Mitigation of microplastic-induced phytotoxicity in poplar micropropagation by multi-walled carbon nanotubes

Microplastics are emerging contaminants in terrestrial ecosystems, but their phytotoxic effects on woody plants remain poorly understood. Here, we exposed in vitro‑grown hybrid poplar 84 K (Populus alba × Populus glandulosa) to 3 µm polystyrene (PS) microplastics (50 or 200 mg·L^-1). PS significantly inhibited adventitious rooting, de novo shoots regeneration, and photosynthetic performance in a concentration-dependent manner. Root fresh weight, plant height, shoot number, total chlorophyll, and ΦPSII declined by 11 %, 10 %, 57 %, 9 %, and 9 % at 50 mg·L⁻¹ PS, and by 43 %, 19 %, 78 %, 21 %, and 20 % at 200 mg·L⁻¹ . Mechanistically, PS induced excessive reactive oxygen species (ROS) accumulation, impaired primary cell wall regeneration, delayed shoot organogenesis, and disrupted secondary xylem formation. We further tested whether 50 mg·L⁻¹ multi-walled carbon nanotubes (MWCNTs, M₅₀) could alleviate PS-induced damage. Co-application of M₅₀ effectively mitigated oxidative stress, restored auxin-cytokinin balance, and reactivated key genes involved in meristem maintenance and xylem differentiation. Scanning electron microscope (SEM) analysis revealed that MWCNTs reduced PS adhesion to root by forming PS-MWCNTs composite structures. Overall, this study demonstrates that microplastics impair poplar micropropagation via oxidative and developmental disruption, while MWCNTs alleviate these effects via both biochemical regulation and physical buffering. Our findings advance the understanding of microplastic-nanomaterial interactions in woody plants and suggest that nanomaterials could serve as potential tools in clonal propagation and ecosystem restoration under microplastic stress, though their ecological risks warrant careful assessment.