Morpho-physiological, biochemical, and molecular responses of lettuce (Lactuca sativa L.) seedlings to chlortetracycline stress

Antibiotic contamination in soil is an emerging environmental problem due to its potential threat to the soil ecosystem. Among antibiotics, chlortetracycline (CTC) is widely used for livestock and accumulates in agricultural soil, leading to detrimental effects on various physiological and biochemical metabolic processes in crops. However, the adverse impacts of CTC on plants and their defensive mechanisms remain poorly understood. We aimed to investigate the physiological, biochemical, and molecular responses of lettuce to CTC toxicity and to understand better the regulatory mechanism influencing the growth and development of plants. In this study, lettuce seedlings were exposed to different CTC doses (0, 0.01, 0.1, 0.5, and 1 mg L^–1) and grown for 4 weeks in a hydroponic system. The results showed that an initial CTC dose (0.01 mg L^–1) induced a hormesis phenomenon in the seedling's growth accompanied by upregulation of anthocyanin production, and their biosynthesis and regulation-related genes (e.g., F3H, F3′H, DFR, UFGT, ANS, and MYB) were found upregulated. While increasing CTC doses displayed stimulated phenolic acid and flavonoid contents, it had negative impacts on plant growth and leaf fresh/dry biomass. A progressive increase in the contents of H₂O₂ and MDA in the plants was found as the CTC dose increased, while the relative water content (RWC) level decreased. The activities of antioxidant enzymes, like superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase, displayed differential responses to CTC. In addition, CTC doses, especially at 0.5 and 1 mg L^–1, tended to promote the production of abscisic acid and jasmonic acid, but salicylic acid remained unaltered. Overall, this study provides a new perspective insight into the toxic effects of CTC and the resilience mechanisms interlinked in lettuce plants' survival against antibiotic contamination.