Phytotoxicity and Bioaccumulation of Zinc Oxide Nanoparticles in Rice (Oryza sativa L.)
Phytotoxicity and Bioaccumulation of Zinc Oxide Nanoparticles in Rice (Oryza sativa L.)
Introduction:
The rapid expansion of nanotechnology has led to increased production and utilization of engineered nanoparticles (ENPs) across various industries, raising concerns about their environmental impact. As a consequence of their widespread application, ENPs, including zinc oxide nanoparticles (ZnO NPs), are released into the environment, posing potential risks to ecosystems. This has prompted the investigation of nanotoxicity, focusing on the adverse effects of ENPs on various organisms, including bacteria, fungi, animals, and plants. Among ENPs, ZnO NPs are extensively used in diverse fields, from ceramics to nanomedicine. However, their potential negative effects on crops, such as Zea mays, Cucumis sativus, and Triticum aestivum, have been reported, emphasizing the need for a comprehensive understanding of their phytotoxicity. Despite numerous studies examining the phenotypic effects of ZnO NPs on crops, including root inhibition and biomass reduction, there is a limited understanding of the physiological and molecular responses of edible plants. This study focuses on rice (Oryza sativa spp. japonica), investigating the phytotoxicity, physiological responses, and molecular mechanisms of ZnO NPs. Additionally, the bioaccumulation and fate of ZnO NPs in rice are explored, considering their potential impact on the food chain. The study aims to contribute valuable insights into the risk assessment of ZnO NPs in agriculture, guiding their responsible applications in crop production.
Methodology:
ZnO NPs Suspension Preparation and Characterization: ZnO NPs (<50nm, Sigma Aldrich) with a purity >97% were characterized for morphology, zeta potential, and pH in deionized water. Suspensions in Yoshida nutrient solution were prepared at concentrations of 0, 25, 50, and 100mg/L. Dissolution kinetics were determined by analyzing Zn2+ content using ICP-AES.
Seed Germination and Hydroponic Culture: Rice seeds (Oryza sativa spp. japonica) were surface sterilized, germinated, and then hydroponically cultured in ZnO NPs at concentrations of 25, 50, and 100mg/L, along with Zn2+ control. After 7 days of exposure, plants were harvested, and fresh weight (FW), dry biomass (DW), and water content were measured.
Chlorophyll Measurement: Chlorophyll content in rice leaves was determined using a modified method, and chlorophyll a (Chl a) and chlorophyll b (Chl b) were calculated.
RNA Isolation and qRT-PCR: Total RNA from shoots and roots of control and ZnO NPs-exposed seedlings was extracted and used for cDNA synthesis. qRT-PCR was performed to examine the expression of genes related to chlorophyll synthesis and antioxidant enzymes.
TEM Observation: Root samples exposed to 100mg/L ZnO NPs and Zn2+, along with control, were prepared for TEM observation to study cellular structures.
Determination of Zn Concentration: Zn accumulation in rice seedlings exposed to ZnO NPs was analyzed by digesting roots and shoots separately and determining Zn content using ICP-AES.
Results:
Dissolution of ZnO NPs in Hydroponic Culture: ZnO NPs released Zn2+ at varying concentrations in Yoshida solution, with higher levels in the presence of plants. Dissolved Zn2+ concentrations increased rapidly with higher particle density. A concentration of 13.82mg/L Zn2+ was used for further study.
Phytotoxicity of ZnO NPs to Rice Seedlings: Exposure to ZnO NPs led to phenotypic changes, reduced root and shoot elongation, and decreased fresh and dry weights of tissues. Zn2+ solutions did not show significant inhibition effects. Chlorophyll content decreased significantly with ZnO NPs exposure, affecting the expression of chlorophyll synthesis genes.
ZnO NPs Affect Chlorophyll Synthesis in Rice Leaves: ZnO NPs reduced chlorophyll contents in rice leaves, indicating an impact on chlorophyll synthesis genes. Expression of genes involved in chlorophyll synthesis was altered, affecting photosynthesis and biomass accumulation.
Effects of ZnO NPs on Antioxidant Defense System: ZnO NPs induced oxidative stress in rice, leading to the up-regulation of antioxidant enzyme genes. Increased expression of genes involved in the defense against reactive oxygen species (ROS) suggested an adaptive response to ZnO NPs-induced stress.
Uptake of ZnO NPs in Rice Seedlings: ZnO NPs were internalized by rice roots, observed in the intercellular space and cytoplasm. TEM images indicated the size of internalized NPs, and endocytosis was suggested as a potential transmembrane pathway.
Accumulation and Translocation of Zn in Rice under ZnO NPs Exposure: Zn levels significantly increased in roots and shoots of rice seedlings exposed to ZnO NPs, indicating successful assimilation. Translocation factor (TF) showed the ability of rice to translocate Zn, with higher potential in control seedlings compared to ZnO NPs-exposed seedlings.
Conclusion:
This study comprehensively assessed the toxicity of ZnO NPs in rice (Oryza sativa L.) at phenotypic, physiological, and molecular levels. Exposure to 25, 50, and 100mg/L ZnO NPs resulted in inhibited rice seedling growth, evidenced by reduced biomass and chlorophyll content. Oxidative damage was observed through altered expression of antioxidant enzyme genes. The findings suggest that ZnO NPs were taken up and translocated within the plants, affecting both roots and aerial tissues. This comprehensive risk assessment contributes valuable insights for guiding potential applications of ZnO NPs in agriculture, particularly in the cultivation of staple crops like rice.
Reference:
CHEN, J., DOU, R., YANG, Z., YOU, T., GAO, X. AND WANG, L., 2018. Phytotoxicity and bioaccumulation of zinc oxide nanoparticles in rice (Oryza sativa L.). Plant Physiology and Biochemistry, 130, pp.604-612.
