Ameliorative effects of strigolactone on tolerance to lead stress on lettuce (Lactuca sativa L.) plants.

Soil contamination by heavy metals, particularly lead (Pb), which is considered the second most toxic metal, poses serious risks to plants and humans due to its accumulation from various anthropogenic activities. Strigolactones (SLs) are a novel class of terpenoid lactones that play a vital role in regulating plant growth and development, particularly under stress conditions. This study aimed to investigate the impact of exogenous SL applications on plant growth and various physiological, biochemical, and molecular parameters in lettuce subjected to Pb stress. Pb stress harmed plant growth, whereas SL treatments improved growth parameters under both control and Pb stress conditions. While Pb stress increased the electrical conductivity (EC), malondialdehyde (MDA) and hydrogen peroxide (H2O2) content, SL applications caused a decrease in these parameters. Pb stress negatively affected chlorophyll content, whereas SL applications reduced negative effect. Pb caused an increase in superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) ascorbate peroxidase (APX), glutathione reductase (GR), glutathione s-transferase (GST) activities and Glucose-6-Phosphate Dehydrogenase (G6PD), 6-Phosphogluconate Dehydrogenase (6GPD). SL treatments significantly increased the activity of antioxidant enzymes in both control and Pb stress conditions. However, Pb stressed plants had lower nitrate reductase activity (NRA) than the control plants while SL treatments increased NRA compared to the non-treatments. Pb stress significantly reduced the uptake of essential nutrients in lettuce seedlings, whereas exogenous SL applications improved nutrient accumulation, particularly under Pb-stressed conditions. Additionally, mRNA expression profiles of nine stress-related genes in different tissues of lettuce were determined. Only Pb stress significantly decreased the expression of genes, particularly LsCCD8 and LsD14, in both tissues. The combined Pb and SL treatment significantly increased the expression of LsMAX2 in both tissues. These results suggest that exogenous SL applications can be an effective strategy to mitigate Pb-induced stress in lettuce by enhancing plant tolerance at physiological, biochemical, and molecular levels.