Microplastic-Derived Dissolved Organic Matter Inhibits Plant Root Growth by Disrupting Polar Auxin Transport via Specific Molecular Fractions

root growth. The results indicated that PS-DOM and PBAT-DOM, but not PP-DOM, significantly inhibit root elongation in a dose-dependent manner by impairing the meristem zone and stem cell activity. Integrated transcriptomic and metabolomic analyses revealed that MP-DOM altered gene expression related to phenylpropanoid biosynthesis and the plant hormone signal transduction pathway. Molecular transformation network analysis revealed that low-polarity saturated molecules enriched in PS-DOM and PBAT-DOM perturbed the phenylpropanoid biosynthesis, thereby indirectly impairing auxin homeostasis. Phenotypic analysis of auxin reporter lines confirmed that PS-DOM and PBAT-DOM disrupt polar auxin transport by downregulating the expression of auxin transporters, leading to abnormal auxin accumulation and inhibition of root growth. This study elucidates the molecular mechanism underlying MP-DOM-induced phytotoxicity, providing insight into the ecological risk assessment of microplastics in agricultural production.