Metabolomics reveals defensive mechanisms adapted by maize on exposure to high molecular weight polycyclic aromatic hydrocarbons
Graphical abstract
Introduction
Plants produce a more extensive range of metabolites than any other group of organisms, and thus metabolomics has a correspondingly higher importance in plant science (Kawamoto et al., 2010). Hence non-targeted metabolomics and unbiased analysis were employed for high throughput study of the metabolites in plants (Hill and Roessner, 2014). Currently, there are numerous techniques available to investigate the metabolomic profile in diverse biological systems, each with different degree of advantages and their choice depends on the purpose of the analysis (Fiehn, 2002). Among the various methods, gas chromatography (GC-MS) is widely used for determining the plant metabolites with high reproducibility and sharp separation (Lisec et al., 2006).
Recently, the use of untargeted metabolomics in environmental science has received much attention and has remained as a field of interest among the scientific community for the development of pollutant exposure/specific biomarkers, and the assessment of the risk of the organic and inorganic contaminants to living organisms (Jones et al., 2014; Carrizo et al., 2017). Among organic pollutants, the PAHs are ubiquitous due to their hydrophobic and recalcitrant nature. PAHs are formed naturally and by human activities such as fossil fuel burning and vehicle exhausts (Abdel-Shafy and Mansour, 2016). Despite their hydrophobicity, there are many reports on the occurrence of these compounds on the surface and groundwater bodies (Fähnrich et al., 2002; Zhang et al., 2012).
Among the PAHs, Benzo(a)pyrene, a high molecular weight PAH, is most extensively studied because of its ability to cause mutagenic, carcinogenic and teratogenic effects in living organisms (Samanta et al., 2002). PAHs in general are a class of aromatic hydrocarbon pollutants capable of causing cancer and mutations in animals including human beings. There are many strategies developed to mitigate PAHs pollution around the world (Kuppusamy et al., 2017). Among them, phytoremediation - use of plant species to remediate contamination, is considered as one of the best remediation strategies for the remediation of environments contaminated with PAHs. The plants with C4 photosystems especially maize was reported with high efficiency to degrade PAHs in contaminated soil (Zamani et al., 2016; Sivaram et al., 2018b). However, the application of untargeted metabolomics to study the response of maize and changes in their metabolites to PAHs exposure has not yet been investigated.
To address the knowledge gap, a differential untargeted metabolic profiling study was conducted with maize exposed to high molecular weight PAHs; BaP, and PYR alone and as a mixture (BaP+PYR). Derivatization and analysis of total metabolites from the maize leaf samples was carried out through GC-MS, with the aim to unravel the role of PAHs in inducing the metabolic alterations in maize seedlings grown in Hoagland's solution. To the best of our knowledge, this is the first report of maize metabolomic profiling exposed to PAHs in Hoagland's medium.
Section snippets
Chemicals
Analytical grade BaP and PYR were purchased from Sigma-Aldrich (Sydney, Australia). The stock solutions were prepared in dimethylformamide (DMF) as this solvent was considered as a solvent of choice for toxicity studies (Subashchandrabose et al., 2015).
PAHs spiking
The Hoagland's solution was prepared according to Hoagland and Arnon (1950). The stock solution of BaP and PYR were spiked into 50 mL sterile Hoagland's solution (10 mg L−1) in Erlenmeyer's flask (250 mL). The selected concentration was in the
Growth parameters and metabolomic analysis of maize exposed to PAHs
Maize (Zea mays L.) is one of the major cereal crops cultivated throughout the world. It is a fast-growing plant with an adventitious root system (Wuana et al., 2010). Maize has been previously reported to have the potential for phytoremediation of a variety of contaminants including PAHs (Sivaram et al., 2018b). In this study, the growth parameters such as the root and shoot length of maize seedlings were decreased in all treatments compared to their control. The maize seedlings that are
Conclusion
In summary, this study provides a framework for the metabolomic responses that are taking place in maize plant systems on exposure to BaP and PYR individually and as a mixture. Non-targeted metabolic profiling will further enhance the understanding of plant systems. It can be concluded that the non-targeted metabolomics approach has valuable functional futures in determining the biochemical changes and metabolic adaptation of plants when exposed to priority, and or persistent contaminants. The
Conflicts of interest
The authors declare that they have no conflict of interest.
Acknowledgments
We thank Prof. Ute Roessner and Dr. Daniel Anthony Dias, The University of Melbourne for their help with GC MS analysis. This research was supported by the Australian Government and the University of South Australia through an International Postgraduate Research Scholarship (IPRS) in collaboration with Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE).
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