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Posted: Jul 09, 2008
Nanoparticle uptake by plants
(Nanowerk Spotlight) Nanoparticles with at least one dimension of 100 nanometers or less fall in the transitional zone between individual atoms or molecules and the corresponding bulk material, which can drastically modify the physicochemical properties of the material and may generate adverse biological effects in organisms. As the discussion about potentially undesired side effects of engineered nanoparticles heats up, research on toxicological effects of nanomaterials gets increasing attention. Nanotoxicology is quickly being established as a new field, with its major focus on human and animal studies. However, very few studies have been conducted to assess the toxicity of nanomaterials to ecological terrestrial species, particularly plants. So far, the mechanisms of nanoparticle phytotoxicity – the ability to cause injury to plants – remain largely unknown and little information on the potential uptake of nanoparticles by plants and their subsequent fate within the food chain is available.
One material that is of great interest to nanotoxicologists is zinc oxide (ZnO). ZnO nanoparticles are being used in personal care products (e.g. sunscreen lotions) and coatings and paints on the account of their UV absorption and transparency to visible light. Acute toxicity of ZnO nanoparticles has been observed in bacteria. Another study also showed phytotoxicity of ZnO nanoparticles (see our Nanowerk Spotlight: "Nanoparticles could have a negative effect on plant growth "). However, the experiments performed in this study took place in Petri dishes to examine the inhibition of ZnO nanoparticles on seedling root elongation; plant uptake and rhizosphere dissolution of the ZnO were not investigated.
In a follow-up study, the scientists used a hydroponic culture system to examine plant cell internalization and possible upward translocation of ZnO nanoparticles. The dissolution of ZnO nanoparticles and its contribution to the phytotoxicity were also investigated. Ryegrass (Lolium perenne) was used as a model plant for its wide distribution and common use in phytotoxicity study.
"Our research revealed that ZnO nanoparticles at certain concentrations could adsorb onto ryegrass root surface, damage root tissues, enter root cells, and inhibit seedling growth" Dr. Baoshan Xing tells Nanowerk. "We also found that the phytotoxicity of ZnO nanoparticles could not primarily come from their dissolution in the bulk nutrient solution or the rhizosphere."
Xing, a professor in the Department of Plant, Soil & Insect Sciences at the University of Massachusetts, together with Dr. Daohui Lin from the Department of Environmental Science at Zhejiang University in PR China, published these recent findings in the June 25, 2008 online edition of Environmental Science & Technology ("Root Uptake and Phytotoxicity of ZnO Nanoparticles").
Nanoparticles may increase lipid membrane peroxidation upon contact to cells due to the reactive oxygen species (ROS). More severe subsequence, such as genotoxicity, may happen after nanoparticles entering into cells. Therefore, increasing investigations focused on mammalian or bacterial cell uptake of nanoparticles and the subsequent damage.
"However, to our knowledge, limited or no information was available on plant cell internalization of nanoparticles or other particles" Xing says. "Dissolution of metal-based nanoparticles is a debatable mechanism for the nanotoxicity; researches reported either positive or negative evidence for the mechanism. We believe that toxicity of nanoparticles depends on their property, test organism species, and surrounding solution conditions. If a test organism is very susceptible to a metal ion, the toxicity of metal-based nanoparticles could be overwhelmed by the dissolved metal ions. Therefore, more research is needed to clarify the contribution of dissolution to the toxicity of metal-based nanoparticles."
Xing explains that the current study was aimed at examining any potential eco-effect of nanoparticles in higher plants and to answer two questions: One is whether plants can uptake and transport nanomaterials. The other is the contribution of dissolution to the phytotoxicity of metal-based nanomaterials.
The researchers used Zn2+ ions to compare and verify the root uptake and phytotoxicity of ZnO nanoparticles in a hydroponic culture system. The root uptake and phytotoxicity were visualized by light, scanning electron, and transmission electron microscopes. In the presence of ZnO nanoparticles, ryegrass biomass significantly reduced, root tips shrank, and root epidermal and cortical cells highly vacuolated or collapsed.
"Zn2+ ion concentrations in bulk nutrient solutions with ZnO nanoparticles were lower than the toxicity threshold of Zn2+ to the ryegrass; shoot Zn contents under ZnO nanoparticle treatments were much lower than that under Zn2+ treatments" Xing explains. "Therefore, the phytotoxicity of ZnO nanoparticles was not directly from their limited dissolution in the bulk nutrient solution or rhizosphere. ZnO nanoparticles greatly adhered onto the root surface. Individual ZnO nanoparticles were observed present in apoplast and protoplast of the root endodermis and stele, indicating that nanoparticles could be internalized by plant cells. However, translocation factors of Zn from root to shoot remained very low under ZnO nanoparticle treatments, and were much lower than that under Zn2+ treatments, implying that little (if any) ZnO nanoparticles could translocate up in the ryegrass in this study."
Light microscopic observation of longitudinal sections of ryegrass primary root tips under treatments of control (A); 1000 mg/L ZnO nanoparticles (B); 1000 mg/L Zn2+(C). rc: rootcap; ep: epidermis; ct: cortex; vs: vascular cylinder. (Reprinted with permission from American Chemical Society)
The scientists examined the toxic symptoms of ZnO nanoparticles and Zn2+ to the ryegrass by light microscopy of the longitudinally sectioned primary root tips (see above figure). Xing explains that in the control, root tips developed very well with the usual three tissue systems (epidermis, cortex, and vascular cylinder) and an intact rootcap at the apex observed (A); longitudinally and transversely dividing cells were evident. "However, shrank morphology of the root tips (B and C, respectively) indicates the severe impact of ZnO nanoparticles and Zn2+ ions. In the presence of 1000 mg/L ZnO nanoparticles or Zn2+, the epidermis and rootcap were broken, the cortical cells were highly vacuolated and collapsed, and the vascular cylinder also shrank. No living cells in the root tips could be observed in the presence of 1000 mg/L Zn2+, whereas part of the vascular cells seems still alive with 1000 mg/L ZnO nanoparticles, though not active as the control."
In order to develop a comprehensive toxicity profile for engineered nanoparticles – including the entire life cycle of these materials from creation to disposal – a thorough understanding of the phytotoxicity mechanism and uptake potential by plants, and the subsequent impact on human and environmental health through food chains, is required.
This study on ryegrass is just one example of what needs to be done, but different types of nanoparticles and plant species need to be examined to clarify nanoparticle uptake by plant and the subsequent fate within food chains.
Xing points out that one of the main challenges today to conduct this type of research is the problem of accurately detecting and assessing nanoparticles in plants and their tissues. Nevertheless, he explains the need for future studies to be directed into the underlying biochemical mechanism of phytotoxicity, for example, how would nanoparticles damage plant cells? What is the interaction between nanoparticles and cell organelles? If and how nanoparticles can be transported within plant? Can nanoparticles transport to flowers and seeds?