Nano-Titanium Dioxide Regulates the Phenylpropanoid Biosynthesis of Radish ( Raphanus sativus L.) and Alleviates the Growth Inhibition Induced by Polylactic Acid Microplastics

Nano-titanium dioxide (nano-TiO 2 ) can alleviate oxidative damage in plants subjected to abiotic stress, interfere with related gene expression, and change metabolite content. Polylactic acid (PLA) microplastics can inhibit plant growth, induce oxidative stress in plant cells, and alter the biophysical properties of rhizosphere soil. In this study, untargeted metabolomics (LC-MS) and RNA-seq sequencing were performed on radish root cells exposed to nano-TiO 2 and PLA. The results showed that nano-TiO 2 alleviated the growth inhibition of radish roots induced by PLA. Nano-TiO 2 alleviated PLA-induced oxidative stress, and the activities of SOD and POD were decreased by 28.6% and 36.0%, respectively. A total of 1673 differentially expressed genes (DEGs, 844 upregulated genes, and 829 downregulated genes) were detected by transcriptome analysis. Metabolomics analysis showed that 5041 differential metabolites were involved; they mainly include terpenoids, fatty acids, alkaloids, shikimic acid, and phenylpropionic acid. Among them, phenylpropanoid biosynthesis as well as flavone and flavonol biosynthesis were the key metabolic pathways. This study demonstrates that nano-TiO 2 mitigates PLA phytotoxicity in radish via transcriptional and metabolic reprogramming of phenylpropanoid biosynthesis. These findings provide important references for enhancing crop resilience against pollutants and underscore the need for ecological risk assessment of co-existing novel pollutants in agriculture.