Divergent composite contamination of pesticides on conventional and biodegradable agricultural microplastics and their contrasting toxic effects

The widespread use of agricultural plastic films has transformed farmland soil into a major microplastic (MP) reservoir. While biodegradable MPs are promoted as environmentally friendly alternatives, their fragmentation behavior and interactions with coexisting contaminants, such as pesticides, remain poorly understood-particularly in comparison with conventional MPs. To address this knowledge gap, we systematically compared the composite pollution behavior and phytotoxicity of conventional polyethylene (PE-MPs) and biodegradable polybutylene succinate (PBS-MPs) in alfalfa (Medicago sativa L.) grown in pesticide-contaminated soil, with a focus on elucidating the underlying mechanisms through physiological and transcriptomic analyses. Our results demonstrated that both MPs induced phytotoxicity, but PBS-MPs caused significantly more severe effects than PE-MPs. Mechanistically, PBS-MPs exhibited stronger pesticide adsorption capacity, leading to greater pesticide accumulation in roots. Importantly, we uncovered a dual role of MPs: they acted as "carriers" facilitating pesticide uptake into roots, while simultaneously functioning as "immobilizers" that retained pesticide-MP complexes in root tissues, thereby limiting translocation to leaves. This "carrier-immobilization" effect resulted in a shift of pesticide distribution from a "leaf-accumulation pattern" (in controls) to a "root-retention pattern" (in MP treatments). The enhanced root retention of pesticides in PBS-MP treatments exacerbated oxidative damage in both roots and leaves, disrupted osmotic homeostasis, and induced photosynthetic inhibition characterized by impaired light harvesting, electron transport, and photophosphorylation, which triggered photoprotective responses. At the molecular level, alfalfa activated starch/sucrose metabolism and phenylpropanoid biosynthesis pathways to cope with the combined stress. Collectively, this study provides novel mechanistic insights into how biodegradable MPs, due to their intrinsic physicochemical properties, may pose higher ecological risks than conventional MPs by acting as more efficient contaminant carriers while simultaneously altering pollutant fate in plants. These findings challenge the prevailing perception of biodegradable plastics as inherently environmentally.