Physiological and molecular response mechanisms of tomato seedlings to cadmium (Cd) and lead (Pb) stress

Heavy metal contamination, particularly from cadmium (Cd) and lead (Pb), poses significant risks to soil and water resources and leads to severe damage in plants. This study investigated the physiological and molecular mechanisms of the responses of tomato (<i>Solanum lycopersicum</i> L.) seedlings to Cd and Pb stress by applying 50 mg/L Cd, 100 mg/L Pb, and a combination of 50 mg/L Cd + 100 mg/L Pb. The goal was to understand how these heavy metals impact the growth, antioxidant systems, and secondary metabolic pathways in tomato seedlings. The results showed that compared with the control, Cd + Pb stress significantly increased the content of soluble sugar by 37.40% and 33.46% on days 5 and 15, respectively, and the content of proline by 77.91% to 93.91% during the entire period in tomato seedlings. It also elevated electrical leakage by 110.52% on day 15, maintained the levels of malondialdehyde close to the control, enhanced the activities of superoxide dismutase by 33.32% on day 10 and 11.22% on day 15, peroxidase by 42.15% on day 10, and catalase by 90.78% on day 10. Additionally, it reduced the contents of hydrogen peroxide by 15.47% to 29.64% and the rate of formation of superoxide anions by 26.34% to 53.47% during the entire period of treatment. The transcriptomic analysis revealed a significant differential expression of the genes involved in pathways, such as phenylalanine, glutathione, arginine and proline, and nitrogen metabolism. These genes included <i>PALs</i>, <i>HDCs</i>, <i>GGCT</i>, <i>ODC1</i>, <i>LAPs</i>, <i>SMS</i>, and <i>SAMDC</i>. Notably, transcription factors, such as <i>ERF109</i>, <i>ARF9</i>, <i>GRF3</i>, <i>GRF4</i>, <i>GRF7</i>, and <i>GRF9</i>, were also significantly regulated. The study concluded that Cd and Pb stress enhanced the osmoregulatory and antioxidant defense systems in tomato seedlings, which may contribute to their tolerance to heavy metal stress. Future research could explore the application of these findings to develop strategies to improve the resistance of plants to contamination with heavy metals.