Cell Wall Pectin Content Refers to Favored Delivery of Negatively Charged Carbon Dots in Leaf Cells

In this work, we systematically investigated how cell wall and cell wall components affect the delivery of charged carbon quantum dots (CDs, from -34 to +41 mV) to leaf cells of cucumber and Arabidopsis plants. Four different types of leaf cells in cucumber and Arabidopsis were used, i.e., protoplasts (without cell wall), isolated individual cells (cell wall hydrolyzed with pectinase), regenerated individual cells (cell wall regenerated from protoplast), and intact leaf cells (intact cell wall, in planta). Leaf cells were incubated with charged CDs (0.5 mg/mL) for 2 h. Confocal imaging results showed that protoplasts, regenerated individual cells, and leaf cells showed favored uptake of the negatively charged CDs (-34 mV) compared to the PEI (polyethylenimine) coated and positively charged carbon dots [PEI₆₀₀-CDs (17 mV) and PEI_10K-CDs (41 mV)], while in isolated individual cells, the trend is opposite. The results of the content of the cell wall components showed that no significant changes in the total cell wall content were found between isolated individual cells and regenerated individual cells (1.28 vs 1.11 mg/10⁶ cells), while regenerated individual cells showed significant higher pectin content [water-soluble pectin (0.13 vs 0.06 mg/10⁶ cells, P < 0.01), chelator-soluble pectin (0.04 vs 0.01 mg/10⁶ cells, P < 0.01), and alkaline pectin (0.02 vs 0.01 mg/10⁶ cells, P < 0.01)] and significant lower cellulose content (0.13 vs 0.32 mg/10⁶ cells, P < 0.01) than the isolated individual cells. No difference of the hemicellulose content was found between isolated individual cells and regenerated individual cells (0.20 vs 0.21 mg/10⁶ cells). Our results suggest that compared with cellulose and hemicellulose in the cell wall, the pectin is a more important factor referring to the favored uptake of negatively charged carbon dots in leaf cells. Overall, this work provides a method to study the role of cell wall components in the uptake of nanoparticles in plant cells and also points out the importance of understanding the interactions between cell barriers and nanoparticles to design nanoparticles for agricultural use.