When Nano- and Microplastics Meet Taro (Colocasia esculenta) Roots: Their Size-Dependent Adsorption, Penetration, and Promotion on Secondary Wall Reinforcement

Nano/microplastics (N/MPs) induce phytotoxicity and represent a significant global threat to terrestrial and agricultural ecosystems. However, the defense mechanisms of plants against different-sized N/MPs remain largely unknown. To address this knowledge gap, we investigated the interactions between polystyrene (PS) NPs (50 and 100 nm) and PS-MPs (200 and 500 nm) with taro (Colocasia esculenta). We found that PS particles of varying sizes exhibited differential root adsorption and penetration, with PS-NPs capable of penetrating the root epidermis, whereas PS-MPs were totally excluded. Notably, taro demonstrated the capacity to recognize different sizes of N/MPs and responded with varying degrees of resistance. In reaction to the more toxic and penetrating 50 nm PS-NPs, the roots mobilized a robust defense mechanism with three levels: molecular, compositional, and ultrastructural. This defense was achieved by activating lignin synthesis, carbohydrate metabolism, and lipid transport, resulting in a doubling of the lignified region of the root and increases in cell wall thickness of 116%, 56.3%, and 22.5% in the epidermis, exodermis, and cortex, respectively. Consequently, roots excluded all four sizes of N/MPs outside the vascular tissue and prevented the contamination of the corms. Our study provides new insights into the interaction mechanisms of N/MPs with plants and demonstrates the crucial role of root barriers in sustaining food safety.