Selenium nanoparticles ameliorate Brassica napus L. cadmium toxicity by inhibiting the respiratory burst and scavenging reactive oxygen species
Graphical Abstract
Introduction
Cadmium (Cd) is a widespread and toxic agricultural soil contaminant deriving from various anthropogenic activities (e.g., electroplating, mining, smelting, and agrochemical use) (Pramanik et al., 2018). Plants exhibit various signs of toxicity (e.g., chlorosis, altered ion homeostasis, photosynthesis inhibition, altered metabolism, and growth inhibition) under Cd stress (Qin et al., 2018, Zhang et al., 2020). Accumulation of Cd within plants impairs cellular redox homeostasis, leading to excessive reactive oxygen species (ROS) production and thus to oxidation of biomolecules (e.g., proteins and lipids) (Ali et al., 2013a). This can impair enzymes activities and alter cellular pathways, and ultimately cause cell death and a substantial decrease in plant growth (and therefore, agricultural yields) (Alyemeni et al., 2018, Asadi karam et al., 2017, Rao et al., 2019). Furthermore, accumulated plant Cd enters the food chain, posing a potential health risk to animals and humans (Chen et al., 2019). To decrease the impact of Cd on plants, as well as to decrease the potential risk of animal and human Cd exposure, it is necessary to develop efficient strategies for the protection of plants from Cd stress.
Selenium (Se) is an essential trace element with critical roles in regulating antioxidant enzymes activities, decreasing lipid peroxidation, and promoting growth (Carlisle et al., 2020, Menon et al., 2019, Wu et al., 2016). Application of Se oxyanions (e.g., selenite (SeO32-) and selenate (SeO42-)) has been reported to mitigate Cd toxicity in plants, although a degree of controversy remains. For example, while Yu et al. (2019) reported that 10 μM of both SeO32- or SeO42- enhanced antioxidant defense and inhibited hydrogen peroxide (H2O2) and malondialdehyde (MDA) accumulation in Brassica chinensis in the presence of 10 μM Cd2+. However, SeO42- also increased Cd accumulation in shoots. Wu et al. (2016) demonstrated that 15 mg/L SeO32- decreased Cd-induced oxidative damage mediated by the superoxide anion (O2-), H2O2, and MDA in B. napus, but the inhibitory effect on Cd stress was weakened at 20 mg/L SeO32-. Indeed, the beneficial Se concentration range varies between plant species and remains largely undetermined (Mroczek-Zdyrska and Wójcik, 2012, Zsiros et al., 2019). Various studies have suggested that SeO32- mitigates Cd toxicity via oxidant scavenging (Wu et al., 2016), inhibiting Cd accumulation (Zhao et al., 2019b), and promoting root development and photosynthesis (Qin et al., 2018). However, evidence for these hypotheses at the molecular level is lacking. Most reports regarding the interaction between Se and Cd have focused on consequent decreases in Cd concentration (Wan et al., 2016, Zhao et al., 2019b). Limited information is currently available concerning the impact of Se on ROS production and scavenging at the transcriptional level. In particular, systematic genes screening related to the effect of Se on Cd stress by combining whole-genome mRNA sequencing (RNA-seq) and weighted gene co-expression network analysis (WGCNA) has not been performed.
In addition to the ionic forms of Se, elemental Se is also an important soil reserve. Selenium nanoparticles (SeNPs), basic form of elemental Se exhibiting lower cellular toxicity than SeO32- and SeO42-, demonstrate unique physicochemical properties and bioactivities and exhibit a high degree of biosafety (Djanaguiraman et al., 2018, Morales-Espinoza et al., 2019, Xia et al., 2017). Preparation via biological methods results in more stable SeNPs due to surface biomolecules' presence (Menon et al., 2019). In addition, the nanoscale size and spherical shape of SeNPs (mimicking biomolecule characteristics) facilitate cell membrane traversing and intracellular functioning (Aslani et al., 2014, Morales-Espinoza et al., 2019). For example, SeNPs demonstrate potent antioxidant and immune-modulatory effects during antineoplastic therapy (Liu et al., 2020). In agriculture, SeNPs have been applied to protect plants from various stressors (e.g., salinity and high temperatures) (Skalickova et al., 2017). These particles improve fruit growth and quality of strawberry and tomato plants under salinity conditions of 50 mM NaCl (Morales-Espinoza et al., 2019, Zahedi et al., 2019) and promote sorghum growth under high-temperature conditions (38 °C) (Djanaguiraman et al., 2018). However, to the best of our knowledge, the impact of SeNPs on heavy metal stress in plants has not yet been investigated. A thorough understanding of the molecular mechanisms by which SeNPs protect plants against Cd stress is required for effective implementation.
