Chuanxin Ma

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. Plants 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.

Phytotoxic Mechanism of Nanoparticles: Destruction of Chloroplasts and Vascular Bundles and Alteration of Nutrient Absorption

This study focused on determining the phytotoxic mechanism of CeO2 nanoparticles (NPs): destroying chloroplasts and vascular bundles and altering absorption of nutrients on conventional and Bt-transgenic cottons. Experiments were designed with three concentrations of CeO2 NPs including: 0, 100 and 500 mg·L−1, and each treatment was three replications. Results indicate that absorbed CeO2 nanoparticles significantly reduced the Zn, Mg, Fe, and P levels in xylem sap compared with the control group and decreased indole-3-acetic acid (IAA) and abscisic acid (ABA) concentrations in the roots of conventional cotton. Transmission electron microscopy (TEM) images revealed that CeO2 NPs were absorbed into the roots and subsequently transported to the stems and leaves of both conventional and Bt-transgenic cotton plants via xylem sap. In addition, the majority of aggregated CeO2 NPs were attached to the external surface of chloroplasts, which were swollen and ruptured, especially in Bt-transgenic cotton. The vascular bundles were destroyed by CeO2 nanoparticles, and more damage was observed in transgenic cotton than conventional cotton.

Defense mechanisms and nutrient displacement in Arabidopsis thaliana upon exposure to CeO2 and In2O3 nanoparticles

Metal-based nanoparticles (NPs) can cause toxicity to terrestrial plants, however there is little understanding of plant defense mechanisms that may counteract nanotoxicity. In the present study, we investigated the defense mechanisms of Arabidopsis thaliana in response to 250 mg L−1 and 1000 mg L−1 cerium oxide (CeO2) and indium oxide (In2O3) NPs exposure. Excessive amounts of total reactive oxygen species (ROS) were measured upon exposure to both NPs, demonstrating clear oxidative stress in Arabidopsis. Analysis of ROS scavenger activity indicated that activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD) were significantly elevated upon exposure to CeO2 NPs, while these elevations were only evident for SOD and POD activities in the In2O3 NP treatments. In addition, the activities of glutathione S-transferase (GST) and glutathione reductase (GR) were increased by approximately 15% and 51% by 1000 mg L−1 CeO2 and In2O3 treatment, respectively. Furthermore, the activities of phenylanine ammonialyase (PAL) and polyphenol oxidase (PPO) were greatly induced in response to both types of NPs. Additionally, both NPs disrupted the uptake of elemental nutrients, as is evident from the significantly lower levels of Fe accumulation in Arabidopsis root tissues exposed to CeO2 and In2O3 NPs. These results were further supported by the differential regulation of three iron-regulating genes, including ferric chelate reductase (FRO), iron-regulated transporter (IRT) and ferritin (FER), at various time points. The findings provide useful mechanistic information for plant detoxification pathways following NP exposure.

Reduced Silver Nanoparticle Phytotoxicity in Crambe abyssinica with Enhanced Glutathione Production by Overexpressing Bacterial γ-Glutamylcysteine Synthase

Silver nanoparticles (Ag NPs) are widely used in consumer products, and their release has raised serious concerns about the risk of their exposure to the environment and to human health. However, biochemical mechanisms by which plants counteract NP toxicity are largely unknown. We have previously engineered Crambe abyssinica plants expressing the bacterial γ-glutamylecysteine synthase (γ-ECS) for enhancing glutathione (GSH) levels. In this study, we investigated if enhanced levels of GSH and its derivatives can protect plants from Ag NPs and AgNO3 (Ag(+) ions). Our results showed that transgenic lines, when exposed to Ag NPs and Ag(+) ions, were significantly more tolerant, attaining a 28%-46% higher biomass and 34-49% more chlorophyll content, as well as maintaining 35-46% higher transpiration rates as compared to those of wild type (WT) plants. Transgenic γ-ECS lines showed 2-6-fold Ag accumulation in shoot tissue and slightly lower or no difference in root tissue relative to levels in WT plants. The levels of malondialdehyde (MDA) in γ-ECS lines were also 27.3-32.5% lower than those in WT Crambe. These results indicate that GSH and related peptides protect plants from Ag nanotoxicity. To our knowledge, this is the first direct report of Ag NP detoxification by GSH in transgenic plants, and these results will be highly useful in developing strategies to counteract the phytotoxicty of metal-based nanoparticles in crop plants.

