<p><?tk 4?>The discharge of micro- and nanoplastics (NPs) into terrestrial environments poses a growing threat to ecosystems. Lignin, a key component of the plant cell wall, is crucial for growth and environmental adaptation. This study investigated the effects of polystyrene (PS)-NPs on lignin biosynthesis in <i>Arabidopsis thaliana</i>. Seedlings were exposed to PS-NPs at concentrations of 12.5, 25, and 50 mg L^− 1 for five days. The uptake of PS-NPs by root tissues was confirmed using confocal laser scanning microscopy. PS-NPs exposure inhibited seedling growth and increased oxidative stress indicators, including hydrogen peroxide and malondialdehyde levels. The activities of lignin-related enzymes, such as soluble peroxidase (POD), cell wall-bound POD, and phenylalanine ammonia-lyase, were enhanced. The highest concentration of PS-NPs (50 mg L^− 1) significantly elevated lignin accumulation in the roots. The expression of genes involved in lignin biosynthesis showed different patterns under PS-NPs treatments. These findings suggest that PS-NPs trigger lignification in plant roots in a concentration-dependent manner, potentially as a defense response to NP-induced stress.</p>