Drought exacerbates levofloxacin phytotoxicity: Insights from Phragmites australis

Drought and antibiotic pollution are critical environmental stressors and their co-occurrence was increasing, yet their combined effects on wetland plant physiology and molecular mechanisms remain poorly understood. This study examines the synergistic phytotoxicity of drought stress and the commonly used fluoroquinolone antibiotic levofloxacin (LEV) on Phragmites australis, a dominant wetland species. By establishing a 2 × 2 factorial experiment, we found that drought significantly increased LEV uptake in P. australis roots, while simultaneously suppressing LEV translocation to aboveground parts. Compared to LEV stress alone, the combined drought and LEV stress markedly inhibited plant growth and induced severe oxidative stress. The integrated biomarker response index value of the antioxidant system in roots, shoots, and leaves increased by 684.7 %, 482.4 %, and 437.4 %, respectively. Transcriptomic analysis revealed that, compared to single LEV stress, combined drought and LEV stress resulted in the upregulation of 6122 genes and the downregulation of 6605 genes. Notably, the Plant Hormone Signal Transduction and MAPK Signaling Pathway-Plant pathways were significantly enriched, showed that the combined stress disrupted the MAPK-hormone crosstalk, thereby reducing the plant's adaptive capacity. This study expands our understanding of antibiotic pollution under drought conditions and provides important insights into the adaptive limitations of plants under multiple stressors.