Size Effect on the Cytotoxicity of Layered Black Phosphorus and Underlying Mechanisms

A systematic cytotoxicity study of layered black phosphorus (BP) is urgently needed before moving forward to its potential biomedical applications. Herein, bulk BP crystals are synthesized and exfoliated into layered BP with different lateral size and thickness. The cytotoxicity of as-exfoliated layered BP is evaluated by a label-free real-time cell analysis technique, displaying a concentration-, size-, and cell type-dependent response. The IC50 values can vary by 40 and 30 times among the BP sizes and cell types, respectively. BP-1 with the largest lateral size and thickness has the highest cytotoxicity; whereas the smallest BP-3 only shows moderate toxicity. The sensitivity of three tested cell lines follows the sequence of 293T > NIH 3T3 > HCoEpiC. Two possible mechanisms for BP to induce cytotoxicity are proposed and verified: (1) the generation of intracellular reactive oxygen species (ROS) is detected by a ROS sensitive probe using the inverted fluorescence microscopy and flow cytometry; (2) the interaction of layered BP and model cell membrane is examined by quartz crystal microbalance with dissipation, illustrating the disruption of cell membrane integrity especially by the largest BP-1. This systematic study of BPs cytotoxicity will shed light on its future biomedical and environmental applications.

Uptake of Engineered Nanoparticles by Food Crops: Characterization, Mechanisms, and Implications

With the rapidly increasing demand for and use of engineered nanoparticles (NPs) in agriculture and related sectors, concerns over the risks to agricultural systems and to crop safety have been the focus of a number of investigations. Significant evidence exists for NP accumulation in soils, including potential particle transformation in the rhizosphere and within terrestrial plants, resulting in subsequent uptake by plants that can yield physiological deficits and molecular alterations that directly undermine crop quality and food safety. In this review, we document in vitro and in vivo characterization of NPs in both growth media and biological matrices; discuss NP uptake patterns, biotransformation, and the underlying mechanisms of nanotoxicity; and summarize the environmental implications of the presence of NPs in agricultural ecosystems. A clear understanding of nano-impacts, including the advantages and disadvantages, on crop plants will help to optimize the safe and sustainable application of nanotechnology in agriculture for the purposes of enhanced yield production, disease suppression, and food quality.

Tannic acid alleviates bulk and nanoparticle Nd2O3 toxicity in pumpkin: a physiological and molecular response

Abstract The effect of dissolved organic matter (DOM) on nanoparticle toxicity to plants is poorly understood. In this study, tannic acid (TA) was selected as a DOM surrogate to explore the mechanisms of neodymium oxide NPs (Nd2O3 NPs) phytotoxicity to pumpkin (Cucurbita maxima). The results from the tested concentrations showed that 100 mg L−1 Nd2O3 NPs were significantly toxic to pumpkin in term of fresh biomass, and the similar results from the bulk particles and the ionic treatments were also evident. Exposure to 100 mg L−1 of Nd2O3 NPs and BPs in 1/5 strength Hoagland’s solution not only significantly inhibited pumpkin growth, but also decreased the S, Ca, K and Mg levels in plant tissues. However, 60 mg L−1 TA significantly moderated the observed phytotoxicity, decreased Nd accumulation in the roots, and notably restored S, Ca, K and Mg levels in NPs and BPs treated pumpkin. TA at 60 mg L−1 increased superoxide dismutase (SOD) activity in both roots (17.5%) and leaves (42.9%), and catalase (CAT) activity (243.1%) in the roots exposed to Nd2O3 NPs. This finding was confirmed by the observed up-regulation of transcript levels of SOD and CAT in Nd2O3 NPs treated pumpkin analyzed by quantitative reverse transcription polymerase chain reaction. These results suggest that TA alleviates Nd2O3 BPs/NPs toxicity through alteration of the particle surface charge, thus reducing the contact and uptake of NPs by pumpkin. In addition, TA promotes antioxidant enzymatic activity by elevating the transcript levels of genes involved in ROS scavenging. Our results shed light on the mechanisms underlying the influence of DOM on the bioavailability and toxicity of NPs to terrestrial plants.

