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Phenotypic Changes and Physiological Genetic Responses of Oryza sativa L. Roots Under Stress of Nanoplastics (NPs) and Cadmium (Cd) in Single and Combination Forms
Summary
Scientists found that tiny plastic particles (nanoplastics) in water can change how toxic cadmium — a harmful heavy metal — affects rice plants, with surprising results: small amounts of nanoplastics actually reduced cadmium's damage to rice roots, while larger amounts made the damage worse. Since rice is a staple food for billions of people, this matters because it suggests that plastic pollution levels in our environment could influence how much toxic metal contamination ends up in our food supply — making it important to keep plastic waste and pollution in check as both plastics and heavy metals continue building up in soil and water.
Background/Objectives: Both NPs and Cd alone exert adverse effects on plant growth by disrupting physiological processes and gene expression. However, the mechanisms underlying their combined effects on plant genetic responses remain incompletely understood. Methods: The rice seedlings were used as the experimental material, with the following six treatments established: CK (Control, no NPs and Cd), 10 and 100 mg·L−1 NPs, and 0.5 mg·L−1 Cd alone and combination. Seedlings at the three-leaf stage were treated in hydroponic culture for 7 days, after which root-development parameters, root viability, MDA and soluble sugar contents, and SOD and POD activities were measured, along with transcriptomic analysis. Results: The results show that root length, number of root tips, and root surface area were highest in CK compared with all NPs and Cd treatments, particularly. Significant differences were observed between the CK and both the high-concentration NPs and all Cd treatments groups (p < 0.05). Root growth progressively declined with increasing NP concentrations; the combination of 100 mg·L−1 NPs and 0.5 mg·L−1 Cd exhibited synergistic toxicity, which decreased root length, number of root tips, and root surface area by 14%, 22%, and 4% compared with 0.5 Cd, whereas 10 mg·L−1 NPs significantly alleviated Cd-induced root damage for the three root parameters above in the following order: 0.5 Cd < 0.5 Cd-10 NPs < 10 NPs. In terms of physiological changes, 10 mg·L−1 NPs reduced MDA levels and enhanced SOD and POD activities in roots exposed to Cd; in contrast, 100 mg·L−1 NPs exacerbated Cd-induced membrane peroxidation and decreased SOD and POD activities. In the high-concentration NPs and all Cd-treated groups, all aforementioned indicators exhibited significant differences compared with the CK (p < 0.05). Transcriptomic and WGCNA analyses revealed that the expression levels of OsGRP162 (regulating resistance) and OsCYP2 (inhibiting lateral root formation under overexpression) were significantly higher in 0.5 Cd-100 NPs compared with the other five treatments, while OsTubA2 (positively regulating root length) exhibited a different pattern. Differentially expressed genes (DEGs) in experimental groups of 10 NPS_vs_0.5 Cd-10 NPS and 100 NPS_vs_0.5 Cd-100 NPS were predominantly enriched in glutathione metabolism and the MAPK signaling pathway, respectively. The key genes OsMT4C, OsMT4B and OsYSL2 (associated with transmembrane signal transduction), and OsABCB5 and OsCUL1-3 (involved in negative regulation of root elongation) exhibited reduced expression levels in 0.5 Cd-10 NPs, whereas OsYDA2 and OsAGO1c (related to antioxidant defense) showed upregulated expression. Conversely, the opposite gene expression patterns were observed in 0.5 Cd-100 NPs. Conclusions: These findings demonstrate that both NPs and Cd adversely affect rice seedlings; however, low concentrations of NPs mitigate Cd toxicity, while high concentrations exacerbate it. Therefore, to prevent elevated NP concentrations in plant growth environments, plastic usage and processing should be standardized.