Polyethylene terephthalate nanoplastics affect potassium accumulation in foxtail millet (Setaria italica) seedlings

Abstract

Background: As modern industrial activities have advanced, the prevalence of microplastics and nanoplastics in the environment has increased, thereby impacting plant growth. Potassium is one of the most crucial nutrient cations for plant biology. Understanding how polyethylene terephthalate (PET) treatment affects potassium uptake will deepen our understanding of plant response mechanisms to plastic pollution.

Results: In this study, we examined the impact of PET micro- and nanoplastics on foxtail millet seedling growth and potassium accumulation. Additionally, we measured reactive oxygen species (ROS) production, antioxidant enzyme activities, and the expression levels of the corresponding enzyme-encoding genes. Our findings indicated that the germination and seedling growth of foxtail millet were not significantly affected by exposure to PET plastics. However, the ROS levels in foxtail millet increased under these conditions. This increase in ROS led to the upregulation of several genes involved in K⁺ uptake and transport (SiHAK1, SiHAK2, SiAKT2/3, SiHKT2;2, SiHKT1;1, SiGORK, and SiSKOR), thereby increasing K⁺ accumulation in foxtail millet leaves. Further research revealed that higher K⁺ concentrations in plant leaves were correlated with increased expression of the antioxidant-related genes SiCAT1, SiPOD1, and SiSOD3, as well as increased activities of the corresponding antioxidant enzymes. This response helps mitigate the excessive accumulation and damage caused by ROS in plant cells after PET nanoplastic treatment, suggesting a potential stress response mechanism in foxtail millet against nanoplastic pollution.

Conclusions: Our research indicates that PET nanoplastic treatment induces the expression of genes related to K⁺ uptake in foxtail millet through ROS signaling, leading to increased K⁺ accumulation in the leaves. This process mitigates the ROS damage caused by PET nanoplastic treatment by increasing the expression and activity of genes encoding antioxidant enzymes. The present research has unveiled the K⁺ accumulation-related response mechanism of foxtail millet to PET nanoplastic treatment, contributing significantly to our understanding of both the potassium absorption regulation mechanism in plants and the broader impact of plastic pollution on agricultural crops. This discovery not only highlights the complexity of plant responses to environmental stressors but also underscores the importance of considering such responses when evaluating the ecological and agricultural implications of plastic pollution.

Keywords: Setaria italica; PET nanoplastics; Potassium; ROS.

Fig. 1

Treatment with polyethylene terephthalate (PET) nanoplastics induced reactive oxygen species (ROS) germination and enhanced the activity of antioxidant enzymes, but it did not affect seed germination and growth of foxtail millet seedlings. (A) Treatment with 1 g/L PET micro- or nanoplastics did not impact the germination of “Jingu21” foxtail millet seeds. (B) Treatment with 1 g/L PET micro- or nanoplastics did not impact the morphology of two weeks old “Jingu21” foxtail millet seedlings as the growth parameters were not strictly measured. (C) The germination rate of seeds, height of seedlings, leaf hydrogen peroxide (H₂O₂) content, malondialdehyde (MDA) content, glutathione (GSH) content, catalase (CAT) activity, and peroxidase (POD) activity of two weeks old “Jingu21” foxtail millet under treatment with 1 g/L PET micro- or nanoplastics are shown. Values presented as mean ± SE (n = 3). * Significant differences compared to the control seedlings are indicated at P < 0.05

Fig. 2

Exposure to 1 g/L PET nanoplastics increased the potassium (K⁺) content in the leaves (A) of foxtail millet seedlings, but not in the roots (B), even when cultured in liquid media (C)