Molecular mechanisms of maize seedling response to La2O3 NP exposure: water uptake, aquaporin gene expression and signal transduction

Due to its increasing demands for use in medical, industrial, and agricultural products, concerns over the risks of lanthanum oxide nanoparticle (La2O3 NP) exposure have increased. As the dominant primary producers in terrestrial ecosystems, higher plants represent a sensitive receptor of concern but the mechanisms of La2O3 NP phytotoxicity remain unknown. In the present study, maize was selected as a model plant and the mechanisms underlying growth inhibition and reduced water uptake upon hydroponic exposure to La2O3 NPs (50–500 mg L−1) were investigated. The root and shoot abscisic acid (ABA) content was increased significantly (1.31–7.47 fold) upon exposure to 50 mg L−1 and 250 mg L−1 La2O3 NPs at 36 h and 72 h. The relative expressions of most aquaporin (AQP) genes in both roots and shoots were downregulated after 72 h and 144 h of 50 mg L−1 and 250 mg L−1 La2O3 NP exposure. Compared to the control, the expression level of PIP2;5 at 72 h was decreased by 93.28% in roots exposed to 250 mg L−1; the expression level of PIP1;2 was decreased by 89.85% at 144 h in shoots exposed to 250 mg L−1 La2O3 NPs. The downregulation of AQP genes led to the reduction of water uptake, subsequently causing significant growth inhibition. For example, upon exposure to 50 mg L−1 La2O3 NPs maize root biomass was decreased by 22.73% at 144 h as compared to the control. Additionally, root morphology was severely altered upon exposure to 250 mg L−1 La2O3 NPs as determined by root thickness, length, and surface integrity. To our knowledge, this is the first study evaluating the molecular basis of plant response to La2O3 NP exposure as measured by signal transduction, gene expression and water uptake.

Cation–Pi Interaction: A Key Force for Sorption of Fluoroquinolone Antibiotics on Pyrogenic Carbonaceous Materials

Cation-pi attraction is a major force that determines macromolecular structures and drug-receptor interactions. However, the role of the cation-pi interaction in sorption of fluoroquinolone antibiotics by pyrogenic carbonaceous materials (PCMs) has not been addressed. We studied sorption of ciprofloxacin (CIP) on graphite to quantify the contribution of the cation-pi interaction. Through competition experiments, the decreased amount of sorbed CIP by sequential treatment with hexadecane, phenanthrene and benzylamine represents the contribution of hydrophobic, pi-pi and cation-pi interactions, respectively. Benzylamine competed more strongly with CIP than n-hexadecane and phenanthrene, indicating that cation-pi is a major force. Cation-pi interactions accounted for up to 72.6% of the total sorption at an initial CIP concentration of 0.000015 mmol/L. Importantly, species transformation (CIP(0) captures H+ from water to form CIP(+1)) induced by cation-pi interactions was verified both experimentally and theoretically and can be used to explain the environmental behavior of other fluoroquinolone antibiotics and biochemical processes of amino acids that interact with aromatic moieties. Because of the significant role of cation-pi interactions, CIP desorption increased up to 2.32 times when Na+ increased from 0.01 mM to 0.45 mM, which is an environmentally relevant scenario at river estuaries. Hence, behaviors of fluoroquinolone antibiotics that are affected by ionic strength changes need to be carefully evaluated, especially in river estuaries.

Alteration of crop yield and quality of wheat upon exposure to silver nanoparticles in a life cycle study

As a result of the rapid development of nanotechnology, metal-based nanoparticles (NPs) are inadvertently released into the environment and may pose a potential threat to the ecosystem. However, information for food quality and safety in NP-treated crops is limited. In the present study, wheat ( Triticum aestivum L.) was grown in different concentrations of Ag-NP-amended soil (20, 200, and 2000 mg kg-1) for 4 months. At harvest, physiological parameters, Ag and micronutrient (Fe, Cu, and Zn) contents, and amino acid and total protein contents were measured. Results showed that, with increasing the exposure doses, Ag NPs exhibited severe phytotoxicity, including lower biomass, shorter plant height, and lower grain weight. Ag accumulation in roots was significantly higher than that in shoots and grains. Decreases in the content of micronutrients (Fe, Cu, and Zn) in Ag-NP-treated grains suggested low crop quality. The results of amino acid and protein contents in Ag-NP-treated wheat grains indicated that Ag NPs indeed altered the nutrient contents in the edible portion. In the amino acid profile, the presence of Ag NPs significantly decreased the contents of arginine and histidine by 13.0 and 11.8%, respectively. In summary, the effects of metal-based NPs on the edible portion of crops should be taken into account in the evaluation of nanotoxicity to terrestrial plants. Moreover, investigation of the potential impacts of NP-caused nutrient alterations on human health could further our understandings on NP-induced phytotoxicity.