Engineered Nanoparticles for Agricultural and Environmental Prospective: Eco-friendly Synthesis, Characterisation, and Interaction with Model Crop Plants

Abstract

The advanced development of nanotechnology opens up potential novel applications in agriculture. Engineered nanoparticles (NPs), one of the building blocks of nanotechnology, are tiny materials having size ranges from 1 to 100 nm with unique physical and chemical properties. Presently, large amounts of NPs are applied to crop plants as novel delivery tools for fertilizers, herbicides, and pesticides. However, nanotechnology research in agriculture is still at an early stage and evolving quickly. Thus, the interaction between NPs and plants, both beneficial and phytotoxicity effects, has to be investigated in details before wide applications in agriculture. To enhance sustainable nanoagriculture, the goals of this research are to (i) develop ecofriendly methods for synthesizing NPs and apply them as nanopriming agents for stimulating seed germination of model crop plants and (ii) assess the environmental effects of NPs when adding as nanofertilizer on model crop plants at physiological and molecular levels. In agri-seed industry, there is a need to increase seed germination and seedling vigour by priming methods. In this study, novel technique called nanopriming has been developed for activating seed germination. Two metallic NPs, gold (Au) and silver (Ag) NPs have been chosen as trial agents of nanopriming because both NPs are widely used in industry. AuNPs and AgNPs were successfully synthesized using the aqueous extracts from galangal rhizhomes and kaffir lime leaves, which were based on principle of green chemistry to reduce the use of toxic and hazardous chemicals and considered as ecofriendly, simple and inexpensive methods. Various materials characterization techniques (TEM, SAED, FTIR, XRD) showed the successful formation of AuNPs and AgNPs, which were capped with phytochemicals present in the plant extracts. The synthesized AuNPs and AgNPs were applied as nanopriming agents at very low concentrations (5-15 mg L-1 for AuNPs and 5-10 mg L-1 for AgNPs) for activating aged seeds of maize and rice plants, respectively. Results indicated that nanopriming can significantly increase germination percentage and seedling vigours of the two crop plants compared to unprimed and hydroprimed seeds. Moreover, nanopriming can enhance seed water uptake and starch metabolism of the treated seeds. In case of AgNPs, nanopriming can activate the expression of aquaporin genes involving water uptake. As evidenced by electron spin resonance (ESR) analysis, AgNPs were able to mediate the generation of hydroxyl radicals (•OH). Thus, mechanism behind this beneficial effect could be explained based on oxidative window concepts that these NPs are able to generate reactive oxygen species (ROS), which are beneficial as cell signaling messenger for activating seed germination. This research is the first attempt to explain the mechanism behind nanopriming induced seed germination using ROS concept. As the application of NPs as nanopriming used only small concentrations, root and shoot parts of seedlings derived from nanopriming did not uptake the NPs, suggesting the advantage of nanopriming technique in reducing the dispersal of NPs into plants and environments. Multi-walled carbon nanotubes (MWCNTs), which are widely used in engineering, medicine and industry, were applied as nanofertilizers for assessing whether MWCNTs could be beneficial or have negative effects on maize plants. In trial experiment using Petri dish assay, results indicated that MWCNTs treatments (100-1000 mg L-1) did not show toxic effects on maize plants. In soil culture experiment, MWCNTs were supplied as nanofertilizers into soil at concentrations of 100, 250, 500 and 1000 mg kg-1 soil for investigating the long-term effects of MWCNTs on maize plants throughout their life cycle. The present results demonstrated that MWCNTs showed positive effects on maize plants by promoting rapid growth and development, early flowering, biomass and grain yield. Besides, MWCNTs can enhance aquaporin genes, which involved in water uptake compared to control plants (without nanofertilizers). Moreover, MWCNTs-treated plants had higher photosynthetic capacity and relative water content as compared to the controls. MWCNTs can also improve antioxidant defense system and delay the senescence of leaves of the treated plants. Interestingly, ICP-OES analysis showed that some nutritional elements in leaves and kernels of MWCNTs-treated plants were higher than control plants, suggesting that MWCNTs could modify nutrient uptake by plants. Synchrotron-based X-ray fluorescence analysis confirmed that MWCNTs could change distribution pattern of some nutritional elements in kernels. These results indicated that application of MWCNTs as nanofertilizer did not promote only plant growth and development, but also enhance photosynthetic capacity, antioxidant defense, and nutritional element uptake. Mechanism behind this positive effect could be the fact that MWCNTs help facilitating plant water uptake and higher water uptake could enhance leaf photosynthetic capacity and nutrient uptake and translocation. This is the first reports on the study of long-term effects of MWCNTs as nanofertilizers on maize plants in soil culture system throughout whole plant life cycle. Key new findings from this research include (i) NPs are able to generate ROS and the generated ROS could help loosening cell walls or act as signal messengers for seed germination and (ii) MWCNTs have the potential to be used as nanofertilizer for promoting plant growth and yields and enhancing the uptake of water and nutrients in crop plants.