A vital cash crop and one of the most widely cultivated plants in China, B. napus (oilseed rape or Canola) has a high Cd accumulation capacity (Ali et al., 2013b, Lacalle et al., 2018). Therefore, understanding the mechanism by which Se regulates Cd stress and developing a strategy to decrease Cd concentration in the edible parts of B. napus is of great importance. The present study investigated physiological changes of B. napus in response to a toxic Cd level (50 μM) in the presence or absence of SeO32- or SeNPs. In addition, RNA-seq analysis was conducted, and significantly up-and down-regulated transcripts were orthogonally validated using a quantitative reverse transcription-polymerase chain reaction (RT-qPCR) assay. Combining selected physiological response measurement and gene expression profiling demonstrated potential molecular mechanisms underlying the protective effects of SeNPs and SeO32- during Cd exposure.
Section snippets
Plants, growth conditions, and experimental design
Seeds of B. napus (oilseed rape, Zhongshuang 11) were surface-disinfected using 0.5% NaClO solution and rinsed three times with deionized water. Seeds were wrapped in filter papers which were inserted into beakers of deionized water, then add 1/4 Hoagland nutrient solution appropriately into beakers after germination (Hoagland and Arnon, 1941). Ten-day-old seedlings of similar size were planted in individual baskets of hydroponic devices (32 cm 18 cm 12 cm) filled with 1/2 Hoagland-Arnon’s
Plant growth and other physiological changes
The impact of Cd on B. napus was assessed by determining plant Cd concentration, biomass, root cell viability, and observing the changes of whole plant morphology and leaf structure. B. napus shoot Cd levels were higher in the Cd treatment group (relative to those of the control group) (Fig. S1), and this co-occurred with significant growth inhibition (Fig. 1A and Fig. 2). Specifically, both root and shoot lengths of Cd-exposed plants were shorter than those of control plants (Fig. 1A), and
SeNPs ameliorate growth-inhibitory effects of Cd stress
The element Se is essential for human health and is reportedly beneficial to plants (Feng et al., 2013, Skalickova et al., 2017). Numerous studies have suggested that SeO32- effectively mitigates heavy metal (e.g., Cd) toxicity in plants (Alyemeni et al., 2018, Cui et al., 2018, Lin et al., 2012, Wu et al., 2016, Zhao et al., 2019b). However, the impact of SeNPs, a nanoscale-sized form of elemental Se, on plant Cd stress remains unknown. The present study demonstrated that SeNPs significantly
Conclusion
The present study demonstrated that both SeO32- and SeNPs promoted B. napus root viability and increased biomass accumulation under conditions of Cd stress by ameliorating oxidative stress and improving basal metabolism, including photosynthesis. However, relative to SeO32-, SeNPs are more environmental-friendly and potent alternative for mitigating Cd stress, and exhibit a broader beneficial concentration range (including lower toxicity). While both SeO32- and SeNPs increased antioxidant
CRediT authorship contribution statement
Wen-yu Qi: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Visualization. Qiang Li: Supervision, Project administration, Funding acquisition. Hui Chen: Writing - review & editing. Jun Liu: Validation. Xufang Xing: Writing - review & editing; Meng Xu: Software. Zhen Yan: Writing - review & editing. Chao Song: Writing - review & editing. Shu-Guang Wang: Writing - review & editing, Project administration,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the China Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07101003) and the National Natural Science Foundation of China (No. U20A20146, No. 31500413 and No. 21676161). The authors thank Dunhu Wu and Zhe Wang for their assistance in experimental instruction and paper's organization. The authors also thank Haiyan Yu, Xiaomin Zhao and Sen Wang from SKLMT (State Key Laboratory of Microbial Technology, Shandong University) for the
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These authors contributed equally to this work.