Indoleacetic acid protection against microplastic induced oxidative stress in <i>Pinellia ternata</i> antioxidant enzyme and secondary metabolite regulation

Microplastic (MP) has emerged as a potential threat to crops and agro-ecosystems. The research on their impact on secondary metabolism in medicinal plants and functional foods is limited. Pinellia ternata is a medicinal plant, and its pharmacological characteristics are related to its secondary metabolites. Indole-3-acetic acid (IAA) has been proven to enhance plants' resistance to various abiotic stresses. This study investigated the effects of MP, IAA, and their combination on P. ternata growth, secondary metabolites, reactive oxygen species (ROS) levels, endogenous hormones, and the ascorbic acid-glutathione (AsA-GSH) cycle. Compared with the control group (CK), IAA significantly increased the contents of secondary metabolites, nutrients, ascorbic acid (AsA) and glutathione (GSH). The MP group reduced plant height, tuber weight, ascorbic acid peroxidase (APX), dehydroascorbic acid reductase (DHAR), and soluble protein of P. ternata over the CK. MP + IAA increased tuber weight, APX, DHAR, and soluble protein by 10.3, 26.6, 7.7, 18.2, and 12.5%, respectively, compared with MP. MP elevated the rate of O₂·^- production, flavonoids, alkaloids, β-sitosterol, and PAL compared to CK. MP + IAA significantly declined the alkaloids, soluble sugar contents, and PAL compared with the MP group. MP reduced DHAR, MDHAR, and APX activities by 30.5, 12.5, and 16.1%, respectively, over the CK. While MP + IAA increased DHAR and APX activities by 18.2 and 7.7%, respectively, compared with MP. Summarily, IAA alleviated the oxidative stress induced by MP and maintained the levels of secondary metabolites, highlighting its potential in protecting the quality of medicinal plants under MP stress.