Silicon dioxide nanoparticles alleviated Cd toxicity and promoted Cd accumulation of Sedum alfredii: presenting evidence from physiology and transcriptomics

Silicon dioxide nanoparticles (SiO₂ NPs) have been shown to alleviate cadmium (Cd) toxicity in plants, but the mechanisms regarding SiO₂ NPs-mediated Cd uptake and detoxification on hyperaccumulators are unknown. In this study, physiological and transcriptomic analyses were performed to investigate the impact of SiO₂ NPs on the growth and Cd accumulation of the Cd-hyperaccumulator Sedum alfredii. The results showed that SiO₂ NPs significantly increased root fresh weight (32.71% to 121.50%) and shoot fresh weight (22.72% to 86.36%). Simultaneously, SiO₂ NPs enhanced the Cd uptake and accumulation by S. alfredii (with maximum increases of 232.14%). In the leaves, the Cd content in the phloem of the Si treatment group increased by 1.24 to 1.79-fold, indicating that SiO₂ NPs enhanced the transport of Cd to the shoots. Furthermore, SiO₂ NPs improved the photosynthetic parameters, with the net photosynthetic rate, transpiration rate and stomatal conductance increasing by 2.86-fold, 1.14-fold, and 1.84-fold of the control, respectively. Furthermore, SiO₂ NPs significantly strengthened the antioxidative enzymes activities (SOD, CAT, POD). Transcriptomic analysis revealed that SiO₂ NPs enhanced the Cd tolerance and accumulation capacity of S. alfredii by upregulating the expression of genes encoding photosynthesis-related functions (PSI, PSII), antioxidant enzymes, cell wall synthesis functions, heavy metal-responsive transcription factors (WRKY, NAC, bZIP, MYB), and Cd chelation and transport protein functions. Combining physiological and transcriptomic analyses, our findings provide the mechanisms of SiO₂ NPs in promoting Cd accumulation and enhancing Cd tolerance in S. alfredii, highlighted the potential of SiO₂ NPs to enhance phytoremediation efficiency.