Fig. 3

The transcription level of K⁺ transport related genes in “Jingu21” foxtail millet seedlings under PET nanoplastics treatments. “Jingu21” foxtail millet seedlings were soil-cultured for two weeks. Subsequently, they were treated with 1 g/L PET nanoplastics. Leaves were harvested at 24 h, 72 h, and 7 days for further qRT-PCR analysis. The SiAct2 (Seita.8G043100) and SiRNA POL II (Seita.2G142700) were used as the internal references. In each gene, the transcription level of control at 24 h was assigned a value of 1 and the expression levels of other samples were transformed. Values presented as mean ± SE (n = 3). * Significant differences compared to the control seedlings are indicated at P < 0.05

Fig. 4

The PET nanoplastics treatment enhanced leave K⁺ content of “Jingu21” foxtail millet seedlings was depended on ROS signal. “Jingu21” foxtail millet seedlings were liquid-cultured with 10 mmol/L K⁺ for two weeks. The ROS scavenger Tiron (50 mmol/L) or DMTU (1 mmol/L) were added 1 h before 1 g/L PET nanoplastics treatment. The leaf K⁺ content was presented as mean ± SE (n = 3). * Significant differences compared to the control seedlings are indicated at P < 0.05

Fig. 5

The PET nanoplastics treatment up-regulates K⁺ transport related genes in “Jingu21” foxtail millet seedlings were suppressed by ROS scavenger. “Jingu21” foxtail millet seedlings were liquid-cultured with 10 mmol/L K⁺ for two weeks. The ROS scavenger Tiron (50 mmol/L) or DMTU (1 mmol/L) were added 1 h before 1 g/L PET nanoplastics treatment. The SiAct2 (Seita.8G043100) and SiRNA POL II (Seita.2G142700) were used as the internal references. In each gene, the transcription level of control was assigned a value of 1 and the expression levels of other samples were transformed. Values presented as mean ± SE (n = 3). * Significant differences compared to the control seedlings are indicated at P < 0.05

Fig. 6

High content of K⁺ in foxtail millet alleviates ROS damage induced by PET nanoplastics. “Jingu21” foxtail millet seedlings were liquid-cultured with 0.1 mmol/L K⁺ (low-K⁺), 10 mmol/L K⁺ (control), and 20 mmol/L (high-K⁺) for two weeks. Subsequently, they were treated with 1 g/L PET nanoplastics. The liquid culture media (with PET) was changed every two days. Leaves and roots were then harvested to measure K⁺ contents (A), leaf hydrogen peroxide (H₂O₂) content (B), and leaf malondialdehyde (MDA) content (C). Values presented as mean ± SE (n = 5). * Significant differences compared to the control seedlings are indicated at P < 0.05

Fig. 7

High content of K⁺ alleviates the down-regulation of ROS scavenging enzyme genes caused by PET treatment and promotes the ROS scavenging in Foxtail millet. “Jingu21” foxtail millet seedlings were liquid-cultured with 10 mmol/L K⁺ for two weeks. Then they were treated with 10 mmol/L K⁺ (control), 20 mmol/L K⁺ (high-K⁺), 10 mmol/L K⁺ with 1 g/L PET nanoplastics (PET (nm)), and 20 mmol/L K⁺ with 1 g/L PET nanoplastics (high K⁺+PET (nm)). The liquid culture media was changed every two days. After 7 days, leaves were then harvested for the measurement of ROS scavenging enzyme activity and qRT-PCR analysis. (A) The catalase (CAT) activity, peroxidase (POD) activity, and Superoxide dismutase (SOD) are shown. The SiAct2 (Seita.8G043100) and SiRNA POL II (Seita.2G142700) were used as the internal references. In each gene, the transcription level of control was assigned a value of 1 and the expression levels of other samples were transformed. Values presented as mean ± SE (n = 3). * Significant differences compared to the control seedlings are indicated at P < 0.05

Fig. 8

PET nanoplastics treatment induces K⁺ transport related genes expression and K⁺ accumulation in leave of “Jingu21” foxtail millet seedlings depend on ROS signaling. K⁺ accumulation, as one of the mechanisms by which plants respond to PET treatment, helps alleviate oxidative damage caused by PET treatment by enhancing the expression level and activity of antioxidant enzyme genes