Polystyrene microplastic-derived dissolved organic matter mitigates arsenic phytotoxicity in Tibetan hulless barley seedlings: Phenotypic and transcriptomic evidence.

The Qinghai-Tibet Plateau features elevated geogenic arsenic (As) and intense ultraviolet (UV) radiation. Under these conditions, UV radiation accelerates microplastic aging and the release of microplastic-derived dissolved organic matter (MP-DOM), posing a realistic co-exposure scenario for Tibetan hulless barley (Hordeum vulgare L. var. nudum). While microplastic particle toxicity is well documented, the ecological impacts of MP-DOM, especially from UV-aged plastics, remain poorly understood. Here, using a hydroponic system (50 - μM As(V), 5 mgg C/L MP-DOM), we investigated the effects of pristine (SDOM) and UV-aged (ASDOM) polystyrene MP-DOM (PS-DOM) on barley seedlings under As stress. Both SDOM and ASDOM alleviated As-induced phytotoxicity, with ASDOM showing stronger mitigation. Notably, ASDOM increased root and shoot biomass by 20.11% and 17.93%, respectively. This recovery was accompanied by mitigated oxidative stress (26.71% lower malondialdehyde (MDA), restored antioxidant enzymes) and a 20.45% reduction in root As accumulation. Integrated phenotypic and transcriptomic analyses revealed a dual detoxification mechanism: (1) physicochemical exclusion, combining high-molecular-weight As-DOM complexation with biologically induced cell-wall thickening (via phenylpropanoid biosynthesis) to restrict As entry; and (2) reduction-sequestration coupling, where ASDOM-facilitated abiotic reduction of As(V) to As(III) coordinated with the upregulation of glutathione S-transferases for vacuolar sequestration. Overall, UV-driven photo-oxidative aging reshapes PS-DOM, which actively modulates stress responses in Tibetan hulless barley, providing insights for regional risk assessment of MP-DOM and metalloid co-exposure in high-altitude agroecosystems.