂O₃ NPs exposure. Molecular response to NP exposure as measured by qPCR showed that both types of elements altered the expression of genes central to the stress response such as the sulfur assimilation and glutathione (GSH) biosynthesis pathway, a series of genes known to be significant in the detoxification of metal toxicity in plants. Interestingly, In₂O₃ NPs exposure resulted in a 3.8–4.6-fold increase in glutathione synthase (GS) transcript production, whereas CeO₂ NPs yielded only a 2-fold increase. It seems likely that the significantly greater gene regulation response upon In₂O₃ NPs exposure was directly related to the decreased phytotoxicity relative to CeO₂ treatment. The use of NP rare earth oxide elements has increased dramatically, yet knowledge on fate and toxicity has lagged behind. To our knowledge, this is the first report evaluating both physiological and molecular plant response from exposure to these important nanoparticles